[PATCH 3.2 101/104] x86: Introduce __uaccess_begin_nospec() and uaccess_try_nospec

[Ben Hutchings]

3.2.101-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Dan Williams <dan.j.williams@xxxxxxxxx>

commit b3bbfb3fb5d25776b8e3f361d2eedaabb0b496cd upstream.

For __get_user() paths, do not allow the kernel to speculate on the value
of a user controlled pointer. In addition to the 'stac' instruction for
Supervisor Mode Access Protection (SMAP), a barrier_nospec() causes the
access_ok() result to resolve in the pipeline before the CPU might take any
speculative action on the pointer value. Given the cost of 'stac' the
speculation barrier is placed after 'stac' to hopefully overlap the cost of
disabling SMAP with the cost of flushing the instruction pipeline.

Since __get_user is a major kernel interface that deals with user
controlled pointers, the __uaccess_begin_nospec() mechanism will prevent
speculative execution past an access_ok() permission check. While
speculative execution past access_ok() is not enough to lead to a kernel
memory leak, it is a necessary precondition.

To be clear, __uaccess_begin_nospec() is addressing a class of potential
problems near __get_user() usages.

Note, that while the barrier_nospec() in __uaccess_begin_nospec() is used
to protect __get_user(), pointer masking similar to array_index_nospec()
will be used for get_user() since it incorporates a bounds check near the
usage.

uaccess_try_nospec provides the same mechanism for get_user_try.

No functional changes.

Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Suggested-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Suggested-by: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: linux-arch@xxxxxxxxxxxxxxx
Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: kernel-hardening@xxxxxxxxxxxxxxxxxx
Cc: gregkh@xxxxxxxxxxxxxxxxxxx
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: alan@xxxxxxxxxxxxxxx
Link: https://lkml.kernel.org/r/151727415922.33451.5796614273104346583.stgit@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
[bwh: Backported to 3.2:
 - There's no SMAP support, so only add uaccess_try_nospec()
 - Use current_thread_info() and save the previous error state, matching
   uaccess_try()]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -462,6 +462,11 @@ struct __large_struct { unsigned long bu
 	current_thread_info()->uaccess_err = 0;				\
 	barrier();
 
+#define uaccess_try_nospec do {						\
+	int prev_err = current_thread_info()->uaccess_err;		\
+	current_thread_info()->uaccess_err = 0;				\
+	barrier_nospec();
+
 #define uaccess_catch(err)						\
 	(err) |= current_thread_info()->uaccess_err;			\
 	current_thread_info()->uaccess_err = prev_err;			\