Re: Patch "x86/boot/compressed/64: Check SEV encryption in the 32-bit boot-path" has been added to the 5.12-stable tree

Re: Patch "x86/boot/compressed/64: Check SEV encryption in the 32-bit boot-path" has been added to the 5.12-stable tree

[Greg KH]

On Fri, May 07, 2021 at 11:22:23PM -0400, Sasha Levin wrote:
> This is a note to let you know that I've just added the patch titled
> 
>     x86/boot/compressed/64: Check SEV encryption in the 32-bit boot-path
> 
> to the 5.12-stable tree which can be found at:
>     http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
> 
> The filename of the patch is:
>      x86-boot-compressed-64-check-sev-encryption-in-the-3.patch
> and it can be found in the queue-5.12 subdirectory.
> 
> If you, or anyone else, feels it should not be added to the stable tree,
> please let <stable@vger.kernel.org> know about it.
> 
> 
> 
> commit 2c622aeb46b16fd945fc681fec16b989940b826d
> Author: Joerg Roedel <jroedel@suse.de>
> Date:   Fri Mar 12 13:38:23 2021 +0100
> 
>     x86/boot/compressed/64: Check SEV encryption in the 32-bit boot-path
>     
>     [ Upstream commit fef81c86262879d4b1176ef51a834c15b805ebb9 ]
>     
>     Check whether the hypervisor reported the correct C-bit when running
>     as an SEV guest. Using a wrong C-bit position could be used to leak
>     sensitive data from the guest to the hypervisor.
>     
>     Signed-off-by: Joerg Roedel <jroedel@suse.de>
>     Signed-off-by: Borislav Petkov <bp@suse.de>
>     Link: https://lkml.kernel.org/r/20210312123824.306-8-joro@8bytes.org
>     Signed-off-by: Sasha Levin <sashal@kernel.org>

This breaks the build (link time) for 5.12, 5.11, and 5.10 trees, so
I'll go drop it for now.

if it needs to come back, can someone submit a working version?

thanks,

greg k-h

[PATCH stable-5.10,5.11,5.12] x86/boot/compressed/64: Check SEV encryption in the 32-bit boot-path

[Joerg Roedel]

[ Upstream commit fef81c86262879d4b1176ef51a834c15b805ebb9 ]

Check whether the hypervisor reported the correct C-bit when running
as an SEV guest. Using a wrong C-bit position could be used to leak
sensitive data from the guest to the hypervisor.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210312123824.306-8-joro@8bytes.org
---
 arch/x86/boot/compressed/head_64.S | 85 ++++++++++++++++++++++++++++++
 1 file changed, 85 insertions(+)

diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index e94874f4bbc1..ae1fe558a2d8 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -172,11 +172,21 @@ SYM_FUNC_START(startup_32)
 	 */
 	call	get_sev_encryption_bit
 	xorl	%edx, %edx
+#ifdef	CONFIG_AMD_MEM_ENCRYPT
 	testl	%eax, %eax
 	jz	1f
 	subl	$32, %eax	/* Encryption bit is always above bit 31 */
 	bts	%eax, %edx	/* Set encryption mask for page tables */
+	/*
+	 * Mark SEV as active in sev_status so that startup32_check_sev_cbit()
+	 * will do a check. The sev_status memory will be fully initialized
+	 * with the contents of MSR_AMD_SEV_STATUS later in
+	 * set_sev_encryption_mask(). For now it is sufficient to know that SEV
+	 * is active.
+	 */
+	movl	$1, rva(sev_status)(%ebp)
 1:
+#endif
 
 	/* Initialize Page tables to 0 */
 	leal	rva(pgtable)(%ebx), %edi
@@ -261,6 +271,9 @@ SYM_FUNC_START(startup_32)
 	movl	%esi, %edx
 1:
 #endif
+	/* Check if the C-bit position is correct when SEV is active */
+	call	startup32_check_sev_cbit
+
 	pushl	$__KERNEL_CS
 	pushl	%eax
 
@@ -786,6 +799,78 @@ SYM_DATA_START_LOCAL(loaded_image_proto)
 SYM_DATA_END(loaded_image_proto)
 #endif
 
+/*
+ * Check for the correct C-bit position when the startup_32 boot-path is used.
+ *
+ * The check makes use of the fact that all memory is encrypted when paging is
+ * disabled. The function creates 64 bits of random data using the RDRAND
+ * instruction. RDRAND is mandatory for SEV guests, so always available. If the
+ * hypervisor violates that the kernel will crash right here.
+ *
+ * The 64 bits of random data are stored to a memory location and at the same
+ * time kept in the %eax and %ebx registers. Since encryption is always active
+ * when paging is off the random data will be stored encrypted in main memory.
+ *
+ * Then paging is enabled. When the C-bit position is correct all memory is
+ * still mapped encrypted and comparing the register values with memory will
+ * succeed. An incorrect C-bit position will map all memory unencrypted, so that
+ * the compare will use the encrypted random data and fail.
+ */
+	__HEAD
+	.code32
+SYM_FUNC_START(startup32_check_sev_cbit)
+#ifdef CONFIG_AMD_MEM_ENCRYPT
+	pushl	%eax
+	pushl	%ebx
+	pushl	%ecx
+	pushl	%edx
+
+	/* Check for non-zero sev_status */
+	movl	rva(sev_status)(%ebp), %eax
+	testl	%eax, %eax
+	jz	4f
+
+	/*
+	 * Get two 32-bit random values - Don't bail out if RDRAND fails
+	 * because it is better to prevent forward progress if no random value
+	 * can be gathered.
+	 */
+1:	rdrand	%eax
+	jnc	1b
+2:	rdrand	%ebx
+	jnc	2b
+
+	/* Store to memory and keep it in the registers */
+	movl	%eax, rva(sev_check_data)(%ebp)
+	movl	%ebx, rva(sev_check_data+4)(%ebp)
+
+	/* Enable paging to see if encryption is active */
+	movl	%cr0, %edx			 /* Backup %cr0 in %edx */
+	movl	$(X86_CR0_PG | X86_CR0_PE), %ecx /* Enable Paging and Protected mode */
+	movl	%ecx, %cr0
+
+	cmpl	%eax, rva(sev_check_data)(%ebp)
+	jne	3f
+	cmpl	%ebx, rva(sev_check_data+4)(%ebp)
+	jne	3f
+
+	movl	%edx, %cr0	/* Restore previous %cr0 */
+
+	jmp	4f
+
+3:	/* Check failed - hlt the machine */
+	hlt
+	jmp	3b
+
+4:
+	popl	%edx
+	popl	%ecx
+	popl	%ebx
+	popl	%eax
+#endif
+	ret
+SYM_FUNC_END(startup32_check_sev_cbit)
+
 /*
  * Stack and heap for uncompression
  */
-- 
2.31.1

Re: [PATCH stable-5.10,5.11,5.12] x86/boot/compressed/64: Check SEV encryption in the 32-bit boot-path

[Joerg Roedel]

On Tue, May 18, 2021 at 01:39:02PM +0200, Joerg Roedel wrote:
> [ Upstream commit fef81c86262879d4b1176ef51a834c15b805ebb9 ]
> 
> Check whether the hypervisor reported the correct C-bit when running
> as an SEV guest. Using a wrong C-bit position could be used to leak
> sensitive data from the guest to the hypervisor.
> 
> Signed-off-by: Joerg Roedel <jroedel@suse.de>
> Signed-off-by: Borislav Petkov <bp@suse.de>
> Link: https://lkml.kernel.org/r/20210312123824.306-8-joro@8bytes.org
> ---
>  arch/x86/boot/compressed/head_64.S | 85 ++++++++++++++++++++++++++++++
>  1 file changed, 85 insertions(+)

This is compile-tested now for 5.10, 5.11 and 5.12. With 5.12 I also did
a boot-test using the 32-bit boot-path and verified it still works as
expected.

Regards,

	Joerg

Re: [PATCH stable-5.10,5.11,5.12] x86/boot/compressed/64: Check SEV encryption in the 32-bit boot-path

[Greg KH]

On Tue, May 18, 2021 at 01:40:18PM +0200, Joerg Roedel wrote:
> On Tue, May 18, 2021 at 01:39:02PM +0200, Joerg Roedel wrote:
> > [ Upstream commit fef81c86262879d4b1176ef51a834c15b805ebb9 ]
> > 
> > Check whether the hypervisor reported the correct C-bit when running
> > as an SEV guest. Using a wrong C-bit position could be used to leak
> > sensitive data from the guest to the hypervisor.
> > 
> > Signed-off-by: Joerg Roedel <jroedel@suse.de>
> > Signed-off-by: Borislav Petkov <bp@suse.de>
> > Link: https://lkml.kernel.org/r/20210312123824.306-8-joro@8bytes.org
> > ---
> >  arch/x86/boot/compressed/head_64.S | 85 ++++++++++++++++++++++++++++++
> >  1 file changed, 85 insertions(+)
> 
> This is compile-tested now for 5.10, 5.11 and 5.12. With 5.12 I also did
> a boot-test using the 32-bit boot-path and verified it still works as
> expected.

Now queued up, thanks.

greg k-h