[v4,11/17] arm64: disable function graph tracing with SCS
diff mbox series

Message ID 20191101221150.116536-12-samitolvanen@google.com
State New
Headers show
Series
  • add support for Clang's Shadow Call Stack
Related show

Commit Message

Sami Tolvanen Nov. 1, 2019, 10:11 p.m. UTC
With CONFIG_FUNCTION_GRAPH_TRACER, function return addresses are
modified in ftrace_graph_caller and prepare_ftrace_return to redirect
control flow to ftrace_return_to_handler. This is incompatible with
SCS.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
---
 arch/arm64/Kconfig | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Comments

Mark Rutland Nov. 4, 2019, 5:11 p.m. UTC | #1
On Fri, Nov 01, 2019 at 03:11:44PM -0700, Sami Tolvanen wrote:
> With CONFIG_FUNCTION_GRAPH_TRACER, function return addresses are
> modified in ftrace_graph_caller and prepare_ftrace_return to redirect
> control flow to ftrace_return_to_handler. This is incompatible with
> SCS.

Can you please elaborate on _how_ this is incompatible in the commit
message?

For example, it's not clear to me if you mean that's functionally
incompatible, or if you're trying to remove return-altering gadgets.

If there's a functional incompatibility, please spell that out a bit
more clearly. Likewise if this is about minimizing the set of places
that can mess with control-flow outside of usual function conventions.

Thanks,
Mark.

> 
> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
> Reviewed-by: Kees Cook <keescook@chromium.org>
> ---
>  arch/arm64/Kconfig | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index e7b57a8a5531..42867174920f 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -148,7 +148,7 @@ config ARM64
>  	select HAVE_FTRACE_MCOUNT_RECORD
>  	select HAVE_FUNCTION_TRACER
>  	select HAVE_FUNCTION_ERROR_INJECTION
> -	select HAVE_FUNCTION_GRAPH_TRACER
> +	select HAVE_FUNCTION_GRAPH_TRACER if !SHADOW_CALL_STACK
>  	select HAVE_GCC_PLUGINS
>  	select HAVE_HW_BREAKPOINT if PERF_EVENTS
>  	select HAVE_IRQ_TIME_ACCOUNTING
> -- 
> 2.24.0.rc1.363.gb1bccd3e3d-goog
>
Sami Tolvanen Nov. 4, 2019, 11:44 p.m. UTC | #2
On Mon, Nov 4, 2019 at 9:11 AM Mark Rutland <mark.rutland@arm.com> wrote:
> Can you please elaborate on _how_ this is incompatible in the commit
> message?
>
> For example, it's not clear to me if you mean that's functionally
> incompatible, or if you're trying to remove return-altering gadgets.
>
> If there's a functional incompatibility, please spell that out a bit
> more clearly. Likewise if this is about minimizing the set of places
> that can mess with control-flow outside of usual function conventions.

Sure, I'll add a better description in v5. In this case, the return
address is modified in the kernel stack, which means the changes are
ignored with SCS.

Sami
Mark Rutland Nov. 5, 2019, 9:15 a.m. UTC | #3
On Mon, Nov 04, 2019 at 03:44:39PM -0800, Sami Tolvanen wrote:
> On Mon, Nov 4, 2019 at 9:11 AM Mark Rutland <mark.rutland@arm.com> wrote:
> > Can you please elaborate on _how_ this is incompatible in the commit
> > message?
> >
> > For example, it's not clear to me if you mean that's functionally
> > incompatible, or if you're trying to remove return-altering gadgets.
> >
> > If there's a functional incompatibility, please spell that out a bit
> > more clearly. Likewise if this is about minimizing the set of places
> > that can mess with control-flow outside of usual function conventions.
> 
> Sure, I'll add a better description in v5. In this case, the return
> address is modified in the kernel stack, which means the changes are
> ignored with SCS.

Ok, that makes sense to me. I'd suggest something like:

| The graph tracer hooks returns by modifying frame records on the
| (regular) stack, but with SCS the return address is taken from the
| shadow stack, and the value in the frame record has no effect. As we
| don't currently have a mechanism to determine the corresponding slot
| on the shadow stack (and to pass this through the ftrace
| infrastructure), for now let's disable the graph tracer when SCS is
| enabled.

... as I suspect with some rework of the trampoline and common ftrace
code we'd be able to correctly manipulate the shadow stack for this.
Similarly, if clang gained -fpatchable-funciton-etnry, we'd get that for
free.

Thanks,
Mark.
Nick Desaulniers Nov. 5, 2019, 8 p.m. UTC | #4
On Tue, Nov 5, 2019 at 11:55 AM Mark Rutland <mark.rutland@arm.com> wrote:
> Similarly, if clang gained -fpatchable-funciton-etnry, we'd get that for
> free.

Filed: https://bugs.llvm.org/show_bug.cgi?id=43912
Sami Tolvanen Nov. 5, 2019, 10:05 p.m. UTC | #5
On Tue, Nov 5, 2019 at 11:55 AM Mark Rutland <mark.rutland@arm.com> wrote:
> On Mon, Nov 04, 2019 at 03:44:39PM -0800, Sami Tolvanen wrote:
> > Sure, I'll add a better description in v5. In this case, the return
> > address is modified in the kernel stack, which means the changes are
> > ignored with SCS.
>
> Ok, that makes sense to me. I'd suggest something like:
>
> | The graph tracer hooks returns by modifying frame records on the
> | (regular) stack, but with SCS the return address is taken from the
> | shadow stack, and the value in the frame record has no effect. As we
> | don't currently have a mechanism to determine the corresponding slot
> | on the shadow stack (and to pass this through the ftrace
> | infrastructure), for now let's disable the graph tracer when SCS is
> | enabled.
>
> ... as I suspect with some rework of the trampoline and common ftrace
> code we'd be able to correctly manipulate the shadow stack for this.
> Similarly, if clang gained -fpatchable-funciton-etnry, we'd get that for
> free.

That sounds good to me. Thanks, Mark.

Sami

Patch
diff mbox series

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index e7b57a8a5531..42867174920f 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -148,7 +148,7 @@  config ARM64
 	select HAVE_FTRACE_MCOUNT_RECORD
 	select HAVE_FUNCTION_TRACER
 	select HAVE_FUNCTION_ERROR_INJECTION
-	select HAVE_FUNCTION_GRAPH_TRACER
+	select HAVE_FUNCTION_GRAPH_TRACER if !SHADOW_CALL_STACK
 	select HAVE_GCC_PLUGINS
 	select HAVE_HW_BREAKPOINT if PERF_EVENTS
 	select HAVE_IRQ_TIME_ACCOUNTING