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

Commit

  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
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: 1958b5fc4010 ("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

diff --git a/arch/x86/boot/compressed/mem_encrypt.S b/arch/x86/boot/compressed/mem_encrypt.S
index eaa843a..a480356 100644 (file)
--- 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