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Message-ID: <151703974570.26578.3809646715924406820.stgit@dwillia2-desk3.amr.corp.intel.com>
Date: Fri, 26 Jan 2018 23:55:45 -0800
From: Dan Williams <dan.j.williams@...el.com>
To: tglx@...utronix.de
Cc: linux-arch@...r.kernel.org, Kees Cook <keescook@...omium.org>,
kernel-hardening@...ts.openwall.com, gregkh@...uxfoundation.org,
x86@...nel.org, Ingo Molnar <mingo@...hat.com>,
Al Viro <viro@...iv.linux.org.uk>, Andy Lutomirski <luto@...nel.org>,
"H. Peter Anvin" <hpa@...or.com>, torvalds@...ux-foundation.org,
alan@...ux.intel.com
Subject: [PATCH v5 06/12] x86,
get_user: use pointer masking to limit speculation
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. 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 <torvalds@...ux-foundation.org>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Al Viro <viro@...iv.linux.org.uk>
Cc: Kees Cook <keescook@...omium.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: "H. Peter Anvin" <hpa@...or.com>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: x86@...nel.org
Signed-off-by: Dan Williams <dan.j.williams@...el.com>
---
arch/x86/lib/getuser.S | 10 ++++++++++
1 file changed, 10 insertions(+)
diff --git a/arch/x86/lib/getuser.S b/arch/x86/lib/getuser.S
index c97d935a29e8..e7a1a9421998 100644
--- a/arch/x86/lib/getuser.S
+++ b/arch/x86/lib/getuser.S
@@ -40,6 +40,8 @@ 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 /* 0 - (uptr < addr_limit) */
+ and %_ASM_DX, %_ASM_AX
ASM_STAC
1: movzbl (%_ASM_AX),%edx
xor %eax,%eax
@@ -54,6 +56,8 @@ 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 /* 0 - (uptr < addr_limit) */
+ and %_ASM_DX, %_ASM_AX
ASM_STAC
2: movzwl -1(%_ASM_AX),%edx
xor %eax,%eax
@@ -68,6 +72,8 @@ 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 /* 0 - (uptr < addr_limit) */
+ and %_ASM_DX, %_ASM_AX
ASM_STAC
3: movl -3(%_ASM_AX),%edx
xor %eax,%eax
@@ -83,6 +89,8 @@ 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 /* 0 - (uptr < addr_limit) */
+ and %_ASM_DX, %_ASM_AX
ASM_STAC
4: movq -7(%_ASM_AX),%rdx
xor %eax,%eax
@@ -94,6 +102,8 @@ 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 /* 0 - (uptr < addr_limit) */
+ and %_ASM_DX, %_ASM_AX
ASM_STAC
4: movl -7(%_ASM_AX),%edx
5: movl -3(%_ASM_AX),%ecx
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