| Message ID | 20220429203644.2868448-22-samitolvanen@google.com (mailing list archive) |
|---|---|
| State | Superseded |
| Headers | show |
| Series | KCFI support | expand |
On Fri, Apr 29, 2022 at 01:36:44PM -0700, Sami Tolvanen wrote: > Add CONFIG_CFI_CLANG error handling and allow the config to be selected > on x86_64. Might be useful to have an example output of all ths somewhere, because unless I go build my own clang again, I can't tell from these patches what actual codegen looks like. Going from the below, I seem to be able to reverse engineer some of it: .long \signature int3 int3 my_func: ENDBR ... ret And then the callsites look like (clang *always* uses r11, right?): cmpl \signature, -6(%r11) je 1f ud2 1: call __x86_indirect_thunk_r11 > Signed-off-by: Sami Tolvanen <samitolvanen@google.com> > --- > arch/x86/Kconfig | 1 + > arch/x86/include/asm/linkage.h | 7 ++++++ > arch/x86/kernel/traps.c | 39 +++++++++++++++++++++++++++++++++- > 3 files changed, 46 insertions(+), 1 deletion(-) > > diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig > index b0142e01002e..01db5c5c4dde 100644 > --- a/arch/x86/Kconfig > +++ b/arch/x86/Kconfig > @@ -108,6 +108,7 @@ config X86 > select ARCH_SUPPORTS_PAGE_TABLE_CHECK if X86_64 > select ARCH_SUPPORTS_NUMA_BALANCING if X86_64 > select ARCH_SUPPORTS_KMAP_LOCAL_FORCE_MAP if NR_CPUS <= 4096 > + select ARCH_SUPPORTS_CFI_CLANG if X86_64 > select ARCH_SUPPORTS_LTO_CLANG > select ARCH_SUPPORTS_LTO_CLANG_THIN > select ARCH_USE_BUILTIN_BSWAP > diff --git a/arch/x86/include/asm/linkage.h b/arch/x86/include/asm/linkage.h > index 85865f1645bd..d20acf5ebae3 100644 > --- a/arch/x86/include/asm/linkage.h > +++ b/arch/x86/include/asm/linkage.h > @@ -25,6 +25,13 @@ > #define RET ret > #endif > > +#ifdef CONFIG_CFI_CLANG > +#define __CFI_TYPE(name) \ > + .fill 10, 1, 0x90 ASM_NL \ > + .4byte __kcfi_typeid_##name ASM_NL \ > + .fill 2, 1, 0xcc > +#endif > + > #else /* __ASSEMBLY__ */ > > #ifdef CONFIG_SLS > diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c > index 1563fb995005..b9e46e6ed83b 100644 > --- a/arch/x86/kernel/traps.c > +++ b/arch/x86/kernel/traps.c > @@ -40,6 +40,7 @@ > #include <linux/hardirq.h> > #include <linux/atomic.h> > #include <linux/ioasid.h> > +#include <linux/cfi.h> > > #include <asm/stacktrace.h> > #include <asm/processor.h> > @@ -295,6 +296,41 @@ static inline void handle_invalid_op(struct pt_regs *regs) > ILL_ILLOPN, error_get_trap_addr(regs)); > } > > +#ifdef CONFIG_CFI_CLANG > +void *arch_get_cfi_target(unsigned long addr, struct pt_regs *regs) > +{ > + char buffer[MAX_INSN_SIZE]; > + int offset; > + struct insn insn; > + unsigned long *target; Reverse xmas please.. > + > + /* > + * The expected CFI check instruction sequence: > + * cmpl <id>, -6(%reg) ; 7 bytes > + * je .Ltmp1 ; 2 bytes > + * ud2 ; <- addr > + * .Ltmp1: > + * > + * Therefore, the target address is in a register that we can > + * decode from the cmpl instruction. > + */ > + if (copy_from_kernel_nofault(buffer, (void *)addr - 9, MAX_INSN_SIZE)) > + return NULL; > + if (insn_decode(&insn, buffer, MAX_INSN_SIZE, INSN_MODE_64)) > + return NULL; insn_decode_kernel() > + if (insn.opcode.value != 0x81) > + return NULL; That's not sufficient to uniquely identify cmp, you also need to look at the modrm to find r==7 I think. > + > + offset = insn_get_modrm_rm_off(&insn, regs); > + if (offset < 0) > + return NULL; > + > + target = (void *)regs + offset; > + > + return (void *)*target; > +} > +#endif > + > static noinstr bool handle_bug(struct pt_regs *regs) > { > bool handled = false; > @@ -312,7 +348,8 @@ static noinstr bool handle_bug(struct pt_regs *regs) > */ > if (regs->flags & X86_EFLAGS_IF) > raw_local_irq_enable(); > - if (report_bug(regs->ip, regs) == BUG_TRAP_TYPE_WARN) { > + if (report_bug(regs->ip, regs) == BUG_TRAP_TYPE_WARN || > + report_cfi(regs->ip, regs) == BUG_TRAP_TYPE_WARN) { This way you'll first get a BUG splat and then tack on the CFI thing. Seems a bit daft to have two splats. > regs->ip += LEN_UD2; > handled = true; > } > -- > 2.36.0.464.gb9c8b46e94-goog >
On Sat, Apr 30, 2022 at 2:24 AM Peter Zijlstra <peterz@infradead.org> wrote: > > On Fri, Apr 29, 2022 at 01:36:44PM -0700, Sami Tolvanen wrote: > > - if (report_bug(regs->ip, regs) == BUG_TRAP_TYPE_WARN) { > > + if (report_bug(regs->ip, regs) == BUG_TRAP_TYPE_WARN || > > + report_cfi(regs->ip, regs) == BUG_TRAP_TYPE_WARN) { > > This way you'll first get a BUG splat and then tack on the CFI thing. The CFI ud2 isn't in the bug table, which means find_bug returns BUG_TRAP_TYPE_NONE and report_bug bails out before printing out anything. Sami
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig index b0142e01002e..01db5c5c4dde 100644 --- a/arch/x86/Kconfig +++ b/arch/x86/Kconfig @@ -108,6 +108,7 @@ config X86 select ARCH_SUPPORTS_PAGE_TABLE_CHECK if X86_64 select ARCH_SUPPORTS_NUMA_BALANCING if X86_64 select ARCH_SUPPORTS_KMAP_LOCAL_FORCE_MAP if NR_CPUS <= 4096 + select ARCH_SUPPORTS_CFI_CLANG if X86_64 select ARCH_SUPPORTS_LTO_CLANG select ARCH_SUPPORTS_LTO_CLANG_THIN select ARCH_USE_BUILTIN_BSWAP diff --git a/arch/x86/include/asm/linkage.h b/arch/x86/include/asm/linkage.h index 85865f1645bd..d20acf5ebae3 100644 --- a/arch/x86/include/asm/linkage.h +++ b/arch/x86/include/asm/linkage.h @@ -25,6 +25,13 @@ #define RET ret #endif +#ifdef CONFIG_CFI_CLANG +#define __CFI_TYPE(name) \ + .fill 10, 1, 0x90 ASM_NL \ + .4byte __kcfi_typeid_##name ASM_NL \ + .fill 2, 1, 0xcc +#endif + #else /* __ASSEMBLY__ */ #ifdef CONFIG_SLS diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c index 1563fb995005..b9e46e6ed83b 100644 --- a/arch/x86/kernel/traps.c +++ b/arch/x86/kernel/traps.c @@ -40,6 +40,7 @@ #include <linux/hardirq.h> #include <linux/atomic.h> #include <linux/ioasid.h> +#include <linux/cfi.h> #include <asm/stacktrace.h> #include <asm/processor.h> @@ -295,6 +296,41 @@ static inline void handle_invalid_op(struct pt_regs *regs) ILL_ILLOPN, error_get_trap_addr(regs)); } +#ifdef CONFIG_CFI_CLANG +void *arch_get_cfi_target(unsigned long addr, struct pt_regs *regs) +{ + char buffer[MAX_INSN_SIZE]; + int offset; + struct insn insn; + unsigned long *target; + + /* + * The expected CFI check instruction sequence: + * cmpl <id>, -6(%reg) ; 7 bytes + * je .Ltmp1 ; 2 bytes + * ud2 ; <- addr + * .Ltmp1: + * + * Therefore, the target address is in a register that we can + * decode from the cmpl instruction. + */ + if (copy_from_kernel_nofault(buffer, (void *)addr - 9, MAX_INSN_SIZE)) + return NULL; + if (insn_decode(&insn, buffer, MAX_INSN_SIZE, INSN_MODE_64)) + return NULL; + if (insn.opcode.value != 0x81) + return NULL; + + offset = insn_get_modrm_rm_off(&insn, regs); + if (offset < 0) + return NULL; + + target = (void *)regs + offset; + + return (void *)*target; +} +#endif + static noinstr bool handle_bug(struct pt_regs *regs) { bool handled = false; @@ -312,7 +348,8 @@ static noinstr bool handle_bug(struct pt_regs *regs) */ if (regs->flags & X86_EFLAGS_IF) raw_local_irq_enable(); - if (report_bug(regs->ip, regs) == BUG_TRAP_TYPE_WARN) { + if (report_bug(regs->ip, regs) == BUG_TRAP_TYPE_WARN || + report_cfi(regs->ip, regs) == BUG_TRAP_TYPE_WARN) { regs->ip += LEN_UD2; handled = true; }
Add CONFIG_CFI_CLANG error handling and allow the config to be selected on x86_64. Signed-off-by: Sami Tolvanen <samitolvanen@google.com> --- arch/x86/Kconfig | 1 + arch/x86/include/asm/linkage.h | 7 ++++++ arch/x86/kernel/traps.c | 39 +++++++++++++++++++++++++++++++++- 3 files changed, 46 insertions(+), 1 deletion(-)