Re: [RFC PATCH v2 20/21] x86: Add support for CONFIG_CFI_CLANG
From: Kees Cook <hidden>
Date: 2022-05-16 23:01:11
Also in:
linux-hardening, lkml, llvm
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:quoted
On Mon, May 16, 2022 at 2:54 AM Peter Zijlstra [off-list ref] wrote:quoted
On Fri, May 13, 2022 at 01:21:58PM -0700, Sami Tolvanen wrote:quoted
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 (local) 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 _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel