[PATCH 4.19 06/17] ARM: 8794/1: uaccess: Prevent speculative use of the current addr_limit

[David Long]

From: Julien Thierry <julien.thierry@xxxxxxx>

Commit 621afc677465db231662ed126ae1f355bf8eac47 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.

Porting commit c2f0ad4fc089cff8 ("arm64: uaccess: Prevent speculative use
of the current addr_limit").

Signed-off-by: Julien Thierry <julien.thierry@xxxxxxx>
Signed-off-by: Russell King <rmk+kernel@xxxxxxxxxxxxxxx>
Signed-off-by: David A. Long <dave.long@xxxxxxxxxx>
---
 arch/arm/include/asm/uaccess.h | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/arm/include/asm/uaccess.h b/arch/arm/include/asm/uaccess.h
index 5451e1f05a19..d65ef85fc617 100644
--- a/arch/arm/include/asm/uaccess.h
+++ b/arch/arm/include/asm/uaccess.h
@@ -69,6 +69,14 @@ extern int __put_user_bad(void);
 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();
+
 	modify_domain(DOMAIN_KERNEL, fs ? DOMAIN_CLIENT : DOMAIN_MANAGER);
 }
 
-- 
2.17.1