linux-kernel - [PATCH 3.16 090/136] parisc: Fix validity check of pointer size argument in new CAS implementation

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Message-ID: <lsq.1518323471.229576546@decadent.org.uk>
Date:   Sun, 11 Feb 2018 04:31:11 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org,
        "John David Anglin" <dave.anglin@...l.net>,
        "Helge Deller" <deller@....de>
Subject: [PATCH 3.16 090/136] parisc: Fix validity check of pointer size
 argument in new CAS implementation

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

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

From: John David Anglin <dave.anglin@...l.net>

commit 05f016d2ca7a4fab99d5d5472168506ddf95e74f upstream.

As noted by Christoph Biedl, passing a pointer size of 4 in the new CAS
implementation causes a kernel crash.  The attached patch corrects the
off by one error in the argument validity check.

In reviewing the code, I noticed that we only perform word operations
with the pointer size argument.  The subi instruction intentionally uses
a word condition on 64-bit kernels.  Nullification was used instead of a
cmpib instruction as the branch should never be taken.  The shlw
pseudo-operation generates a depw,z instruction and it clears the target
before doing a shift left word deposit.  Thus, we don't need to clip the
upper 32 bits of this argument on 64-bit kernels.

Tested with a gcc testsuite run with a 64-bit kernel.  The gcc atomic
code in libgcc is the only direct user of the new CAS implementation
that I am aware of.

Signed-off-by: John David Anglin <dave.anglin@...l.net>
Signed-off-by: Helge Deller <deller@....de>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 arch/parisc/kernel/syscall.S | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

--- a/arch/parisc/kernel/syscall.S
+++ b/arch/parisc/kernel/syscall.S
@@ -690,15 +690,15 @@ cas_action:
 	/* ELF32 Process entry path */
 lws_compare_and_swap_2:
 #ifdef CONFIG_64BIT
-	/* Clip the input registers */
+	/* Clip the input registers. We don't need to clip %r23 as we
+	   only use it for word operations */
 	depdi	0, 31, 32, %r26
 	depdi	0, 31, 32, %r25
 	depdi	0, 31, 32, %r24
-	depdi	0, 31, 32, %r23
 #endif

 	/* Check the validity of the size pointer */
-	subi,>>= 4, %r23, %r0
+	subi,>>= 3, %r23, %r0
 	b,n	lws_exit_nosys

 	/* Jump to the functions which will load the old and new values into