Messages in this thread | | | Date | Tue, 27 Feb 2024 00:20:03 +0200 | Subject | Re: [tip: x86/urgent] x86/bugs: Add asm helpers for executing VERW | From | Nikolay Borisov <> |
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On 27.02.24 г. 0:10 ч., Pawan Gupta wrote: > On Mon, Feb 26, 2024 at 09:17:30AM +0200, Nikolay Borisov wrote: >>> diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h >>> index 262e655..077083e 100644 >>> --- a/arch/x86/include/asm/nospec-branch.h >>> +++ b/arch/x86/include/asm/nospec-branch.h >>> @@ -315,6 +315,17 @@ >>> #endif >>> .endm >>> +/* >>> + * Macro to execute VERW instruction that mitigate transient data sampling >>> + * attacks such as MDS. On affected systems a microcode update overloaded VERW >>> + * instruction to also clear the CPU buffers. VERW clobbers CFLAGS.ZF. >>> + * >>> + * Note: Only the memory operand variant of VERW clears the CPU buffers. >>> + */ >>> +.macro CLEAR_CPU_BUFFERS >>> + ALTERNATIVE "", __stringify(verw _ASM_RIP(mds_verw_sel)), X86_FEATURE_CLEAR_CPU_BUF >> >> Any particular reason why this uses RIP-relative vs an absolute address >> mode? > > Early versions of the series had the VERW arg pointing to the macro > itself, that is why relative addressing was used. That got changed in a > later version with all VERW sites pointing to a single memory location. > >> I know in our private exchange you said there is no significance but >> for example older kernels have a missing relocation support in alternatives. >> This of course can be worked around by slightly changing the logic of the >> macro which means different kernels will have slightly different macros. > > Do you anticipate a problem with that? If yes, I can send a patch to use > fixed addressing in upstream as well.
I experienced crashes on older kernels before realizing that the relocation wasn't resolved correctly by the alternative framework. Instead i simply changed the macro to jmp 1f, where the next instruction is the verw ( I did send a backport for 5.4) and it works. Recently there's been a push to make as much of the kernel assembly as possible PIC so having a rip-relative addressing helps. Whether that makes any material difference - I cannot say.
Here's my backport version for reference:
https://lore.kernel.org/stable/20240226122237.198921-3-nik.borisov@suse.com/
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