kevin/linux
1
Fork 0
Code

x86/head/64: Switch to KERNEL_CS as soon as new GDT is installed

Browse Source 
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 bcce829083 ("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: bcce829083 ("x86/sev: Detect/setup SEV/SME features earlier in boot")
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Joerg Roedel <jroedel@suse.de>
Link: https://lore.kernel.org/all/6ff1f28af2829cc9aea357ebee285825f90a431f.1684340801.git.thomas.lendacky%40amd.com
This commit is contained in:
Tom Lendacky 2023-05-17 11:26:41 -05:00 committed by Dave Hansen
parent edc0a2b595
commit a37f2699c3
1 changed files with 9 additions and 9 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

18
arch/x86/kernel/head_64.S
View File

@ -77,6 +77,15 @@ 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
@ -90,15 +99,6 @@ 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