[PATCH] x86: Prevent uninitialized stack byte read in apply_alternatives()

[PATCH] x86: Prevent uninitialized stack byte read in apply_alternatives()

[Mateusz Jurczyk]

In the current form of the code, if a->replacementlen is 0, the reference
to *insnbuf for comparison touches potentially garbage memory. While it
doesn't affect the execution flow due to the subsequent a->replacementlen
comparison, it is (rightly) detected as use of uninitialized memory by a
runtime instrumentation currently under my development, and could be
detected as such by other tools in the future, too (e.g. KMSAN).

Fix the "false-positive" by reordering the conditions to first check the
replacement instruction length before referencing specific opcode bytes.

Signed-off-by: Mateusz Jurczyk <mjurczyk@google.com>
---
 arch/x86/kernel/alternative.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index c5b8f760473c..d03ba6bc97d8 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -410,7 +410,7 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
 		insnbuf_sz = a->replacementlen;
 
 		/* 0xe8 is a relative jump; fix the offset. */
-		if (*insnbuf == 0xe8 && a->replacementlen == 5) {
+		if (a->replacementlen == 5 && *insnbuf == 0xe8) {
 			*(s32 *)(insnbuf + 1) += replacement - instr;
 			DPRINTK("Fix CALL offset: 0x%x, CALL 0x%lx",
 				*(s32 *)(insnbuf + 1),
-- 
2.13.0.219.gdb65acc882-goog

Re: [PATCH] x86: Prevent uninitialized stack byte read in apply_alternatives()

[Borislav Petkov]

On Wed, May 24, 2017 at 02:51:20PM +0200, Mateusz Jurczyk wrote:
> In the current form of the code, if a->replacementlen is 0, the reference
> to *insnbuf for comparison touches potentially garbage memory. While it
> doesn't affect the execution flow due to the subsequent a->replacementlen
> comparison, it is (rightly) detected as use of uninitialized memory by a
> runtime instrumentation currently under my development, and could be
> detected as such by other tools in the future, too (e.g. KMSAN).
> 
> Fix the "false-positive" by reordering the conditions to first check the
> replacement instruction length before referencing specific opcode bytes.
> 
> Signed-off-by: Mateusz Jurczyk <mjurczyk@google.com>
> ---
>  arch/x86/kernel/alternative.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index c5b8f760473c..d03ba6bc97d8 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -410,7 +410,7 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
>  		insnbuf_sz = a->replacementlen;
>  
>  		/* 0xe8 is a relative jump; fix the offset. */

Sure but put in short the reason to look at the instruction length first
in the comment above so that it is clear why we're doing it this way.

> -		if (*insnbuf == 0xe8 && a->replacementlen == 5) {
> +		if (a->replacementlen == 5 && *insnbuf == 0xe8) {
>  			*(s32 *)(insnbuf + 1) += replacement - instr;
>  			DPRINTK("Fix CALL offset: 0x%x, CALL 0x%lx",
>  				*(s32 *)(insnbuf + 1),
> -- 

Thanks.

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
-- 

[PATCH v2] x86: Prevent uninitialized stack byte read in apply_alternatives()

[Mateusz Jurczyk]

In the current form of the code, if a->replacementlen is 0, the reference
to *insnbuf for comparison touches potentially garbage memory. While it
doesn't affect the execution flow due to the subsequent a->replacementlen
comparison, it is (rightly) detected as use of uninitialized memory by a
runtime instrumentation currently under my development, and could be
detected as such by other tools in the future, too (e.g. KMSAN).

Fix the "false-positive" by reordering the conditions to first check the
replacement instruction length before referencing specific opcode bytes.

Signed-off-by: Mateusz Jurczyk <mjurczyk@google.com>
---
Changes in v2:
  - Add an explaining comment, as per Borislav Petkov's request.

 arch/x86/kernel/alternative.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index c5b8f760473c..32e14d137416 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -409,8 +409,13 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
 		memcpy(insnbuf, replacement, a->replacementlen);
 		insnbuf_sz = a->replacementlen;
 
-		/* 0xe8 is a relative jump; fix the offset. */
-		if (*insnbuf == 0xe8 && a->replacementlen == 5) {
+		/*
+		 * 0xe8 is a relative jump; fix the offset.
+		 *
+		 * Instruction length is checked before the opcode to avoid
+		 * accessing uninitialized bytes for zero-length replacements.
+		 */
+		if (a->replacementlen == 5 && *insnbuf == 0xe8) {
 			*(s32 *)(insnbuf + 1) += replacement - instr;
 			DPRINTK("Fix CALL offset: 0x%x, CALL 0x%lx",
 				*(s32 *)(insnbuf + 1),
-- 
2.13.0.219.gdb65acc882-goog

Re: [PATCH v2] x86: Prevent uninitialized stack byte read in apply_alternatives()

[Borislav Petkov]

On Wed, May 24, 2017 at 03:55:00PM +0200, Mateusz Jurczyk wrote:
> In the current form of the code, if a->replacementlen is 0, the reference
> to *insnbuf for comparison touches potentially garbage memory. While it
> doesn't affect the execution flow due to the subsequent a->replacementlen
> comparison, it is (rightly) detected as use of uninitialized memory by a
> runtime instrumentation currently under my development, and could be
> detected as such by other tools in the future, too (e.g. KMSAN).
> 
> Fix the "false-positive" by reordering the conditions to first check the
> replacement instruction length before referencing specific opcode bytes.
> 
> Signed-off-by: Mateusz Jurczyk <mjurczyk@google.com>
> ---
> Changes in v2:
>   - Add an explaining comment, as per Borislav Petkov's request.
> 
>  arch/x86/kernel/alternative.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index c5b8f760473c..32e14d137416 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -409,8 +409,13 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
>  		memcpy(insnbuf, replacement, a->replacementlen);
>  		insnbuf_sz = a->replacementlen;
>  
> -		/* 0xe8 is a relative jump; fix the offset. */
> -		if (*insnbuf == 0xe8 && a->replacementlen == 5) {
> +		/*
> +		 * 0xe8 is a relative jump; fix the offset.
> +		 *
> +		 * Instruction length is checked before the opcode to avoid
> +		 * accessing uninitialized bytes for zero-length replacements.
> +		 */
> +		if (a->replacementlen == 5 && *insnbuf == 0xe8) {
>  			*(s32 *)(insnbuf + 1) += replacement - instr;
>  			DPRINTK("Fix CALL offset: 0x%x, CALL 0x%lx",
>  				*(s32 *)(insnbuf + 1),
> -- 

Reviewed-by: Borislav Petkov <bp@suse.de>

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
-- 

[tip:x86/urgent] x86/alternatives: Prevent uninitialized stack byte read in apply_alternatives()

[tip-bot for Mateusz Jurczyk]

Commit-ID:  fc152d22d6e9fac95a9a990e6c29510bdf1b9425
Gitweb:     http://git.kernel.org/tip/fc152d22d6e9fac95a9a990e6c29510bdf1b9425
Author:     Mateusz Jurczyk <mjurczyk@google.com>
AuthorDate: Wed, 24 May 2017 15:55:00 +0200
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Wed, 24 May 2017 16:18:12 +0200

x86/alternatives: Prevent uninitialized stack byte read in apply_alternatives()

In the current form of the code, if a->replacementlen is 0, the reference
to *insnbuf for comparison touches potentially garbage memory. While it
doesn't affect the execution flow due to the subsequent a->replacementlen
comparison, it is (rightly) detected as use of uninitialized memory by a
runtime instrumentation currently under my development, and could be
detected as such by other tools in the future, too (e.g. KMSAN).

Fix the "false-positive" by reordering the conditions to first check the
replacement instruction length before referencing specific opcode bytes.

Signed-off-by: Mateusz Jurczyk <mjurczyk@google.com>
Reviewed-by: Borislav Petkov <bp@suse.de>
Cc: Andy Lutomirski <luto@kernel.org>
Link: http://lkml.kernel.org/r/20170524135500.27223-1-mjurczyk@google.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
 arch/x86/kernel/alternative.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index c5b8f76..32e14d1 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -409,8 +409,13 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
 		memcpy(insnbuf, replacement, a->replacementlen);
 		insnbuf_sz = a->replacementlen;
 
-		/* 0xe8 is a relative jump; fix the offset. */
-		if (*insnbuf == 0xe8 && a->replacementlen == 5) {
+		/*
+		 * 0xe8 is a relative jump; fix the offset.
+		 *
+		 * Instruction length is checked before the opcode to avoid
+		 * accessing uninitialized bytes for zero-length replacements.
+		 */
+		if (a->replacementlen == 5 && *insnbuf == 0xe8) {
 			*(s32 *)(insnbuf + 1) += replacement - instr;
 			DPRINTK("Fix CALL offset: 0x%x, CALL 0x%lx",
 				*(s32 *)(insnbuf + 1),