Messages in this thread | | | Date | Thu, 29 Feb 2024 10:19:09 +0100 | Subject | Re: [PATCH] x86/bugs: Use fixed addressing for VERW operand | From | Jiri Slaby <> |
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On 29. 02. 24, 10:14, Pawan Gupta wrote: > On Wed, Feb 28, 2024 at 05:39:27PM -0800, Pawan Gupta wrote: >> On Tue, Feb 27, 2024 at 10:43:53AM +0100, Jiri Slaby wrote: >>> On 27. 02. 24, 9:47, Nikolay Borisov wrote: >>>> >>>> >>>> On 27.02.24 г. 1:52 ч., Pawan Gupta wrote: >>>>> Macro used for MDS mitigation executes VERW with relative addressing for >>>>> the operand. This is unnecessary and creates a problem for backports on >>>>> older kernels that don't support relocations in alternatives. Relocation >>>>> support was added by commit 270a69c4485d ("x86/alternative: Support >>>>> relocations in alternatives"). Also asm for fixed addressing is much >>>>> more cleaner than relative RIP addressing. >>>>> >>>>> Simplify the asm by using fixed addressing for VERW operand. >>>>> >>>>> Fixes: baf8361e5455 ("x86/bugs: Add asm helpers for executing VERW") >>>>> Reported-by: Nikolay Borisov <nik.borisov@suse.com> >>>>> Closes: https://lore.kernel.org/lkml/20558f89-299b-472e-9a96-171403a83bd6@suse.com/ >>>>> Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com> >>>>> --- >>>>> arch/x86/include/asm/nospec-branch.h | 2 +- >>>>> 1 file changed, 1 insertion(+), 1 deletion(-) >>>>> >>>>> diff --git a/arch/x86/include/asm/nospec-branch.h >>>>> b/arch/x86/include/asm/nospec-branch.h >>>>> index 2aa52cab1e46..ab19c7f1167b 100644 >>>>> --- a/arch/x86/include/asm/nospec-branch.h >>>>> +++ b/arch/x86/include/asm/nospec-branch.h >>>>> @@ -323,7 +323,7 @@ >>>>> * 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 >>>>> + ALTERNATIVE "", __stringify(verw mds_verw_sel), >>>>> X86_FEATURE_CLEAR_CPU_BUF >>>> >>>> Actually thinking about it more and discussing with Jiri (cc'ed), will >>>> this work with KASLR enabled? >>> >>> I might of course be wrong. We appear to rely on the asm+linker here. >> >> You were right, with KASLR enabled, instructions with fixed addressing >> in alternatives don't get relocated. I guess we will have to keep >> rip-relative as is. Thanks for catching that. > > Looks like this is not settled yet, it was naive of me to trust gdb on > /proc/kcore earlier with KASLR enabled. > > With the below debug patch it appears the relocation with fixed > addresses is working as expected with KASLR enabled.
As I wrote already, asm+linker converts the fixed address to rip-rela anyway:
https://lore.kernel.org/all/fd8f2df0-563e-4f5c-aca4-bc92a14e9426@kernel.org/
I also raised questions in there: ==== The assembler generates a relocation for the fixed address anyway. And the linker resolves it as rip-relative. At least the pair from my binutils-2.42.
But if it generates a rip-relative address, is < 6.5 with no support of rip-rel in alternatives still fine?
Another question: can we rely on the assembler to generate a relocation and on the linker to resolve it as rip-relative? ====
thanks, -- js suse labs
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