arm64: uaccess: Prevent speculative use of the current addr_limit

commit c2f0ad4fc089 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: Alex Shi <alex.shi@linaro.org>
Conflicts:
	no set_thread_flag(TIF_FSCHECK) in arch/arm64/include/asm/uaccess.h

diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
index 3531fec..00025c5 100644 (file)
--- a/arch/arm64/include/asm/uaccess.h
+++ b/arch/arm64/include/asm/uaccess.h
@@ -70,6 +70,13 @@ 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.