| Message ID | 20221129141803.1746898-5-ardb@kernel.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | arm64: Add return address protection to asm code | expand |
Hi Ard, On Tue, Nov 29, 2022 at 03:18:03PM +0100, Ard Biesheuvel wrote: > Use the newly added asm macros to protect and restore the return address > in the ftrace call wrappers, based on whichever method is active (PAC > and/or shadow call stack). > > If the graph tracer is in use, this covers both the return address *to* > the ftrace call site as well as the return address *at* the call site, > and the latter will either be restored in return_to_handler(), or before > returning to the call site. > > Signed-off-by: Ard Biesheuvel <ardb@kernel.org> > --- > arch/arm64/kernel/entry-ftrace.S | 28 +++++++++++++++++++- > 1 file changed, 27 insertions(+), 1 deletion(-) As a heads-up, this code has recently changed quite significantly, and this won't apply to the version queued in arm64's for-next/{ftrace,core} branches. I had a direction of travel in mind with some changes for better stacktracing, which won't work with the approach here, so I'd prefer we do this a bit differently; more on that below. > diff --git a/arch/arm64/kernel/entry-ftrace.S b/arch/arm64/kernel/entry-ftrace.S > index 795344ab4ec45889..c744e4dd8c90a352 100644 > --- a/arch/arm64/kernel/entry-ftrace.S > +++ b/arch/arm64/kernel/entry-ftrace.S > @@ -35,6 +35,11 @@ > * is missing from the LR and existing chain of frame records. > */ > .macro ftrace_regs_entry, allregs=0 > +#ifdef CONFIG_FUNCTION_GRAPH_TRACER > + protect_return_address x9 > +#endif > + protect_return_address x30 I think if we're going to protect the callsite's original LR (x9 here), we should do that regardless of CONFIG_FUNCTION_GRAPH_TRACER; what matters is whether that's vulnerable rather than whether we intend to modify it, so I don't think it makes sene to protect it conditionally based on CONFIG_FUNCTION_GRAPH_TRACER. I'm a bit worried this might confuse some ftrace code manipulating the return address (e.g. manipulation of the ftrace graph return stack), as I don't think that's all PAC-clean, and might need some modification. > + > /* Make room for pt_regs, plus a callee frame */ > sub sp, sp, #(PT_REGS_SIZE + 16) > > @@ -89,7 +94,9 @@ SYM_CODE_START(ftrace_caller) > b ftrace_common > SYM_CODE_END(ftrace_caller) > > -SYM_CODE_START(ftrace_common) > +SYM_CODE_START_LOCAL(ftrace_common) > + alternative_insn nop, "xpaci x30", ARM64_HAS_ADDRESS_AUTH, IS_ENABLED(CONFIG_ARM64_PTR_AUTH_KERNEL) > + > sub x0, x30, #AARCH64_INSN_SIZE // ip (callsite's BL insn) > mov x1, x9 // parent_ip (callsite's LR) > ldr_l x2, function_trace_op // op > @@ -115,9 +122,27 @@ SYM_INNER_LABEL(ftrace_call, SYM_L_GLOBAL) > ldr x30, [sp, #S_LR] > ldr x9, [sp, #S_PC] > > +#ifdef CONFIG_FUNCTION_GRAPH_TRACER > + /* grab the original return address from the stack */ > + ldr x10, [sp, #PT_REGS_SIZE + 8] > +#endif I'm planning to teach the stack unwinder how to unwind through ftrace_regs, such that we wouldn't need to duplicate the LR in a frame record here, and so we'd *only* have the copy inside the struct ftrace_regs. I think we don't need the copy here if we sign the callsite's LR against the base of the struct ftrace_regs. That way ftrace_graph_func() can sign the updated return address, and this code wouldn't need to care. The ftrace_regs have a copy of x18 that we can use to manipulate the SCS. > + > /* Restore the callsite's SP */ > add sp, sp, #PT_REGS_SIZE + 16 > > + restore_return_address x9 > +#ifdef CONFIG_FUNCTION_GRAPH_TRACER > + /* compare the original return address with the actual one */ > + cmp x10, x30 > + b.ne 0f > + > + /* > + * If they are the same, unprotect it now. If it was modified, it will > + * be dealt with in return_to_handler() below. > + */ > + restore_return_address x30 > +0: > +#endif > ret x9 This means if the return address is clobbered, we'll blindly trust it without authentication, which IMO undermines the point of signing it in the first place. As above, I'd prefer that we had ftrace_graph_func() fix things up so that we can unconditionally authenticate things here, which would be a bit stronger and simpler to reason about. Thanks, Mark. > SYM_CODE_END(ftrace_common) > > @@ -329,6 +354,7 @@ SYM_CODE_START(return_to_handler) > ldp x6, x7, [sp, #48] > add sp, sp, #64 > > + restore_return_address x30 > ret > SYM_CODE_END(return_to_handler) > #endif /* CONFIG_FUNCTION_GRAPH_TRACER */ > -- > 2.35.1 >
Hi Mark, On Wed, 30 Nov 2022 at 15:04, Mark Rutland <mark.rutland@arm.com> wrote: > > Hi Ard, > > On Tue, Nov 29, 2022 at 03:18:03PM +0100, Ard Biesheuvel wrote: > > Use the newly added asm macros to protect and restore the return address > > in the ftrace call wrappers, based on whichever method is active (PAC > > and/or shadow call stack). > > > > If the graph tracer is in use, this covers both the return address *to* > > the ftrace call site as well as the return address *at* the call site, > > and the latter will either be restored in return_to_handler(), or before > > returning to the call site. > > > > Signed-off-by: Ard Biesheuvel <ardb@kernel.org> > > --- > > arch/arm64/kernel/entry-ftrace.S | 28 +++++++++++++++++++- > > 1 file changed, 27 insertions(+), 1 deletion(-) > > As a heads-up, this code has recently changed quite significantly, and this > won't apply to the version queued in arm64's for-next/{ftrace,core} branches. > > I had a direction of travel in mind with some changes for better stacktracing, > which won't work with the approach here, so I'd prefer we do this a bit > differently; more on that below. > > > diff --git a/arch/arm64/kernel/entry-ftrace.S b/arch/arm64/kernel/entry-ftrace.S > > index 795344ab4ec45889..c744e4dd8c90a352 100644 > > --- a/arch/arm64/kernel/entry-ftrace.S > > +++ b/arch/arm64/kernel/entry-ftrace.S > > @@ -35,6 +35,11 @@ > > * is missing from the LR and existing chain of frame records. > > */ > > .macro ftrace_regs_entry, allregs=0 > > +#ifdef CONFIG_FUNCTION_GRAPH_TRACER > > + protect_return_address x9 > > +#endif > > + protect_return_address x30 > > I think if we're going to protect the callsite's original LR (x9 here), we > should do that regardless of CONFIG_FUNCTION_GRAPH_TRACER; what matters is > whether that's vulnerable rather than whether we intend to modify it, so I > don't think it makes sene to protect it conditionally based on > CONFIG_FUNCTION_GRAPH_TRACER. > My reasoning was that if we are not going to return from it (in return_to_handler()), we can rely on the interrupted function to sign/authenticate it as usual. So the only reason for signing it here is so that we can authenticate it in return_to_handler() if that exists on the call path, removing a potentially vulnerable sequence from that function. > I'm a bit worried this might confuse some ftrace code manipulating the return > address (e.g. manipulation of the ftrace graph return stack), as I don't think > that's all PAC-clean, and might need some modification. > This is the reason for the xpaci instruction below. > > + > > /* Make room for pt_regs, plus a callee frame */ > > sub sp, sp, #(PT_REGS_SIZE + 16) > > > > @@ -89,7 +94,9 @@ SYM_CODE_START(ftrace_caller) > > b ftrace_common > > SYM_CODE_END(ftrace_caller) > > > > -SYM_CODE_START(ftrace_common) > > +SYM_CODE_START_LOCAL(ftrace_common) > > + alternative_insn nop, "xpaci x30", ARM64_HAS_ADDRESS_AUTH, IS_ENABLED(CONFIG_ARM64_PTR_AUTH_KERNEL) > > + > > sub x0, x30, #AARCH64_INSN_SIZE // ip (callsite's BL insn) > > mov x1, x9 // parent_ip (callsite's LR) > > ldr_l x2, function_trace_op // op > > @@ -115,9 +122,27 @@ SYM_INNER_LABEL(ftrace_call, SYM_L_GLOBAL) > > ldr x30, [sp, #S_LR] > > ldr x9, [sp, #S_PC] > > > > +#ifdef CONFIG_FUNCTION_GRAPH_TRACER > > + /* grab the original return address from the stack */ > > + ldr x10, [sp, #PT_REGS_SIZE + 8] > > +#endif > > I'm planning to teach the stack unwinder how to unwind through ftrace_regs, > such that we wouldn't need to duplicate the LR in a frame record here, and so > we'd *only* have the copy inside the struct ftrace_regs. > Does doing so solve anything beyond reducing the stack footprint by 16 bytes? > I think we don't need the copy here if we sign the callsite's LR against the > base of the struct ftrace_regs. That way ftrace_graph_func() can sign the > updated return address, and this code wouldn't need to care. The ftrace_regs > have a copy of x18 that we can use to manipulate the SCS. > The updated return address will be signed when returning to the call site, and we never return from it here or anywhere else, so I don't think we need to sign it to begin with. What we need to sign here is the LR value that return_to_handler() will use, so ideally, we'd only sign the callsite's LR if we know we will be returning via return_to_handler(). > > + > > /* Restore the callsite's SP */ > > add sp, sp, #PT_REGS_SIZE + 16 > > > > + restore_return_address x9 > > +#ifdef CONFIG_FUNCTION_GRAPH_TRACER > > + /* compare the original return address with the actual one */ > > + cmp x10, x30 > > + b.ne 0f > > + > > + /* > > + * If they are the same, unprotect it now. If it was modified, it will > > + * be dealt with in return_to_handler() below. > > + */ > > + restore_return_address x30 > > +0: > > +#endif > > ret x9 > > This means if the return address is clobbered, we'll blindly trust it without > authentication, which IMO undermines the point of signing it in the first > place. > How do you mean? x9 is authenticated here, and x30 will either be authenticated here or in return_to_handler() > As above, I'd prefer that we had ftrace_graph_func() fix things up so that we > can unconditionally authenticate things here, which would be a bit stronger and > simpler to reason about. > I think having all in one place makes it much easier to reason about, tbh. Adding additional handling of the PAC state as well as the shadow call stack in ftrace_graph_func() seems much more fiddly to me.
On Wed, Nov 30, 2022 at 03:26:19PM +0100, Ard Biesheuvel wrote: > On Wed, 30 Nov 2022 at 15:04, Mark Rutland <mark.rutland@arm.com> wrote: > > On Tue, Nov 29, 2022 at 03:18:03PM +0100, Ard Biesheuvel wrote: > > > Use the newly added asm macros to protect and restore the return address > > > in the ftrace call wrappers, based on whichever method is active (PAC > > > and/or shadow call stack). > > > > > > If the graph tracer is in use, this covers both the return address *to* > > > the ftrace call site as well as the return address *at* the call site, > > > and the latter will either be restored in return_to_handler(), or before > > > returning to the call site. > > > > > > Signed-off-by: Ard Biesheuvel <ardb@kernel.org> > > > --- > > > arch/arm64/kernel/entry-ftrace.S | 28 +++++++++++++++++++- > > > 1 file changed, 27 insertions(+), 1 deletion(-) > > > > As a heads-up, this code has recently changed quite significantly, and this > > won't apply to the version queued in arm64's for-next/{ftrace,core} branches. > > > > I had a direction of travel in mind with some changes for better stacktracing, > > which won't work with the approach here, so I'd prefer we do this a bit > > differently; more on that below. > > > > > diff --git a/arch/arm64/kernel/entry-ftrace.S b/arch/arm64/kernel/entry-ftrace.S > > > index 795344ab4ec45889..c744e4dd8c90a352 100644 > > > --- a/arch/arm64/kernel/entry-ftrace.S > > > +++ b/arch/arm64/kernel/entry-ftrace.S > > > @@ -35,6 +35,11 @@ > > > * is missing from the LR and existing chain of frame records. > > > */ > > > .macro ftrace_regs_entry, allregs=0 > > > +#ifdef CONFIG_FUNCTION_GRAPH_TRACER > > > + protect_return_address x9 > > > +#endif > > > + protect_return_address x30 > > > > I think if we're going to protect the callsite's original LR (x9 here), we > > should do that regardless of CONFIG_FUNCTION_GRAPH_TRACER; what matters is > > whether that's vulnerable rather than whether we intend to modify it, so I > > don't think it makes sene to protect it conditionally based on > > CONFIG_FUNCTION_GRAPH_TRACER. > > My reasoning was that if we are not going to return from it (in > return_to_handler()), we can rely on the interrupted function to > sign/authenticate it as usual. So the only reason for signing it here > is so that we can authenticate it in return_to_handler() if that > exists on the call path, removing a potentially vulnerable sequence > from that function. What I was trying to point out is that there is a window where this is spilled to the stack (and hence is potentially vulnerable) between ftrace_{caller,regs_caller}() and the end of ftrace_common(). So if we don't protect this when CONFIG_FUNCTION_GRAPH_TRACER=n, it could be clobbered during that window (e.g. while function tracers are invoked), *before* we return back into the instrumented function and sign the (potentially already clobbered) value. Hence, my thinking is that we should sign this regardless of CONFIG_FUNCTION_GRAPH_TRACER to mitigate that case. I agree that we also want it to be signed while it's in the graph return stack (i.e. until the instrumented function returns back to return_to_handler()). In general, we should sign the value if it's going to be spilled to the stack. > > I'm a bit worried this might confuse some ftrace code manipulating the return > > address (e.g. manipulation of the ftrace graph return stack), as I don't think > > that's all PAC-clean, and might need some modification. > > This is the reason for the xpaci instruction below. Unfortunately, that alone isn't sufficient. What I was alluding to is that this change means the ftrace graph return stack contains signed addresses, and other code doesn't expect that. For example, arm64's stacktrace code currently depends on the graph return stack containing plain pointers, and so that gets broken as of this patch when function graph tracing is enabled: | # uname -a | # Linux buildroot 6.1.0-rc7-00003-g44a67f0b8ac7 #1 SMP PREEMPT Wed Nov 30 17:19:38 GMT 2022 aarch64 GNU/Linux | # cat /proc/self/stack | [<0>] proc_pid_stack+0xc0/0x130 | [<0>] proc_single_show+0x68/0x120 | [<0>] seq_read_iter+0x16c/0x45c | [<0>] seq_read+0x98/0xd0 | [<0>] vfs_read+0xc8/0x2c0 | [<0>] ksys_read+0x78/0x110 | [<0>] __arm64_sys_read+0x24/0x30 | [<0>] invoke_syscall+0x50/0x120 | [<0>] el0_svc_common.constprop.0+0xd4/0xf4 | [<0>] do_el0_svc+0x34/0xd0 | [<0>] el0_svc+0x2c/0x84 | [<0>] el0t_64_sync_handler+0xf4/0x120 | [<0>] el0t_64_sync+0x18c/0x190 | # echo function_graph > /sys/kernel/debug/tracing/current_tracer | # cat /proc/self/stack | [<0>] 0xf5f98000083dff40 | [<0>] 0xd6b88000083e0f68 | [<0>] 0x21ac800008381ad0 | [<0>] 0xd0bc800008381e58 | [<0>] 0x22b280000834bc28 | [<0>] 0xf0ca80000834c5c8 | [<0>] 0x299080000834c684 | [<0>] 0xb1a1800008029cf0 | [<0>] 0x9bd0800008029e94 | [<0>] 0x1788800008029ee8 | [<0>] 0xa08680000916dd5c | [<0>] el0t_64_sync_handler+0xf4/0x120 | [<0>] el0t_64_sync+0x18c/0x190 That's unfortunate (and would break RELIABLE_STACKTRACE, which we're slowly getting towards being able to implement), but it's simple enough to account for in the stacktrace code. I have a fear that there are other cases where code tries to consume the graph return stack (or to match against entries within it), which would be similarly broken. I vaguely recall that we had issues of that shape in the past when we tried to adjust the reported PC value, and would need to go page that in to check that we don't open a similar issue here. > > > + > > > /* Make room for pt_regs, plus a callee frame */ > > > sub sp, sp, #(PT_REGS_SIZE + 16) > > > > > > @@ -89,7 +94,9 @@ SYM_CODE_START(ftrace_caller) > > > b ftrace_common > > > SYM_CODE_END(ftrace_caller) > > > > > > -SYM_CODE_START(ftrace_common) > > > +SYM_CODE_START_LOCAL(ftrace_common) > > > + alternative_insn nop, "xpaci x30", ARM64_HAS_ADDRESS_AUTH, IS_ENABLED(CONFIG_ARM64_PTR_AUTH_KERNEL) > > > + > > > sub x0, x30, #AARCH64_INSN_SIZE // ip (callsite's BL insn) > > > mov x1, x9 // parent_ip (callsite's LR) > > > ldr_l x2, function_trace_op // op > > > @@ -115,9 +122,27 @@ SYM_INNER_LABEL(ftrace_call, SYM_L_GLOBAL) > > > ldr x30, [sp, #S_LR] > > > ldr x9, [sp, #S_PC] > > > > > > +#ifdef CONFIG_FUNCTION_GRAPH_TRACER > > > + /* grab the original return address from the stack */ > > > + ldr x10, [sp, #PT_REGS_SIZE + 8] > > > +#endif > > > > I'm planning to teach the stack unwinder how to unwind through ftrace_regs, > > such that we wouldn't need to duplicate the LR in a frame record here, and so > > we'd *only* have the copy inside the struct ftrace_regs. > > Does doing so solve anything beyond reducing the stack footprint by 16 bytes? My concern is functional rather than stack space. Having the single copy means that it's not possible for the two copies to become out-of-sync, and so the unwinder will always return the actual return address even when it has been rewritten. Thats important for livepatching where that may be changed for function redirection rather than tracing (and so there's not a return path to balance against), and similarly for ftrace direct calls / trampolines, which we might need to implement a variant of. I've already implemented similar logic for unwinding through the pt_regs, and I'm planning to clean that up and get it out after v6.2-rc1: https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/log/?h=arm64/stacktrace/metadata > > I think we don't need the copy here if we sign the callsite's LR against the > > base of the struct ftrace_regs. That way ftrace_graph_func() can sign the > > updated return address, and this code wouldn't need to care. The ftrace_regs > > have a copy of x18 that we can use to manipulate the SCS. > > The updated return address will be signed when returning to the call > site, and we never return from it here or anywhere else, so I don't > think we need to sign it to begin with. As above, there's a window where it's spilled ot the stack, and I think we should protect it for that window where it has been spilled. Otherwise it can be clobbered prior to being signed. > What we need to sign here is the LR value that return_to_handler() > will use, so ideally, we'd only sign the callsite's LR if we know we > will be returning via return_to_handler(). > > > > + > > > /* Restore the callsite's SP */ > > > add sp, sp, #PT_REGS_SIZE + 16 > > > > > > + restore_return_address x9 > > > +#ifdef CONFIG_FUNCTION_GRAPH_TRACER > > > + /* compare the original return address with the actual one */ > > > + cmp x10, x30 > > > + b.ne 0f > > > + > > > + /* > > > + * If they are the same, unprotect it now. If it was modified, it will > > > + * be dealt with in return_to_handler() below. > > > + */ > > > + restore_return_address x30 > > > +0: > > > +#endif > > > ret x9 > > > > This means if the return address is clobbered, we'll blindly trust it without > > authentication, which IMO undermines the point of signing it in the first > > place. > > How do you mean? x9 is authenticated here, and x30 will either be > authenticated here or in return_to_handler() I had confused myself here; sorry for the noise. > > As above, I'd prefer that we had ftrace_graph_func() fix things up so that we > > can unconditionally authenticate things here, which would be a bit stronger and > > simpler to reason about. > > > > I think having all in one place makes it much easier to reason about, > tbh. Adding additional handling of the PAC state as well as the shadow > call stack in ftrace_graph_func() seems much more fiddly to me. I appreciate that concern, but my intuition here is the inverse; I'd like to avoid the conditionality in the regular tracing path to make that clearly balanced and (from my perspective) easier to reason about. I'm happy if we have to do a bit more work in ftrace_graph_func() and return_to_handler() since those are already more special anyway. Thanks, Mark.
On Wed, 30 Nov 2022 at 18:45, Mark Rutland <mark.rutland@arm.com> wrote: > > On Wed, Nov 30, 2022 at 03:26:19PM +0100, Ard Biesheuvel wrote: > > On Wed, 30 Nov 2022 at 15:04, Mark Rutland <mark.rutland@arm.com> wrote: > > > On Tue, Nov 29, 2022 at 03:18:03PM +0100, Ard Biesheuvel wrote: > > > > Use the newly added asm macros to protect and restore the return address > > > > in the ftrace call wrappers, based on whichever method is active (PAC > > > > and/or shadow call stack). > > > > > > > > If the graph tracer is in use, this covers both the return address *to* > > > > the ftrace call site as well as the return address *at* the call site, > > > > and the latter will either be restored in return_to_handler(), or before > > > > returning to the call site. > > > > > > > > Signed-off-by: Ard Biesheuvel <ardb@kernel.org> > > > > --- > > > > arch/arm64/kernel/entry-ftrace.S | 28 +++++++++++++++++++- > > > > 1 file changed, 27 insertions(+), 1 deletion(-) > > > > > > As a heads-up, this code has recently changed quite significantly, and this > > > won't apply to the version queued in arm64's for-next/{ftrace,core} branches. > > > > > > I had a direction of travel in mind with some changes for better stacktracing, > > > which won't work with the approach here, so I'd prefer we do this a bit > > > differently; more on that below. > > > > > > > diff --git a/arch/arm64/kernel/entry-ftrace.S b/arch/arm64/kernel/entry-ftrace.S > > > > index 795344ab4ec45889..c744e4dd8c90a352 100644 > > > > --- a/arch/arm64/kernel/entry-ftrace.S > > > > +++ b/arch/arm64/kernel/entry-ftrace.S > > > > @@ -35,6 +35,11 @@ > > > > * is missing from the LR and existing chain of frame records. > > > > */ > > > > .macro ftrace_regs_entry, allregs=0 > > > > +#ifdef CONFIG_FUNCTION_GRAPH_TRACER > > > > + protect_return_address x9 > > > > +#endif > > > > + protect_return_address x30 > > > > > > I think if we're going to protect the callsite's original LR (x9 here), we > > > should do that regardless of CONFIG_FUNCTION_GRAPH_TRACER; what matters is > > > whether that's vulnerable rather than whether we intend to modify it, so I > > > don't think it makes sene to protect it conditionally based on > > > CONFIG_FUNCTION_GRAPH_TRACER. > > > > My reasoning was that if we are not going to return from it (in > > return_to_handler()), we can rely on the interrupted function to > > sign/authenticate it as usual. So the only reason for signing it here > > is so that we can authenticate it in return_to_handler() if that > > exists on the call path, removing a potentially vulnerable sequence > > from that function. > > What I was trying to point out is that there is a window where this is spilled > to the stack (and hence is potentially vulnerable) between > ftrace_{caller,regs_caller}() and the end of ftrace_common(). > > So if we don't protect this when CONFIG_FUNCTION_GRAPH_TRACER=n, it could be > clobbered during that window (e.g. while function tracers are invoked), > *before* we return back into the instrumented function and sign the > (potentially already clobbered) value. > Agreed. But to clarify, the intent of this series is not to add protection to ftrace, the intent is to get rid of the gadgets from the ftrace code that can be abused even if you don't use ftrace at all. > Hence, my thinking is that we should sign this regardless of > CONFIG_FUNCTION_GRAPH_TRACER to mitigate that case. I agree that we also want > it to be signed while it's in the graph return stack (i.e. until the > instrumented function returns back to return_to_handler()). In general, we > should sign the value if it's going to be spilled to the stack. > Sure, but it solves a different problem. > > > I'm a bit worried this might confuse some ftrace code manipulating the return > > > address (e.g. manipulation of the ftrace graph return stack), as I don't think > > > that's all PAC-clean, and might need some modification. > > > > This is the reason for the xpaci instruction below. > > Unfortunately, that alone isn't sufficient. > > What I was alluding to is that this change means the ftrace graph return stack > contains signed addresses, and other code doesn't expect that. For example, > arm64's stacktrace code currently depends on the graph return stack containing > plain pointers, and so that gets broken as of this patch when function graph > tracing is enabled: > > | # uname -a > | # Linux buildroot 6.1.0-rc7-00003-g44a67f0b8ac7 #1 SMP PREEMPT Wed Nov 30 17:19:38 GMT 2022 aarch64 GNU/Linux > | # cat /proc/self/stack > | [<0>] proc_pid_stack+0xc0/0x130 > | [<0>] proc_single_show+0x68/0x120 > | [<0>] seq_read_iter+0x16c/0x45c > | [<0>] seq_read+0x98/0xd0 > | [<0>] vfs_read+0xc8/0x2c0 > | [<0>] ksys_read+0x78/0x110 > | [<0>] __arm64_sys_read+0x24/0x30 > | [<0>] invoke_syscall+0x50/0x120 > | [<0>] el0_svc_common.constprop.0+0xd4/0xf4 > | [<0>] do_el0_svc+0x34/0xd0 > | [<0>] el0_svc+0x2c/0x84 > | [<0>] el0t_64_sync_handler+0xf4/0x120 > | [<0>] el0t_64_sync+0x18c/0x190 > | # echo function_graph > /sys/kernel/debug/tracing/current_tracer > | # cat /proc/self/stack > | [<0>] 0xf5f98000083dff40 > | [<0>] 0xd6b88000083e0f68 > | [<0>] 0x21ac800008381ad0 > | [<0>] 0xd0bc800008381e58 > | [<0>] 0x22b280000834bc28 > | [<0>] 0xf0ca80000834c5c8 > | [<0>] 0x299080000834c684 > | [<0>] 0xb1a1800008029cf0 > | [<0>] 0x9bd0800008029e94 > | [<0>] 0x1788800008029ee8 > | [<0>] 0xa08680000916dd5c > | [<0>] el0t_64_sync_handler+0xf4/0x120 > | [<0>] el0t_64_sync+0x18c/0x190 > > That's unfortunate (and would break RELIABLE_STACKTRACE, which we're slowly > getting towards being able to implement), but it's simple enough to account for > in the stacktrace code. > Indeed. Those functions should just strip the PAC bits, no? > I have a fear that there are other cases where code tries to consume the graph > return stack (or to match against entries within it), which would be similarly > broken. I vaguely recall that we had issues of that shape in the past when we > tried to adjust the reported PC value, and would need to go page that in to > check that we don't open a similar issue here. > OK > > > > + > > > > /* Make room for pt_regs, plus a callee frame */ > > > > sub sp, sp, #(PT_REGS_SIZE + 16) > > > > > > > > @@ -89,7 +94,9 @@ SYM_CODE_START(ftrace_caller) > > > > b ftrace_common > > > > SYM_CODE_END(ftrace_caller) > > > > > > > > -SYM_CODE_START(ftrace_common) > > > > +SYM_CODE_START_LOCAL(ftrace_common) > > > > + alternative_insn nop, "xpaci x30", ARM64_HAS_ADDRESS_AUTH, IS_ENABLED(CONFIG_ARM64_PTR_AUTH_KERNEL) > > > > + > > > > sub x0, x30, #AARCH64_INSN_SIZE // ip (callsite's BL insn) > > > > mov x1, x9 // parent_ip (callsite's LR) > > > > ldr_l x2, function_trace_op // op > > > > @@ -115,9 +122,27 @@ SYM_INNER_LABEL(ftrace_call, SYM_L_GLOBAL) > > > > ldr x30, [sp, #S_LR] > > > > ldr x9, [sp, #S_PC] > > > > > > > > +#ifdef CONFIG_FUNCTION_GRAPH_TRACER > > > > + /* grab the original return address from the stack */ > > > > + ldr x10, [sp, #PT_REGS_SIZE + 8] > > > > +#endif > > > > > > I'm planning to teach the stack unwinder how to unwind through ftrace_regs, > > > such that we wouldn't need to duplicate the LR in a frame record here, and so > > > we'd *only* have the copy inside the struct ftrace_regs. > > > > Does doing so solve anything beyond reducing the stack footprint by 16 bytes? > > My concern is functional rather than stack space. Having the single copy means > that it's not possible for the two copies to become out-of-sync, and so the > unwinder will always return the actual return address even when it has been > rewritten. Thats important for livepatching where that may be changed for > function redirection rather than tracing (and so there's not a return path to > balance against), and similarly for ftrace direct calls / trampolines, which we > might need to implement a variant of. > > I've already implemented similar logic for unwinding through the pt_regs, and > I'm planning to clean that up and get it out after v6.2-rc1: > > https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/log/?h=arm64/stacktrace/metadata > > > > I think we don't need the copy here if we sign the callsite's LR against the > > > base of the struct ftrace_regs. That way ftrace_graph_func() can sign the > > > updated return address, and this code wouldn't need to care. The ftrace_regs > > > have a copy of x18 that we can use to manipulate the SCS. > > > > The updated return address will be signed when returning to the call > > site, and we never return from it here or anywhere else, so I don't > > think we need to sign it to begin with. > > As above, there's a window where it's spilled ot the stack, and I think we > should protect it for that window where it has been spilled. Otherwise it can > be clobbered prior to being signed. > Yes, if ftrace is in use. > > What we need to sign here is the LR value that return_to_handler() > > will use, so ideally, we'd only sign the callsite's LR if we know we > > will be returning via return_to_handler(). > > > > > > + > > > > /* Restore the callsite's SP */ > > > > add sp, sp, #PT_REGS_SIZE + 16 > > > > > > > > + restore_return_address x9 > > > > +#ifdef CONFIG_FUNCTION_GRAPH_TRACER > > > > + /* compare the original return address with the actual one */ > > > > + cmp x10, x30 > > > > + b.ne 0f > > > > + > > > > + /* > > > > + * If they are the same, unprotect it now. If it was modified, it will > > > > + * be dealt with in return_to_handler() below. > > > > + */ > > > > + restore_return_address x30 > > > > +0: > > > > +#endif > > > > ret x9 > > > > > > This means if the return address is clobbered, we'll blindly trust it without > > > authentication, which IMO undermines the point of signing it in the first > > > place. > > > > How do you mean? x9 is authenticated here, and x30 will either be > > authenticated here or in return_to_handler() > > I had confused myself here; sorry for the noise. > > > > As above, I'd prefer that we had ftrace_graph_func() fix things up so that we > > > can unconditionally authenticate things here, which would be a bit stronger and > > > simpler to reason about. > > > > > > > I think having all in one place makes it much easier to reason about, > > tbh. Adding additional handling of the PAC state as well as the shadow > > call stack in ftrace_graph_func() seems much more fiddly to me. > > I appreciate that concern, but my intuition here is the inverse; I'd like to > avoid the conditionality in the regular tracing path to make that clearly > balanced and (from my perspective) easier to reason about. > > I'm happy if we have to do a bit more work in ftrace_graph_func() and > return_to_handler() since those are already more special anyway. > Fair enough. As long as the asm routines have a SCS pop or AUTIASP between reloading x30 and returning to it, I don't have any problems with that.
On Thu, Dec 01, 2022 at 02:09:41PM +0100, Ard Biesheuvel wrote: > On Wed, 30 Nov 2022 at 18:45, Mark Rutland <mark.rutland@arm.com> wrote: > > > > On Wed, Nov 30, 2022 at 03:26:19PM +0100, Ard Biesheuvel wrote: > > > On Wed, 30 Nov 2022 at 15:04, Mark Rutland <mark.rutland@arm.com> wrote: > > > > On Tue, Nov 29, 2022 at 03:18:03PM +0100, Ard Biesheuvel wrote: > > > > > Use the newly added asm macros to protect and restore the return address > > > > > in the ftrace call wrappers, based on whichever method is active (PAC > > > > > and/or shadow call stack). > > > > > > > > > > If the graph tracer is in use, this covers both the return address *to* > > > > > the ftrace call site as well as the return address *at* the call site, > > > > > and the latter will either be restored in return_to_handler(), or before > > > > > returning to the call site. > > > > > > > > > > Signed-off-by: Ard Biesheuvel <ardb@kernel.org> > > > > > --- > > > > > arch/arm64/kernel/entry-ftrace.S | 28 +++++++++++++++++++- > > > > > 1 file changed, 27 insertions(+), 1 deletion(-) > > > > > > > > As a heads-up, this code has recently changed quite significantly, and this > > > > won't apply to the version queued in arm64's for-next/{ftrace,core} branches. > > > > > > > > I had a direction of travel in mind with some changes for better stacktracing, > > > > which won't work with the approach here, so I'd prefer we do this a bit > > > > differently; more on that below. > > > > > > > > > diff --git a/arch/arm64/kernel/entry-ftrace.S b/arch/arm64/kernel/entry-ftrace.S > > > > > index 795344ab4ec45889..c744e4dd8c90a352 100644 > > > > > --- a/arch/arm64/kernel/entry-ftrace.S > > > > > +++ b/arch/arm64/kernel/entry-ftrace.S > > > > > @@ -35,6 +35,11 @@ > > > > > * is missing from the LR and existing chain of frame records. > > > > > */ > > > > > .macro ftrace_regs_entry, allregs=0 > > > > > +#ifdef CONFIG_FUNCTION_GRAPH_TRACER > > > > > + protect_return_address x9 > > > > > +#endif > > > > > + protect_return_address x30 > > > > > > > > I think if we're going to protect the callsite's original LR (x9 here), we > > > > should do that regardless of CONFIG_FUNCTION_GRAPH_TRACER; what matters is > > > > whether that's vulnerable rather than whether we intend to modify it, so I > > > > don't think it makes sene to protect it conditionally based on > > > > CONFIG_FUNCTION_GRAPH_TRACER. > > > > > > My reasoning was that if we are not going to return from it (in > > > return_to_handler()), we can rely on the interrupted function to > > > sign/authenticate it as usual. So the only reason for signing it here > > > is so that we can authenticate it in return_to_handler() if that > > > exists on the call path, removing a potentially vulnerable sequence > > > from that function. > > > > What I was trying to point out is that there is a window where this is spilled > > to the stack (and hence is potentially vulnerable) between > > ftrace_{caller,regs_caller}() and the end of ftrace_common(). > > > > So if we don't protect this when CONFIG_FUNCTION_GRAPH_TRACER=n, it could be > > clobbered during that window (e.g. while function tracers are invoked), > > *before* we return back into the instrumented function and sign the > > (potentially already clobbered) value. > > Agreed. > > But to clarify, the intent of this series is not to add protection to > ftrace, the intent is to get rid of the gadgets from the ftrace code > that can be abused even if you don't use ftrace at all. Ok; sorry for missing that; I'll need to think a little harder. > > Hence, my thinking is that we should sign this regardless of > > CONFIG_FUNCTION_GRAPH_TRACER to mitigate that case. I agree that we also want > > it to be signed while it's in the graph return stack (i.e. until the > > instrumented function returns back to return_to_handler()). In general, we > > should sign the value if it's going to be spilled to the stack. > > Sure, but it solves a different problem. Fair enough! I think we're agreed that something which solves both issues makes sense, even if that's not necessary for the gadgetisation issue specifically? > > > > I'm a bit worried this might confuse some ftrace code manipulating the return > > > > address (e.g. manipulation of the ftrace graph return stack), as I don't think > > > > that's all PAC-clean, and might need some modification. > > > > > > This is the reason for the xpaci instruction below. > > > > Unfortunately, that alone isn't sufficient. > > > > What I was alluding to is that this change means the ftrace graph return stack > > contains signed addresses, and other code doesn't expect that. For example, > > arm64's stacktrace code currently depends on the graph return stack containing > > plain pointers, and so that gets broken as of this patch when function graph > > tracing is enabled: > > > > | # uname -a > > | # Linux buildroot 6.1.0-rc7-00003-g44a67f0b8ac7 #1 SMP PREEMPT Wed Nov 30 17:19:38 GMT 2022 aarch64 GNU/Linux > > | # cat /proc/self/stack > > | [<0>] proc_pid_stack+0xc0/0x130 > > | [<0>] proc_single_show+0x68/0x120 > > | [<0>] seq_read_iter+0x16c/0x45c > > | [<0>] seq_read+0x98/0xd0 > > | [<0>] vfs_read+0xc8/0x2c0 > > | [<0>] ksys_read+0x78/0x110 > > | [<0>] __arm64_sys_read+0x24/0x30 > > | [<0>] invoke_syscall+0x50/0x120 > > | [<0>] el0_svc_common.constprop.0+0xd4/0xf4 > > | [<0>] do_el0_svc+0x34/0xd0 > > | [<0>] el0_svc+0x2c/0x84 > > | [<0>] el0t_64_sync_handler+0xf4/0x120 > > | [<0>] el0t_64_sync+0x18c/0x190 > > | # echo function_graph > /sys/kernel/debug/tracing/current_tracer > > | # cat /proc/self/stack > > | [<0>] 0xf5f98000083dff40 > > | [<0>] 0xd6b88000083e0f68 > > | [<0>] 0x21ac800008381ad0 > > | [<0>] 0xd0bc800008381e58 > > | [<0>] 0x22b280000834bc28 > > | [<0>] 0xf0ca80000834c5c8 > > | [<0>] 0x299080000834c684 > > | [<0>] 0xb1a1800008029cf0 > > | [<0>] 0x9bd0800008029e94 > > | [<0>] 0x1788800008029ee8 > > | [<0>] 0xa08680000916dd5c > > | [<0>] el0t_64_sync_handler+0xf4/0x120 > > | [<0>] el0t_64_sync+0x18c/0x190 > > > > That's unfortunate (and would break RELIABLE_STACKTRACE, which we're slowly > > getting towards being able to implement), but it's simple enough to account for > > in the stacktrace code. > > > > Indeed. Those functions should just strip the PAC bits, no? For that case, yup. That was roughly what I meant about it being simple to deal with in the stacktrace code. :) > > I have a fear that there are other cases where code tries to consume the graph > > return stack (or to match against entries within it), which would be similarly > > broken. I vaguely recall that we had issues of that shape in the past when we > > tried to adjust the reported PC value, and would need to go page that in to > > check that we don't open a similar issue here. > > OK FWIW, I'm happy to go audit that, I just wanted to make sure we didn't forget to do so, since it's not obvious that there are potential issues there. [...] > > > > > /* Restore the callsite's SP */ > > > > > add sp, sp, #PT_REGS_SIZE + 16 > > > > > > > > > > + restore_return_address x9 > > > > > +#ifdef CONFIG_FUNCTION_GRAPH_TRACER > > > > > + /* compare the original return address with the actual one */ > > > > > + cmp x10, x30 > > > > > + b.ne 0f > > > > > + > > > > > + /* > > > > > + * If they are the same, unprotect it now. If it was modified, it will > > > > > + * be dealt with in return_to_handler() below. > > > > > + */ > > > > > + restore_return_address x30 > > > > > +0: > > > > > +#endif > > > > > ret x9 > > > > As above, I'd prefer that we had ftrace_graph_func() fix things up so that we > > > > can unconditionally authenticate things here, which would be a bit stronger and > > > > simpler to reason about. > > > > > > > > > > I think having all in one place makes it much easier to reason about, > > > tbh. Adding additional handling of the PAC state as well as the shadow > > > call stack in ftrace_graph_func() seems much more fiddly to me. > > > > I appreciate that concern, but my intuition here is the inverse; I'd like to > > avoid the conditionality in the regular tracing path to make that clearly > > balanced and (from my perspective) easier to reason about. > > > > I'm happy if we have to do a bit more work in ftrace_graph_func() and > > return_to_handler() since those are already more special anyway. > > > > Fair enough. As long as the asm routines have a SCS pop or AUTIASP > between reloading x30 and returning to it, I don't have any problems > with that. Sure; I think that's workable. I have a rough shape in mind, so I'll have a go at that as an example and try to get back to you shortly. With that in mind, I think we should also fix up qcom_link_stack_sanitisation(), since that ends up creating a gadget of the form: MOV X30, Xn RET ... and that can be fixed by leaving it to the compiler to save/restore x30, whereupon it should create a frame record and all the usual PAC goodness. Example patch below (reformatted into the usual arm64 inline asm style). Thanks, Mark. ---->8---- diff --git a/arch/arm64/kernel/proton-pack.c b/arch/arm64/kernel/proton-pack.c index bfce41c2a53b3..9fc54facf1ccb 100644 --- a/arch/arm64/kernel/proton-pack.c +++ b/arch/arm64/kernel/proton-pack.c @@ -250,12 +250,13 @@ static noinstr void qcom_link_stack_sanitisation(void) { u64 tmp; - asm volatile("mov %0, x30 \n" - ".rept 16 \n" - "bl . + 4 \n" - ".endr \n" - "mov x30, %0 \n" - : "=&r" (tmp)); + asm volatile( + " .rept 16 \n" + " bl . + 4 \n" + " .endr \n" + : "=&r" (tmp) + : + : "x30"); } static bp_hardening_cb_t spectre_v2_get_sw_mitigation_cb(void)
On Thu, 1 Dec 2022 at 15:40, Mark Rutland <mark.rutland@arm.com> wrote: > > On Thu, Dec 01, 2022 at 02:09:41PM +0100, Ard Biesheuvel wrote: > > On Wed, 30 Nov 2022 at 18:45, Mark Rutland <mark.rutland@arm.com> wrote: > > > > > > On Wed, Nov 30, 2022 at 03:26:19PM +0100, Ard Biesheuvel wrote: > > > > On Wed, 30 Nov 2022 at 15:04, Mark Rutland <mark.rutland@arm.com> wrote: > > > > > On Tue, Nov 29, 2022 at 03:18:03PM +0100, Ard Biesheuvel wrote: > > > > > > Use the newly added asm macros to protect and restore the return address > > > > > > in the ftrace call wrappers, based on whichever method is active (PAC > > > > > > and/or shadow call stack). > > > > > > > > > > > > If the graph tracer is in use, this covers both the return address *to* > > > > > > the ftrace call site as well as the return address *at* the call site, > > > > > > and the latter will either be restored in return_to_handler(), or before > > > > > > returning to the call site. > > > > > > > > > > > > Signed-off-by: Ard Biesheuvel <ardb@kernel.org> > > > > > > --- > > > > > > arch/arm64/kernel/entry-ftrace.S | 28 +++++++++++++++++++- > > > > > > 1 file changed, 27 insertions(+), 1 deletion(-) > > > > > > > > > > As a heads-up, this code has recently changed quite significantly, and this > > > > > won't apply to the version queued in arm64's for-next/{ftrace,core} branches. > > > > > > > > > > I had a direction of travel in mind with some changes for better stacktracing, > > > > > which won't work with the approach here, so I'd prefer we do this a bit > > > > > differently; more on that below. > > > > > > > > > > > diff --git a/arch/arm64/kernel/entry-ftrace.S b/arch/arm64/kernel/entry-ftrace.S > > > > > > index 795344ab4ec45889..c744e4dd8c90a352 100644 > > > > > > --- a/arch/arm64/kernel/entry-ftrace.S > > > > > > +++ b/arch/arm64/kernel/entry-ftrace.S > > > > > > @@ -35,6 +35,11 @@ > > > > > > * is missing from the LR and existing chain of frame records. > > > > > > */ > > > > > > .macro ftrace_regs_entry, allregs=0 > > > > > > +#ifdef CONFIG_FUNCTION_GRAPH_TRACER > > > > > > + protect_return_address x9 > > > > > > +#endif > > > > > > + protect_return_address x30 > > > > > > > > > > I think if we're going to protect the callsite's original LR (x9 here), we > > > > > should do that regardless of CONFIG_FUNCTION_GRAPH_TRACER; what matters is > > > > > whether that's vulnerable rather than whether we intend to modify it, so I > > > > > don't think it makes sene to protect it conditionally based on > > > > > CONFIG_FUNCTION_GRAPH_TRACER. > > > > > > > > My reasoning was that if we are not going to return from it (in > > > > return_to_handler()), we can rely on the interrupted function to > > > > sign/authenticate it as usual. So the only reason for signing it here > > > > is so that we can authenticate it in return_to_handler() if that > > > > exists on the call path, removing a potentially vulnerable sequence > > > > from that function. > > > > > > What I was trying to point out is that there is a window where this is spilled > > > to the stack (and hence is potentially vulnerable) between > > > ftrace_{caller,regs_caller}() and the end of ftrace_common(). > > > > > > So if we don't protect this when CONFIG_FUNCTION_GRAPH_TRACER=n, it could be > > > clobbered during that window (e.g. while function tracers are invoked), > > > *before* we return back into the instrumented function and sign the > > > (potentially already clobbered) value. > > > > Agreed. > > > > But to clarify, the intent of this series is not to add protection to > > ftrace, the intent is to get rid of the gadgets from the ftrace code > > that can be abused even if you don't use ftrace at all. > > Ok; sorry for missing that; I'll need to think a little harder. > You said it :-) > > > Hence, my thinking is that we should sign this regardless of > > > CONFIG_FUNCTION_GRAPH_TRACER to mitigate that case. I agree that we also want > > > it to be signed while it's in the graph return stack (i.e. until the > > > instrumented function returns back to return_to_handler()). In general, we > > > should sign the value if it's going to be spilled to the stack. > > > > Sure, but it solves a different problem. > > Fair enough! > > I think we're agreed that something which solves both issues makes sense, even > if that's not necessary for the gadgetisation issue specifically? > Of course. So the issue we are talking about here is the fact that you might be able to attack the ftrace infrastructure while it is being used so that the function return from ftrace_common() is made to point somewhere else. I agree that this is something we might want to harden, and I also wonder whether we should perhaps insert three NOPs instead of two, or teach the compiler to put its PACIASP right after so that we can use BR instead of RET to perform the return. But again, this is ground that I am currently not attempting to cover. > > > > > I'm a bit worried this might confuse some ftrace code manipulating the return > > > > > address (e.g. manipulation of the ftrace graph return stack), as I don't think > > > > > that's all PAC-clean, and might need some modification. > > > > > > > > This is the reason for the xpaci instruction below. > > > > > > Unfortunately, that alone isn't sufficient. > > > > > > What I was alluding to is that this change means the ftrace graph return stack > > > contains signed addresses, and other code doesn't expect that. For example, > > > arm64's stacktrace code currently depends on the graph return stack containing > > > plain pointers, and so that gets broken as of this patch when function graph > > > tracing is enabled: > > > > > > | # uname -a > > > | # Linux buildroot 6.1.0-rc7-00003-g44a67f0b8ac7 #1 SMP PREEMPT Wed Nov 30 17:19:38 GMT 2022 aarch64 GNU/Linux > > > | # cat /proc/self/stack > > > | [<0>] proc_pid_stack+0xc0/0x130 > > > | [<0>] proc_single_show+0x68/0x120 > > > | [<0>] seq_read_iter+0x16c/0x45c > > > | [<0>] seq_read+0x98/0xd0 > > > | [<0>] vfs_read+0xc8/0x2c0 > > > | [<0>] ksys_read+0x78/0x110 > > > | [<0>] __arm64_sys_read+0x24/0x30 > > > | [<0>] invoke_syscall+0x50/0x120 > > > | [<0>] el0_svc_common.constprop.0+0xd4/0xf4 > > > | [<0>] do_el0_svc+0x34/0xd0 > > > | [<0>] el0_svc+0x2c/0x84 > > > | [<0>] el0t_64_sync_handler+0xf4/0x120 > > > | [<0>] el0t_64_sync+0x18c/0x190 > > > | # echo function_graph > /sys/kernel/debug/tracing/current_tracer > > > | # cat /proc/self/stack > > > | [<0>] 0xf5f98000083dff40 > > > | [<0>] 0xd6b88000083e0f68 > > > | [<0>] 0x21ac800008381ad0 > > > | [<0>] 0xd0bc800008381e58 > > > | [<0>] 0x22b280000834bc28 > > > | [<0>] 0xf0ca80000834c5c8 > > > | [<0>] 0x299080000834c684 > > > | [<0>] 0xb1a1800008029cf0 > > > | [<0>] 0x9bd0800008029e94 > > > | [<0>] 0x1788800008029ee8 > > > | [<0>] 0xa08680000916dd5c > > > | [<0>] el0t_64_sync_handler+0xf4/0x120 > > > | [<0>] el0t_64_sync+0x18c/0x190 > > > > > > That's unfortunate (and would break RELIABLE_STACKTRACE, which we're slowly > > > getting towards being able to implement), but it's simple enough to account for > > > in the stacktrace code. > > > > > > > Indeed. Those functions should just strip the PAC bits, no? > > For that case, yup. That was roughly what I meant about it being simple to deal > with in the stacktrace code. :) > Right. So given that this is an issue for PAC but not for shadow call stack, we might consider a shorter term fix where we push/pop these addresses to the shadow call stack, and address the PAC clearing more comprehensively once we get around to it. > > > I have a fear that there are other cases where code tries to consume the graph > > > return stack (or to match against entries within it), which would be similarly > > > broken. I vaguely recall that we had issues of that shape in the past when we > > > tried to adjust the reported PC value, and would need to go page that in to > > > check that we don't open a similar issue here. > > > > OK > > FWIW, I'm happy to go audit that, I just wanted to make sure we didn't forget > to do so, since it's not obvious that there are potential issues there. > Great. > > > > > > /* Restore the callsite's SP */ > > > > > > add sp, sp, #PT_REGS_SIZE + 16 > > > > > > > > > > > > + restore_return_address x9 > > > > > > +#ifdef CONFIG_FUNCTION_GRAPH_TRACER > > > > > > + /* compare the original return address with the actual one */ > > > > > > + cmp x10, x30 > > > > > > + b.ne 0f > > > > > > + > > > > > > + /* > > > > > > + * If they are the same, unprotect it now. If it was modified, it will > > > > > > + * be dealt with in return_to_handler() below. > > > > > > + */ > > > > > > + restore_return_address x30 > > > > > > +0: > > > > > > +#endif > > > > > > ret x9 > > > > > > As above, I'd prefer that we had ftrace_graph_func() fix things up so that we > > > > > can unconditionally authenticate things here, which would be a bit stronger and > > > > > simpler to reason about. > > > > > > > > > > > > > I think having all in one place makes it much easier to reason about, > > > > tbh. Adding additional handling of the PAC state as well as the shadow > > > > call stack in ftrace_graph_func() seems much more fiddly to me. > > > > > > I appreciate that concern, but my intuition here is the inverse; I'd like to > > > avoid the conditionality in the regular tracing path to make that clearly > > > balanced and (from my perspective) easier to reason about. > > > > > > I'm happy if we have to do a bit more work in ftrace_graph_func() and > > > return_to_handler() since those are already more special anyway. > > > > > > > Fair enough. As long as the asm routines have a SCS pop or AUTIASP > > between reloading x30 and returning to it, I don't have any problems > > with that. > > Sure; I think that's workable. I have a rough shape in mind, so I'll have a go > at that as an example and try to get back to you shortly. > Thanks. > With that in mind, I think we should also fix up > qcom_link_stack_sanitisation(), since that ends up creating a gadget of the form: > > MOV X30, Xn > RET > > ... and that can be fixed by leaving it to the compiler to save/restore x30, > whereupon it should create a frame record and all the usual PAC goodness. > Example patch below (reformatted into the usual arm64 inline asm style). > > Thanks, > Mark. > > ---->8---- > diff --git a/arch/arm64/kernel/proton-pack.c b/arch/arm64/kernel/proton-pack.c > index bfce41c2a53b3..9fc54facf1ccb 100644 > --- a/arch/arm64/kernel/proton-pack.c > +++ b/arch/arm64/kernel/proton-pack.c > @@ -250,12 +250,13 @@ static noinstr void qcom_link_stack_sanitisation(void) > { > u64 tmp; > > - asm volatile("mov %0, x30 \n" > - ".rept 16 \n" > - "bl . + 4 \n" > - ".endr \n" > - "mov x30, %0 \n" > - : "=&r" (tmp)); > + asm volatile( > + " .rept 16 \n" > + " bl . + 4 \n" > + " .endr \n" > + : "=&r" (tmp) > + : > + : "x30"); > } > Yeah, I'm sure that's the last one :-)
On Thu, Dec 01, 2022 at 04:05:35PM +0100, Ard Biesheuvel wrote: > On Thu, 1 Dec 2022 at 15:40, Mark Rutland <mark.rutland@arm.com> wrote: > > > > On Thu, Dec 01, 2022 at 02:09:41PM +0100, Ard Biesheuvel wrote: > > > On Wed, 30 Nov 2022 at 18:45, Mark Rutland <mark.rutland@arm.com> wrote: > > > > > > > > On Wed, Nov 30, 2022 at 03:26:19PM +0100, Ard Biesheuvel wrote: > > > > > On Wed, 30 Nov 2022 at 15:04, Mark Rutland <mark.rutland@arm.com> wrote: > > > > > > On Tue, Nov 29, 2022 at 03:18:03PM +0100, Ard Biesheuvel wrote: > > > > > > > Use the newly added asm macros to protect and restore the return address > > > > > > > in the ftrace call wrappers, based on whichever method is active (PAC > > > > > > > and/or shadow call stack). > > > > > > > > > > > > > > If the graph tracer is in use, this covers both the return address *to* > > > > > > > the ftrace call site as well as the return address *at* the call site, > > > > > > > and the latter will either be restored in return_to_handler(), or before > > > > > > > returning to the call site. > > > > > > > > > > > > > > Signed-off-by: Ard Biesheuvel <ardb@kernel.org> > > > > > > > --- > > > > > > > arch/arm64/kernel/entry-ftrace.S | 28 +++++++++++++++++++- > > > > > > > 1 file changed, 27 insertions(+), 1 deletion(-) > > > > > > > > > > > > As a heads-up, this code has recently changed quite significantly, and this > > > > > > won't apply to the version queued in arm64's for-next/{ftrace,core} branches. > > > > > > > > > > > > I had a direction of travel in mind with some changes for better stacktracing, > > > > > > which won't work with the approach here, so I'd prefer we do this a bit > > > > > > differently; more on that below. > > > > > > > > > > > > > diff --git a/arch/arm64/kernel/entry-ftrace.S b/arch/arm64/kernel/entry-ftrace.S > > > > > > > index 795344ab4ec45889..c744e4dd8c90a352 100644 > > > > > > > --- a/arch/arm64/kernel/entry-ftrace.S > > > > > > > +++ b/arch/arm64/kernel/entry-ftrace.S > > > > > > > @@ -35,6 +35,11 @@ > > > > > > > * is missing from the LR and existing chain of frame records. > > > > > > > */ > > > > > > > .macro ftrace_regs_entry, allregs=0 > > > > > > > +#ifdef CONFIG_FUNCTION_GRAPH_TRACER > > > > > > > + protect_return_address x9 > > > > > > > +#endif > > > > > > > + protect_return_address x30 > > > > > > > > > > > > I think if we're going to protect the callsite's original LR (x9 here), we > > > > > > should do that regardless of CONFIG_FUNCTION_GRAPH_TRACER; what matters is > > > > > > whether that's vulnerable rather than whether we intend to modify it, so I > > > > > > don't think it makes sene to protect it conditionally based on > > > > > > CONFIG_FUNCTION_GRAPH_TRACER. > > > > > > > > > > My reasoning was that if we are not going to return from it (in > > > > > return_to_handler()), we can rely on the interrupted function to > > > > > sign/authenticate it as usual. So the only reason for signing it here > > > > > is so that we can authenticate it in return_to_handler() if that > > > > > exists on the call path, removing a potentially vulnerable sequence > > > > > from that function. > > > > > > > > What I was trying to point out is that there is a window where this is spilled > > > > to the stack (and hence is potentially vulnerable) between > > > > ftrace_{caller,regs_caller}() and the end of ftrace_common(). > > > > > > > > So if we don't protect this when CONFIG_FUNCTION_GRAPH_TRACER=n, it could be > > > > clobbered during that window (e.g. while function tracers are invoked), > > > > *before* we return back into the instrumented function and sign the > > > > (potentially already clobbered) value. > > > > > > Agreed. > > > > > > But to clarify, the intent of this series is not to add protection to > > > ftrace, the intent is to get rid of the gadgets from the ftrace code > > > that can be abused even if you don't use ftrace at all. > > > > Ok; sorry for missing that; I'll need to think a little harder. > > > > You said it :-) > > > > > Hence, my thinking is that we should sign this regardless of > > > > CONFIG_FUNCTION_GRAPH_TRACER to mitigate that case. I agree that we also want > > > > it to be signed while it's in the graph return stack (i.e. until the > > > > instrumented function returns back to return_to_handler()). In general, we > > > > should sign the value if it's going to be spilled to the stack. > > > > > > Sure, but it solves a different problem. > > > > Fair enough! > > > > I think we're agreed that something which solves both issues makes sense, even > > if that's not necessary for the gadgetisation issue specifically? > > Of course. Great -- just wanted to check there wasn't an inverse problem I'd missed! > So the issue we are talking about here is the fact that you might be > able to attack the ftrace infrastructure while it is being used so > that the function return from ftrace_common() is made to point > somewhere else. Yup. I suspect the risk is must lower due to the smaller amount of code there, but given things like fprobe and BPF hooks, there might be code that gets injected there which isn't as careful as we'd like, so it would be nice to protect. > I agree that this is something we might want to > harden, and I also wonder whether we should perhaps insert three NOPs > instead of two, or teach the compiler to put its PACIASP right after > so that we can use BR instead of RET to perform the return. I think that approach is a mixed bag :/ I was hoping that we could reduce the set of BTI-compatible instructions we have, and I'd like to get to a point where we can set SCTLR_ELx.BT1=1 so that PACIASP isn't an implicit BTI in the kernel. That way we'd be in a similar boat to x86 after redundant ENDBRs are removed, with forward-edge protection being strengthened and EXPORT-control being strengthened. That needs new compiler help to output separate BTI and PACIASP instructions, but otherwise that's relatively simple, and could significantly reduce the set of gadgetizable functions regardless of whether ftrace is in use. Given that, I'm not keen on adding an extra BTI-compatible instruction into function prologues. > But again, this is ground that I am currently not attempting to cover. > > > > > > > I'm a bit worried this might confuse some ftrace code manipulating the return > > > > > > address (e.g. manipulation of the ftrace graph return stack), as I don't think > > > > > > that's all PAC-clean, and might need some modification. > > > > > > > > > > This is the reason for the xpaci instruction below. > > > > > > > > Unfortunately, that alone isn't sufficient. > > > > > > > > What I was alluding to is that this change means the ftrace graph return stack > > > > contains signed addresses, and other code doesn't expect that. For example, > > > > arm64's stacktrace code currently depends on the graph return stack containing > > > > plain pointers, and so that gets broken as of this patch when function graph > > > > tracing is enabled: > > > > > > > > | # uname -a > > > > | # Linux buildroot 6.1.0-rc7-00003-g44a67f0b8ac7 #1 SMP PREEMPT Wed Nov 30 17:19:38 GMT 2022 aarch64 GNU/Linux > > > > | # cat /proc/self/stack > > > > | [<0>] proc_pid_stack+0xc0/0x130 > > > > | [<0>] proc_single_show+0x68/0x120 > > > > | [<0>] seq_read_iter+0x16c/0x45c > > > > | [<0>] seq_read+0x98/0xd0 > > > > | [<0>] vfs_read+0xc8/0x2c0 > > > > | [<0>] ksys_read+0x78/0x110 > > > > | [<0>] __arm64_sys_read+0x24/0x30 > > > > | [<0>] invoke_syscall+0x50/0x120 > > > > | [<0>] el0_svc_common.constprop.0+0xd4/0xf4 > > > > | [<0>] do_el0_svc+0x34/0xd0 > > > > | [<0>] el0_svc+0x2c/0x84 > > > > | [<0>] el0t_64_sync_handler+0xf4/0x120 > > > > | [<0>] el0t_64_sync+0x18c/0x190 > > > > | # echo function_graph > /sys/kernel/debug/tracing/current_tracer > > > > | # cat /proc/self/stack > > > > | [<0>] 0xf5f98000083dff40 > > > > | [<0>] 0xd6b88000083e0f68 > > > > | [<0>] 0x21ac800008381ad0 > > > > | [<0>] 0xd0bc800008381e58 > > > > | [<0>] 0x22b280000834bc28 > > > > | [<0>] 0xf0ca80000834c5c8 > > > > | [<0>] 0x299080000834c684 > > > > | [<0>] 0xb1a1800008029cf0 > > > > | [<0>] 0x9bd0800008029e94 > > > > | [<0>] 0x1788800008029ee8 > > > > | [<0>] 0xa08680000916dd5c > > > > | [<0>] el0t_64_sync_handler+0xf4/0x120 > > > > | [<0>] el0t_64_sync+0x18c/0x190 > > > > > > > > That's unfortunate (and would break RELIABLE_STACKTRACE, which we're slowly > > > > getting towards being able to implement), but it's simple enough to account for > > > > in the stacktrace code. > > > > > > > > > > Indeed. Those functions should just strip the PAC bits, no? > > > > For that case, yup. That was roughly what I meant about it being simple to deal > > with in the stacktrace code. :) > > Right. So given that this is an issue for PAC but not for shadow call > stack, we might consider a shorter term fix where we push/pop these > addresses to the shadow call stack, and address the PAC clearing more > comprehensively once we get around to it. I'm not necessarily opposed to that, and TBH we might not need the address in the graph return stack to be signed, since the graph return stack itself is a shadow stack. I think we can restructure things such that the values on the graph return stack would remain unsigned, but we'd still always protect spills to the regular stack *AND* the assembly would be structured to ensure to remove the return gadgets. As before, I'll have a go at that and try to get it out shortly. [...] > > With that in mind, I think we should also fix up > > qcom_link_stack_sanitisation(), since that ends up creating a gadget of the form: > > > > MOV X30, Xn > > RET > Yeah, I'm sure that's the last one :-) :) Mark.
diff --git a/arch/arm64/kernel/entry-ftrace.S b/arch/arm64/kernel/entry-ftrace.S index 795344ab4ec45889..c744e4dd8c90a352 100644 --- a/arch/arm64/kernel/entry-ftrace.S +++ b/arch/arm64/kernel/entry-ftrace.S @@ -35,6 +35,11 @@ * is missing from the LR and existing chain of frame records. */ .macro ftrace_regs_entry, allregs=0 +#ifdef CONFIG_FUNCTION_GRAPH_TRACER + protect_return_address x9 +#endif + protect_return_address x30 + /* Make room for pt_regs, plus a callee frame */ sub sp, sp, #(PT_REGS_SIZE + 16) @@ -89,7 +94,9 @@ SYM_CODE_START(ftrace_caller) b ftrace_common SYM_CODE_END(ftrace_caller) -SYM_CODE_START(ftrace_common) +SYM_CODE_START_LOCAL(ftrace_common) + alternative_insn nop, "xpaci x30", ARM64_HAS_ADDRESS_AUTH, IS_ENABLED(CONFIG_ARM64_PTR_AUTH_KERNEL) + sub x0, x30, #AARCH64_INSN_SIZE // ip (callsite's BL insn) mov x1, x9 // parent_ip (callsite's LR) ldr_l x2, function_trace_op // op @@ -115,9 +122,27 @@ SYM_INNER_LABEL(ftrace_call, SYM_L_GLOBAL) ldr x30, [sp, #S_LR] ldr x9, [sp, #S_PC] +#ifdef CONFIG_FUNCTION_GRAPH_TRACER + /* grab the original return address from the stack */ + ldr x10, [sp, #PT_REGS_SIZE + 8] +#endif + /* Restore the callsite's SP */ add sp, sp, #PT_REGS_SIZE + 16 + restore_return_address x9 +#ifdef CONFIG_FUNCTION_GRAPH_TRACER + /* compare the original return address with the actual one */ + cmp x10, x30 + b.ne 0f + + /* + * If they are the same, unprotect it now. If it was modified, it will + * be dealt with in return_to_handler() below. + */ + restore_return_address x30 +0: +#endif ret x9 SYM_CODE_END(ftrace_common) @@ -329,6 +354,7 @@ SYM_CODE_START(return_to_handler) ldp x6, x7, [sp, #48] add sp, sp, #64 + restore_return_address x30 ret SYM_CODE_END(return_to_handler) #endif /* CONFIG_FUNCTION_GRAPH_TRACER */
Use the newly added asm macros to protect and restore the return address in the ftrace call wrappers, based on whichever method is active (PAC and/or shadow call stack). If the graph tracer is in use, this covers both the return address *to* the ftrace call site as well as the return address *at* the call site, and the latter will either be restored in return_to_handler(), or before returning to the call site. Signed-off-by: Ard Biesheuvel <ardb@kernel.org> --- arch/arm64/kernel/entry-ftrace.S | 28 +++++++++++++++++++- 1 file changed, 27 insertions(+), 1 deletion(-)