arm64: uaccess: Prevent speculative use of the current addr_limit

From: Will Deacon <will.deacon@arm.com>

commit c2f0ad4fc089cff81cef6a13d04b399980ecbfcc upstream.

A mispredicted conditional call to set_fs could result in the wrong
addr_limit being forwarded under speculation to a subsequent access_ok
check, potentially forming part of a spectre-v1 attack using uaccess
routines.

This patch prevents this forwarding from taking place, but putting heavy
barriers in set_fs after writing the addr_limit.

Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Mark Rutland <mark.rutland@arm.com> [v4.9 backport]
Tested-by: Greg Hackmann <ghackmann@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
index a9db051..900397e 100644 (file)
--- a/arch/arm64/include/asm/uaccess.h
+++ b/arch/arm64/include/asm/uaccess.h
@@ -68,6 +68,13 @@ static inline void set_fs(mm_segment_t fs)
        current_thread_info()->addr_limit = fs;
 
        /*
+        * Prevent a mispredicted conditional call to set_fs from forwarding
+        * the wrong address limit to access_ok under speculation.
+        */
+       dsb(nsh);
+       isb();
+
+       /*
         * Enable/disable UAO so that copy_to_user() etc can access
         * kernel memory with the unprivileged instructions.
         */