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x86/boot: Fix kexec booting failure in the SEV bit detection code

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  1958b5fc40 ("x86/boot: Add early boot support when running with SEV active")

can occasionally cause system resets when kexec-ing a second kernel even
if SEV is not active.

That's because get_sev_encryption_bit() uses 32-bit rIP-relative
addressing to read the value of enc_bit - a variable which caches a
previously detected encryption bit position - but kexec may allocate
the early boot code to a higher location, beyond the 32-bit addressing
limit.

In this case, garbage will be read and get_sev_encryption_bit() will
return the wrong value, leading to accessing memory with the wrong
encryption setting.

Therefore, remove enc_bit, and thus get rid of the need to do 32-bit
rIP-relative addressing in the first place.

 [ bp: massage commit message heavily. ]

Fixes: 1958b5fc40 ("x86/boot: Add early boot support when running with SEV active")
Suggested-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Kairui Song <kasong@redhat.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Cc: linux-kernel@vger.kernel.org
Cc: tglx@linutronix.de
Cc: mingo@redhat.com
Cc: hpa@zytor.com
Cc: brijesh.singh@amd.com
Cc: kexec@lists.infradead.org
Cc: dyoung@redhat.com
Cc: bhe@redhat.com
Cc: ghook@redhat.com
Link: https://lkml.kernel.org/r/20180927123845.32052-1-kasong@redhat.com
This commit is contained in:
Kairui Song 2018-09-27 20:38:45 +08:00 committed by Borislav Petkov
parent 05ab1d8a4b
commit bdec8d7fa5
1 changed files with 0 additions and 19 deletions
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19
arch/x86/boot/compressed/mem_encrypt.S
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@ -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