Messages in this thread | | | Date | Mon, 22 May 2023 15:12:05 +0200 | From | Joerg Roedel <> | Subject | Re: [PATCH] x86/head/64: Switch to KERNEL_CS as soon as new GDT is installed |
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On Wed, May 17, 2023 at 11:26:41AM -0500, Tom Lendacky wrote: > The call to startup_64_setup_env() will install a new GDT but does not > actually switch to using the KERNEL_CS entry until returning from the > function call. > > Commit bcce82908333 ("x86/sev: Detect/setup SEV/SME features earlier in > boot") moved the call to sme_enable() earlier in the boot process and in > between the call to startup_64_setup_env() and the switch to KERNEL_CS. > An SEV-ES or an SEV-SNP guest will trigger #VC exceptions during the call > to sme_enable() and if the CS pushed on the stack as part of the exception > and used by IRETQ is not mapped by the new GDT, then problems occur. > Today, the current CS when entering startup_64 is the kernel CS value > because it was set up by the decompressor code, so no issue is seen. > > However, a recent patchset that looked to avoid using the legacy > decompressor during an EFI boot exposed this bug. At entry to startup_64, > the CS value is that of EFI and is not mapped in the new kernel GDT. So > when a #VC exception occurs, the CS value used by IRETQ is not valid and > the guest boot crashes. > > Fix this issue by moving the block that switches to the KERNEL_CS value to > be done immediately after returning from startup_64_setup_env(). > > Fixes: bcce82908333 ("x86/sev: Detect/setup SEV/SME features earlier in boot") > Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com> > --- > arch/x86/kernel/head_64.S | 18 +++++++++--------- > 1 file changed, 9 insertions(+), 9 deletions(-)
Makes sense, thanks for the fix.
Reviewed-by: Joerg Roedel <jroedel@suse.de>
> > diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S > index 9cd77d319555..c5b9289837dc 100644 > --- a/arch/x86/kernel/head_64.S > +++ b/arch/x86/kernel/head_64.S > @@ -79,6 +79,15 @@ SYM_CODE_START_NOALIGN(startup_64) > call startup_64_setup_env > popq %rsi > > + /* Now switch to __KERNEL_CS so IRET works reliably */ > + pushq $__KERNEL_CS > + leaq .Lon_kernel_cs(%rip), %rax > + pushq %rax > + lretq > + > +.Lon_kernel_cs: > + UNWIND_HINT_END_OF_STACK > + > #ifdef CONFIG_AMD_MEM_ENCRYPT > /* > * Activate SEV/SME memory encryption if supported/enabled. This needs to > @@ -92,15 +101,6 @@ SYM_CODE_START_NOALIGN(startup_64) > popq %rsi > #endif > > - /* Now switch to __KERNEL_CS so IRET works reliably */ > - pushq $__KERNEL_CS > - leaq .Lon_kernel_cs(%rip), %rax > - pushq %rax > - lretq > - > -.Lon_kernel_cs: > - UNWIND_HINT_END_OF_STACK > - > /* Sanitize CPU configuration */ > call verify_cpu > > -- > 2.40.0 >
-- Jörg Rödel jroedel@suse.de
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