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Date: Mon, 16 May 2022 12:39:19 -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 11:30 AM Peter Zijlstra <peterz@...radead.org> wrote: > > On Mon, May 16, 2022 at 10:15:00AM -0700, Sami Tolvanen wrote: > > 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? > > Well, yes, but also that combination isn't very interesting. See, > > https://lore.kernel.org/all/20220420004241.2093-1-joao@overdrivepizza.com/T/#m5d67fb010d488b2f8eee33f1eb39d12f769e4ad2 > > and the patch I did down-thread: > > https://lkml.kernel.org/r/YoJKhHluN4n0kZDm@hirez.programming.kicks-ass.net > > If we have IBT, then FineIBT is a much better option than kCFI+IBT. > Removing that superfluous endbr also shrinks the whole thing by 4 bytes. > > So I'm fine with the compiler generating working code for that > combination; but please get me an option to supress it in order to save > those pointless bytes. All this CFI stuff is enough bloat as it is. Sure, I'll take a look at what's the best way to accomplish this. > > > > ; 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. > > Yes, please fix that. Again see that same patch for why this is a > problem. Objtool can trivially find retpoline calls, but finding this > kCFI gadget is going to be hard work. If you ensure they're > unconditionally stuck together, then the problem goes away find one, > finds the other. You can use .kcfi_traps to locate the check right now, but I agree, it's not quite ideal. Sami
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