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Date: Mon, 16 May 2022 15:59:41 -0700 From: Kees Cook <keescook@...omium.org> To: Peter Zijlstra <peterz@...radead.org> Cc: Sami Tolvanen <samitolvanen@...gle.com>, linux-kernel@...r.kernel.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 08:30:47PM +0200, Peter Zijlstra 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. I'm still not convinced about this, but I'm on the fence. Cons: - FineIBT does callee-based hash verification, which means any attacker-constructed memory region just has to have an endbr and nops at "shellcode - 9". KCFI would need the region to have the hash at "shellcode - 6" and an endbr at "shellcode". However, that hash is well known, so it's not much protection. - Potential performance hit due to making an additional "call" outside the cache lines of both caller and callee. Pros: - FineIBT can be done without read access to the kernel text, which will be nice in the exec-only future. I'd kind of like the "dynamic FineIBT conversion" to be a config option, at least at first. We could at least do performance comparisons between them. > 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. So, in the case of "built for IBT but running on a system without IBT", no rewrite happens, and no endbr is present (i.e. address-taken functions have endbr emission suppressed)? Stock kernel build: function: [normal code] caller: call __x86_indirect_thunk_r11 IBT kernel build: function: endbr [normal code] caller: call __x86_indirect_thunk_r11 CFI kernel build: __cfi_function: [int3/mov/int3 preamble] function: [normal code] caller: cmpl \hash, -6(%r11) je .Ltmp1 ud2 .Ltmp1: call __x86_indirect_thunk_r11 CFI+IBT kernel build: __cfi_function: [int3/mov/int3 preamble] function: endbr [normal code] caller: cmpl \hash, -6(%r11) je .Ltmp1 ud2 .Ltmp1: call __x86_indirect_thunk_r11 CFI+IBT+FineIBT kernel build: __cfi_function: [int3/mov/int3 preamble] function: /* no endbr emitted */ [normal code] caller: cmpl \hash, -6(%r11) je .Ltmp1 ud2 .Ltmp1: call __x86_indirect_thunk_r11 at boot, if IBT is detected: - replace __cfi_function with: endbr call __fineibt_\hash - replace caller with: movl \hash, %r10d sub $9, %r11 nop2 call *%r11 - inject all the __fineibt_\hash elements via module_alloc() __fineibt_\hash: xor \hash, %r10 jz 1f ud2 1: ret int3 -- Kees Cook
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