Messages in this thread | ![/](/images/icornerl.gif) | | Date | Thu, 29 Feb 2024 01:34:16 -0800 | From | Pawan Gupta <> | Subject | Re: [PATCH] x86/bugs: Use fixed addressing for VERW operand |
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On Thu, Feb 29, 2024 at 10:19:09AM +0100, Jiri Slaby wrote: > 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?
In that case backports < 6.5 can do:
+.macro CLEAR_CPU_BUFFERS + ALTERNATIVE "jmp .Lskip_verw_\@", "", X86_FEATURE_CLEAR_CPU_BUF + verw _ASM_RIP(mds_verw_sel) +.Lskip_verw_\@: +.endm
As done in Nikolay's 5.4 backport:
https://lore.kernel.org/stable/20240226122237.198921-3-nik.borisov@suse.com/
> Another question: can we rely on the assembler to generate a relocation > and on the linker to resolve it as rip-relative?
This is definitely not my area of expertise, but with the above approach VERW should be inlined always, and rip-relative should be resolved as with any other instruction.
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