Re: [PATCH] x86/head/64: Switch to KERNEL_CS as soon as new GDT is installed

From: Joerg Roedel
Date: Mon May 22 2023 - 09:12:25 EST


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@xxxxxxx>
> ---
> 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@xxxxxxx>

>
> 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@xxxxxxx

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