[PATCH] x86/boot: Fix kexec booting failure after SEV early boot support

[PATCH] x86/boot: Fix kexec booting failure after SEV early boot support

[Kairui Song]

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@suse.de>
Signed-off-by: Kairui Song <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
-- 
2.17.1

Re: [PATCH] x86/boot: Fix kexec booting failure after SEV early boot support

[Lendacky, Thomas]

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@suse.de>
> Signed-off-by: Kairui Song <kasong@redhat.com>

Reviewed-by: Tom Lendacky <thomas.lendacky@amd.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
> 

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

[tip-bot for Kairui Song]

Commit-ID:  bdec8d7fa55e6f5314ed72e5a0b435d90ff90548
Gitweb:     https://git.kernel.org/tip/bdec8d7fa55e6f5314ed72e5a0b435d90ff90548
Author:     Kairui Song <kasong@redhat.com>
AuthorDate: Thu, 27 Sep 2018 20:38:45 +0800
Committer:  Borislav Petkov <bp@suse.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@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
---
 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