[PATCH 4.18 49/53] x86/boot: Fix kexec booting failure in the SEV bit detection code

From: Greg Kroah-Hartman
Date: Thu Oct 18 2018 - 13:59:29 EST

4.18-stable review patch. If anyone has any objections, please let me know.


From: Kairui Song <kasong@xxxxxxxxxx>

[ Upstream commit bdec8d7fa55e6f5314ed72e5a0b435d90ff90548 ]


1958b5fc4010 ("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

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: 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>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Reviewed-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: tglx@xxxxxxxxxxxxx
Cc: mingo@xxxxxxxxxx
Cc: hpa@xxxxxxxxx
Cc: brijesh.singh@xxxxxxx
Cc: kexec@xxxxxxxxxxxxxxxxxxx
Cc: dyoung@xxxxxxxxxx
Cc: bhe@xxxxxxxxxx
Cc: ghook@xxxxxxxxxx
Link: https://lkml.kernel.org/r/20180927123845.32052-1-kasong@xxxxxxxxxx
Signed-off-by: Sasha Levin <alexander.levin@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
arch/x86/boot/compressed/mem_encrypt.S | 19 -------------------
1 file changed, 19 deletions(-)

--- 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

xor %eax, %eax
- movl %eax, enc_bit(%edi)

- pop %edi
pop %edx
pop %ecx
pop %ebx
@@ -113,8 +96,6 @@ ENTRY(set_sev_encryption_mask)

- .int 0xffffffff

.balign 8