On 09/27/2018 07:38 AM, Kairui Song wrote:
> Commit 1958b5fc4010 ("x86/boot: Add early boot support when running
> with SEV active") is causing kexec becomes sometimes unstable even if
> SEV is not active. kexec reboot won't start a second kernel bypassing
> BIOS boot process, instead, the system got reset.
>
> That's because, in get_sev_encryption_bit function, we are using
> 32-bit RIP-relative addressing to read the value of enc_bit, but
> kexec may alloc the early boot up code to a higher location, which
> is beyond 32-bit addressing limit. Some garbage will be read and
> get_sev_encryption_bit will return the wrong value, which leads to
> wrong memory page flag.
>
> This patch removes the use of enc_bit, as currently, enc_bit's only
> purpose is to avoid duplicated encryption bit reading, but the overhead
> of reading encryption bit is so tiny, so no need to cache that.
>
> Fixes: 1958b5fc4010 ("x86/boot: Add early boot support when running with SEV active")
> Suggested-by: Borislav Petkov <bp@xxxxxxx>
> Signed-off-by: Kairui Song <kasong@xxxxxxxxxx>
Reviewed-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
> ---
> arch/x86/boot/compressed/mem_encrypt.S | 19 -------------------
> 1 file changed, 19 deletions(-)
>
> diff --git a/arch/x86/boot/compressed/mem_encrypt.S b/arch/x86/boot/compressed/mem_encrypt.S
> index eaa843a52907..a480356e0ed8 100644
> --- a/arch/x86/boot/compressed/mem_encrypt.S
> +++ b/arch/x86/boot/compressed/mem_encrypt.S
> @@ -25,20 +25,6 @@ ENTRY(get_sev_encryption_bit)
> push %ebx
> push %ecx
> push %edx
> - push %edi
> -
> - /*
> - * RIP-relative addressing is needed to access the encryption bit
> - * variable. Since we are running in 32-bit mode we need this call/pop
> - * sequence to get the proper relative addressing.
> - */
> - call 1f
> -1: popl %edi
> - subl $1b, %edi
> -
> - movl enc_bit(%edi), %eax
> - cmpl $0, %eax
> - jge .Lsev_exit
>
> /* Check if running under a hypervisor */
> movl $1, %eax
> @@ -69,15 +55,12 @@ ENTRY(get_sev_encryption_bit)
>
> movl %ebx, %eax
> andl $0x3f, %eax /* Return the encryption bit location */
> - movl %eax, enc_bit(%edi)
> jmp .Lsev_exit
>
> .Lno_sev:
> xor %eax, %eax
> - movl %eax, enc_bit(%edi)
>
> .Lsev_exit:
> - pop %edi
> pop %edx
> pop %ecx
> pop %ebx
> @@ -113,8 +96,6 @@ ENTRY(set_sev_encryption_mask)
> ENDPROC(set_sev_encryption_mask)
>
> .data
> -enc_bit:
> - .int 0xffffffff
>
> #ifdef CONFIG_AMD_MEM_ENCRYPT
> .balign 8
>
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