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Date: Mon, 16 May 2022 10:15:00 -0700 From: Sami Tolvanen <samitolvanen@...gle.com> To: Peter Zijlstra <peterz@...radead.org> 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 v2 20/21] x86: Add support for CONFIG_CFI_CLANG On Mon, May 16, 2022 at 2:54 AM Peter Zijlstra <peterz@...radead.org> wrote: > > On Fri, May 13, 2022 at 01:21:58PM -0700, Sami Tolvanen wrote: > > With CONFIG_CFI_CLANG, the compiler injects a type preamble > > immediately before each function and a check to validate the target > > function type before indirect calls: > > > > ; type preamble > > __cfi_function: > > int3 > > int3 > > mov <id>, %eax > > int3 > > int3 > > function: > > ... > > When I enable CFI_CLANG and X86_KERNEL_IBT I get: > > 0000000000000c80 <__cfi_io_schedule_timeout>: > c80: cc int3 > c81: cc int3 > c82: b8 b5 b1 39 b3 mov $0xb339b1b5,%eax > c87: cc int3 > c88: cc int3 > > 0000000000000c89 <io_schedule_timeout>: > c89: f3 0f 1e fa endbr64 > > > That seems unfortunate. Would it be possible to get an additional > compiler option to suppress the endbr for all symbols that get a __cfi_ > preaamble? What's the concern with the endbr? Dropping it would currently break the CFI+IBT combination on newer hardware, no? > Also, perhaps s/CFI_CLANG/KERNEL_CFI/ or somesuch, so that GCC might > also implement this same scheme (in time)? I'm fine with renaming the config. > > ; indirect call check > > cmpl <id>, -6(%r11) > > je .Ltmp1 > > ud2 > > .Ltmp1: > > call __x86_indirect_thunk_r11 > > The first one I try and find looks like: > > 26: 41 81 7b fa a6 96 9e 38 cmpl $0x389e96a6,-0x6(%r11) > 2e: 74 02 je 32 <__traceiter_sched_kthread_stop+0x29> > 30: 0f 0b ud2 > 32: 4c 89 f6 mov %r14,%rsi > 35: e8 00 00 00 00 call 3a <__traceiter_sched_kthread_stop+0x31> 36: R_X86_64_PLT32 __x86_indirect_thunk_r11-0x4 > > This must not be. If I'm to rewrite that lot to: > > movl $\hash, %r10d > sub $9, %r11 > call *%r11 > .nop 4 > > Then there must not be spurious instruction in between the ud2 and the > indirect call/retpoline thing. With the current compiler patch, LLVM sets up function arguments after the CFI check. if it's a problem, we can look into changing that. Sami
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