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Date: Sat, 30 Apr 2022 11:24:27 +0200 From: Peter Zijlstra <peterz@...radead.org> To: Sami Tolvanen <samitolvanen@...gle.com> Cc: linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>, Josh Poimboeuf <jpoimboe@...hat.com>, x86@...nel.org, Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>, Mark Rutland <mark.rutland@....com>, Nathan Chancellor <nathan@...nel.org>, Nick Desaulniers <ndesaulniers@...gle.com>, Joao Moreira <joao@...rdrivepizza.com>, Sedat Dilek <sedat.dilek@...il.com>, Steven Rostedt <rostedt@...dmis.org>, linux-hardening@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, llvm@...ts.linux.dev Subject: Re: [RFC PATCH 21/21] x86: Add support for CONFIG_CFI_CLANG 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@...gle.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 >
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