[PATCH 4.9 63/92] x86/get_user: Use pointer masking to limit speculation

[Greg Kroah-Hartman]

4.9-stable review patch.  If anyone has any objections, please let me know.

--

From: Dan Williams 


(cherry picked from commit c7f631cb07e7da06ac1d231ca178452339e32a94)

Quoting Linus:

I do think that it would be a good idea to very expressly document
the fact that it's not that the user access itself is unsafe. I do
agree that things like "get_user()" want to be protected, but not
because of any direct bugs or problems with get_user() and friends,
but simply because get_user() is an excellent source of a pointer
that is obviously controlled from a potentially attacking user
space. So it's a prime candidate for then finding _subsequent_
accesses that can then be used to perturb the cache.

Unlike the __get_user() case get_user() includes the address limit check
near the pointer de-reference. With that locality the speculation can be
mitigated with pointer narrowing rather than a barrier, i.e.
array_index_nospec(). Where the narrowing is performed by:

cmp %limit, %ptr
sbb %mask, %mask
and %mask, %ptr

With respect to speculation the value of %ptr is either less than %limit
or NULL.

Co-developed-by: Linus Torvalds 
Signed-off-by: Dan Williams 
Signed-off-by: Thomas Gleixner 
Cc: linux-a...@vger.kernel.org
Cc: Kees Cook 
Cc: kernel-harden...@lists.openwall.com
Cc: gre...@linuxfoundation.org
Cc: Al Viro 
Cc: Andy Lutomirski 
Cc: torva...@linux-foundation.org
Cc: a...@linux.intel.com
Link: 
https://lkml.kernel.org/r/151727417469.33451.11804043010080838495.st...@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: David Woodhouse 
Signed-off-by: Greg Kroah-Hartman 
---
 arch/x86/lib/getuser.S |   10 ++
 1 file changed, 10 insertions(+)

--- a/arch/x86/lib/getuser.S
+++ b/arch/x86/lib/getuser.S
@@ -39,6 +39,8 @@ ENTRY(__get_user_1)
mov PER_CPU_VAR(current_task), %_ASM_DX
cmp TASK_addr_limit(%_ASM_DX),%_ASM_AX
jae bad_get_user
+   sbb %_ASM_DX, %_ASM_DX  /* array_index_mask_nospec() */
+   and %_ASM_DX, %_ASM_AX
ASM_STAC
 1: movzbl (%_ASM_AX),%edx
xor %eax,%eax
@@ -53,6 +55,8 @@ ENTRY(__get_user_2)
mov PER_CPU_VAR(current_task), %_ASM_DX
cmp TASK_addr_limit(%_ASM_DX),%_ASM_AX
jae bad_get_user
+   sbb %_ASM_DX, %_ASM_DX  /* array_index_mask_nospec() */
+   and %_ASM_DX, %_ASM_AX
ASM_STAC
 2: movzwl -1(%_ASM_AX),%edx
xor %eax,%eax
@@ -67,6 +71,8 @@ ENTRY(__get_user_4)
mov PER_CPU_VAR(current_task), %_ASM_DX
cmp TASK_addr_limit(%_ASM_DX),%_ASM_AX
jae bad_get_user
+   sbb %_ASM_DX, %_ASM_DX  /* array_index_mask_nospec() */
+   and %_ASM_DX, %_ASM_AX
ASM_STAC
 3: movl -3(%_ASM_AX),%edx
xor %eax,%eax
@@ -82,6 +88,8 @@ ENTRY(__get_user_8)
mov PER_CPU_VAR(current_task), %_ASM_DX
cmp TASK_addr_limit(%_ASM_DX),%_ASM_AX
jae bad_get_user
+   sbb %_ASM_DX, %_ASM_DX  /* array_index_mask_nospec() */
+   and %_ASM_DX, %_ASM_AX
ASM_STAC
 4: movq -7(%_ASM_AX),%rdx
xor %eax,%eax
@@ -93,6 +101,8 @@ ENTRY(__get_user_8)
mov PER_CPU_VAR(current_task), %_ASM_DX
cmp TASK_addr_limit(%_ASM_DX),%_ASM_AX
jae bad_get_user_8
+   sbb %_ASM_DX, %_ASM_DX  /* array_index_mask_nospec() */
+   and %_ASM_DX, %_ASM_AX
ASM_STAC
 4: movl -7(%_ASM_AX),%edx
 5: movl -3(%_ASM_AX),%ecx

[PATCH 4.9 63/92] x86/get_user: Use pointer masking to limit speculation

[Greg Kroah-Hartman]

4.9-stable review patch.  If anyone has any objections, please let me know.

--

From: Dan Williams 


(cherry picked from commit c7f631cb07e7da06ac1d231ca178452339e32a94)

Quoting Linus:

I do think that it would be a good idea to very expressly document
the fact that it's not that the user access itself is unsafe. I do
agree that things like "get_user()" want to be protected, but not
because of any direct bugs or problems with get_user() and friends,
but simply because get_user() is an excellent source of a pointer
that is obviously controlled from a potentially attacking user
space. So it's a prime candidate for then finding _subsequent_
accesses that can then be used to perturb the cache.

Unlike the __get_user() case get_user() includes the address limit check
near the pointer de-reference. With that locality the speculation can be
mitigated with pointer narrowing rather than a barrier, i.e.
array_index_nospec(). Where the narrowing is performed by:

cmp %limit, %ptr
sbb %mask, %mask
and %mask, %ptr

With respect to speculation the value of %ptr is either less than %limit
or NULL.

Co-developed-by: Linus Torvalds 
Signed-off-by: Dan Williams 
Signed-off-by: Thomas Gleixner 
Cc: linux-a...@vger.kernel.org
Cc: Kees Cook 
Cc: kernel-harden...@lists.openwall.com
Cc: gre...@linuxfoundation.org
Cc: Al Viro 
Cc: Andy Lutomirski 
Cc: torva...@linux-foundation.org
Cc: a...@linux.intel.com
Link: 
https://lkml.kernel.org/r/151727417469.33451.11804043010080838495.st...@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: David Woodhouse 
Signed-off-by: Greg Kroah-Hartman 
---
 arch/x86/lib/getuser.S |   10 ++
 1 file changed, 10 insertions(+)

--- a/arch/x86/lib/getuser.S
+++ b/arch/x86/lib/getuser.S
@@ -39,6 +39,8 @@ ENTRY(__get_user_1)
mov PER_CPU_VAR(current_task), %_ASM_DX
cmp TASK_addr_limit(%_ASM_DX),%_ASM_AX
jae bad_get_user
+   sbb %_ASM_DX, %_ASM_DX  /* array_index_mask_nospec() */
+   and %_ASM_DX, %_ASM_AX
ASM_STAC
 1: movzbl (%_ASM_AX),%edx
xor %eax,%eax
@@ -53,6 +55,8 @@ ENTRY(__get_user_2)
mov PER_CPU_VAR(current_task), %_ASM_DX
cmp TASK_addr_limit(%_ASM_DX),%_ASM_AX
jae bad_get_user
+   sbb %_ASM_DX, %_ASM_DX  /* array_index_mask_nospec() */
+   and %_ASM_DX, %_ASM_AX
ASM_STAC
 2: movzwl -1(%_ASM_AX),%edx
xor %eax,%eax
@@ -67,6 +71,8 @@ ENTRY(__get_user_4)
mov PER_CPU_VAR(current_task), %_ASM_DX
cmp TASK_addr_limit(%_ASM_DX),%_ASM_AX
jae bad_get_user
+   sbb %_ASM_DX, %_ASM_DX  /* array_index_mask_nospec() */
+   and %_ASM_DX, %_ASM_AX
ASM_STAC
 3: movl -3(%_ASM_AX),%edx
xor %eax,%eax
@@ -82,6 +88,8 @@ ENTRY(__get_user_8)
mov PER_CPU_VAR(current_task), %_ASM_DX
cmp TASK_addr_limit(%_ASM_DX),%_ASM_AX
jae bad_get_user
+   sbb %_ASM_DX, %_ASM_DX  /* array_index_mask_nospec() */
+   and %_ASM_DX, %_ASM_AX
ASM_STAC
 4: movq -7(%_ASM_AX),%rdx
xor %eax,%eax
@@ -93,6 +101,8 @@ ENTRY(__get_user_8)
mov PER_CPU_VAR(current_task), %_ASM_DX
cmp TASK_addr_limit(%_ASM_DX),%_ASM_AX
jae bad_get_user_8
+   sbb %_ASM_DX, %_ASM_DX  /* array_index_mask_nospec() */
+   and %_ASM_DX, %_ASM_AX
ASM_STAC
 4: movl -7(%_ASM_AX),%edx
 5: movl -3(%_ASM_AX),%ecx