linux-kernel - [tip:x86/urgent] x86/boot: Fix kexec booting failure in the SEV bit detection code

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Date:   Thu, 27 Sep 2018 12:06:56 -0700
From:   tip-bot for Kairui Song <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     kasong@...hat.com, linux-kernel@...r.kernel.org,
        thomas.lendacky@....com, bp@...e.de, tglx@...utronix.de,
        hpa@...or.com, mingo@...nel.org
Subject: [tip:x86/urgent] x86/boot: Fix kexec booting failure in the SEV bit
 detection code

Commit-ID:  bdec8d7fa55e6f5314ed72e5a0b435d90ff90548
Gitweb:     https://git.kernel.org/tip/bdec8d7fa55e6f5314ed72e5a0b435d90ff90548
Author:     Kairui Song <kasong@...hat.com>
AuthorDate: Thu, 27 Sep 2018 20:38:45 +0800
Committer:  Borislav Petkov <bp@...e.de>
CommitDate: Thu, 27 Sep 2018 19:35:03 +0200

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@...e.de>
Signed-off-by: Kairui Song <kasong@...hat.com>
Signed-off-by: Borislav Petkov <bp@...e.de>
Reviewed-by: Tom Lendacky <thomas.lendacky@....com>
Cc: linux-kernel@...r.kernel.org
Cc: tglx@...utronix.de
Cc: mingo@...hat.com
Cc: hpa@...or.com
Cc: brijesh.singh@....com
Cc: kexec@...ts.infradead.org
Cc: dyoung@...hat.com
Cc: bhe@...hat.com
Cc: ghook@...hat.com
Link: https://lkml.kernel.org/r/20180927123845.32052-1-kasong@redhat.com
---
 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