linux-kernel - [PATCH 4.4 094/105] s390: always save and restore all registers on context switch

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Fri, 15 Dec 2017 10:45:32 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Heiko Carstens <heiko.carstens@...ibm.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: [PATCH 4.4 094/105] s390: always save and restore all registers on context switch

4.4-stable review patch.  If anyone has any objections, please let me know.

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

From: Heiko Carstens <heiko.carstens@...ibm.com>

commit fbbd7f1a51965b50dd12924841da0d478f3da71b upstream.

The switch_to() macro has an optimization to avoid saving and
restoring register contents that aren't needed for kernel threads.

There is however the possibility that a kernel thread execve's a user
space program. In such a case the execve'd process can partially see
the contents of the previous process, which shouldn't be allowed.

To avoid this, simply always save and restore register contents on
context switch.

Cc: <stable@...r.kernel.org> # v2.6.37+
Fixes: fdb6d070effba ("switch_to: dont restore/save access & fpu regs for kernel threads")
Signed-off-by: Heiko Carstens <heiko.carstens@...ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@...ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 arch/s390/include/asm/switch_to.h |   19 +++++++++----------
 1 file changed, 9 insertions(+), 10 deletions(-)

--- a/arch/s390/include/asm/switch_to.h
+++ b/arch/s390/include/asm/switch_to.h
@@ -29,17 +29,16 @@ static inline void restore_access_regs(u
 }
 
 #define switch_to(prev,next,last) do {					\
-	if (prev->mm) {							\
-		save_fpu_regs();					\
-		save_access_regs(&prev->thread.acrs[0]);		\
-		save_ri_cb(prev->thread.ri_cb);				\
-	}								\
+	/* save_fpu_regs() sets the CIF_FPU flag, which enforces	\
+	 * a restore of the floating point / vector registers as	\
+	 * soon as the next task returns to user space			\
+	 */								\
+	save_fpu_regs();						\
+	save_access_regs(&prev->thread.acrs[0]);			\
+	save_ri_cb(prev->thread.ri_cb);					\
 	update_cr_regs(next);						\
-	if (next->mm) {							\
-		set_cpu_flag(CIF_FPU);					\
-		restore_access_regs(&next->thread.acrs[0]);		\
-		restore_ri_cb(next->thread.ri_cb, prev->thread.ri_cb);	\
-	}								\
+	restore_access_regs(&next->thread.acrs[0]);			\
+	restore_ri_cb(next->thread.ri_cb, prev->thread.ri_cb);		\
 	prev = __switch_to(prev,next);					\
 } while (0)