4.4-stable review patch. If anyone has any objections, please let me know.
------------------
From: Dan Williams <dan.j.willi...@intel.com>
(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 <torva...@linux-foundation.org>
Signed-off-by: Dan Williams <dan.j.willi...@intel.com>
Signed-off-by: Thomas Gleixner <t...@linutronix.de>
Cc: linux-a...@vger.kernel.org
Cc: Kees Cook <keesc...@chromium.org>
Cc: kernel-harden...@lists.openwall.com
Cc: gre...@linuxfoundation.org
Cc: Al Viro <v...@zeniv.linux.org.uk>
Cc: Andy Lutomirski <l...@kernel.org>
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 <d...@amazon.co.uk>
[jwang: port to 4.4]
Signed-off-by: Jack Wang <jinpu.w...@profitbricks.com>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>
---
arch/x86/lib/getuser.S | 10 ++++++++++
1 file changed, 10 insertions(+)
--- a/arch/x86/lib/getuser.S
+++ b/arch/x86/lib/getuser.S
@@ -38,6 +38,8 @@ ENTRY(__get_user_1)
GET_THREAD_INFO(%_ASM_DX)
cmp TI_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
@@ -51,6 +53,8 @@ ENTRY(__get_user_2)
GET_THREAD_INFO(%_ASM_DX)
cmp TI_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
@@ -64,6 +68,8 @@ ENTRY(__get_user_4)
GET_THREAD_INFO(%_ASM_DX)
cmp TI_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
@@ -78,6 +84,8 @@ ENTRY(__get_user_8)
GET_THREAD_INFO(%_ASM_DX)
cmp TI_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
@@ -89,6 +97,8 @@ ENTRY(__get_user_8)
GET_THREAD_INFO(%_ASM_DX)
cmp TI_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
