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

[Joerg Roedel]

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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