[3.5.y.z extended stable] Patch "ipc, msg: fix message length check for negative values" has been added to staging queue

Luis Henriques luis.henriques at canonical.com
Tue Nov 19 13:18:30 UTC 2013

This is a note to let you know that I have just added a patch titled

    ipc, msg: fix message length check for negative values

to the linux-3.5.y-queue branch of the 3.5.y.z extended stable tree 
which can be found at:


If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.5.y.z tree, see



>From dd8a5d84708799e2e7fb1fd291695764abd1fe05 Mon Sep 17 00:00:00 2001
From: Mathias Krause <minipli at googlemail.com>
Date: Tue, 12 Nov 2013 15:11:47 -0800
Subject: ipc, msg: fix message length check for negative values

commit 4e9b45a19241354daec281d7a785739829b52359 upstream.

On 64 bit systems the test for negative message sizes is bogus as the
size, which may be positive when evaluated as a long, will get truncated
to an int when passed to load_msg().  So a long might very well contain a
positive value but when truncated to an int it would become negative.

That in combination with a small negative value of msg_ctlmax (which will
be promoted to an unsigned type for the comparison against msgsz, making
it a big positive value and therefore make it pass the check) will lead to
two problems: 1/ The kmalloc() call in alloc_msg() will allocate a too
small buffer as the addition of alen is effectively a subtraction.  2/ The
copy_from_user() call in load_msg() will first overflow the buffer with
userland data and then, when the userland access generates an access
violation, the fixup handler copy_user_handle_tail() will try to fill the
remainder with zeros -- roughly 4GB.  That almost instantly results in a
system crash or reset.

  ,-[ Reproducer (needs to be run as root) ]--
  | #include <sys/stat.h>
  | #include <sys/msg.h>
  | #include <unistd.h>
  | #include <fcntl.h>
  | int main(void) {
  |     long msg = 1;
  |     int fd;
  |     fd = open("/proc/sys/kernel/msgmax", O_WRONLY);
  |     write(fd, "-1", 2);
  |     close(fd);
  |     msgsnd(0, &msg, 0xfffffff0, IPC_NOWAIT);
  |     return 0;
  | }

Fix the issue by preventing msgsz from getting truncated by consistently
using size_t for the message length.  This way the size checks in
do_msgsnd() could still be passed with a negative value for msg_ctlmax but
we would fail on the buffer allocation in that case and error out.

Also change the type of m_ts from int to size_t to avoid similar nastiness
in other code paths -- it is used in similar constructs, i.e.  signed vs.
unsigned checks.  It should never become negative under normal
circumstances, though.

Setting msg_ctlmax to a negative value is an odd configuration and should
be prevented.  As that might break existing userland, it will be handled
in a separate commit so it could easily be reverted and reworked without
reintroducing the above described bug.

Hardening mechanisms for user copy operations would have catched that bug
early -- e.g.  checking slab object sizes on user copy operations as the
usercopy feature of the PaX patch does.  Or, for that matter, detect the
long vs.  int sign change due to truncation, as the size overflow plugin
of the very same patch does.

[akpm at linux-foundation.org: fix i386 min() warnings]
Signed-off-by: Mathias Krause <minipli at googlemail.com>
Cc: Pax Team <pageexec at freemail.hu>
Cc: Davidlohr Bueso <davidlohr at hp.com>
Cc: Brad Spengler <spender at grsecurity.net>
Cc: Manfred Spraul <manfred at colorfullife.com>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
[ luis: backported to 3.5:
  - adjusted context
  - dropped changes to alloc_msg() and copy_msg() ]
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
 include/linux/msg.h |  6 +++---
 ipc/msgutil.c       | 12 ++++++------
 ipc/util.h          |  4 ++--
 3 files changed, 11 insertions(+), 11 deletions(-)

diff --git a/include/linux/msg.h b/include/linux/msg.h
index 56abf155..70fc369 100644
--- a/include/linux/msg.h
+++ b/include/linux/msg.h
@@ -76,9 +76,9 @@ struct msginfo {

 /* one msg_msg structure for each message */
 struct msg_msg {
-	struct list_head m_list;
-	long  m_type;
-	int m_ts;           /* message text size */
+	struct list_head m_list;
+	long m_type;
+	size_t m_ts;		/* message text size */
 	struct msg_msgseg* next;
 	void *security;
 	/* the actual message follows immediately */
diff --git a/ipc/msgutil.c b/ipc/msgutil.c
index 26143d3..52be05a 100644
--- a/ipc/msgutil.c
+++ b/ipc/msgutil.c
@@ -39,15 +39,15 @@ struct msg_msgseg {
 	/* the next part of the message follows immediately */

-#define DATALEN_MSG	(PAGE_SIZE-sizeof(struct msg_msg))
-#define DATALEN_SEG	(PAGE_SIZE-sizeof(struct msg_msgseg))
+#define DATALEN_MSG	((size_t)PAGE_SIZE-sizeof(struct msg_msg))
+#define DATALEN_SEG	((size_t)PAGE_SIZE-sizeof(struct msg_msgseg))

-struct msg_msg *load_msg(const void __user *src, int len)
+struct msg_msg *load_msg(const void __user *src, size_t len)
 	struct msg_msg *msg;
 	struct msg_msgseg **pseg;
 	int err;
-	int alen;
+	size_t alen;

 	alen = len;
 	if (alen > DATALEN_MSG)
@@ -101,9 +101,9 @@ out_err:
 	return ERR_PTR(err);

-int store_msg(void __user *dest, struct msg_msg *msg, int len)
+int store_msg(void __user *dest, struct msg_msg *msg, size_t len)
-	int alen;
+	size_t alen;
 	struct msg_msgseg *seg;

 	alen = len;
diff --git a/ipc/util.h b/ipc/util.h
index 6f5c20b..0bfc934 100644
--- a/ipc/util.h
+++ b/ipc/util.h
@@ -138,8 +138,8 @@ int ipc_parse_version (int *cmd);

 extern void free_msg(struct msg_msg *msg);
-extern struct msg_msg *load_msg(const void __user *src, int len);
-extern int store_msg(void __user *dest, struct msg_msg *msg, int len);
+extern struct msg_msg *load_msg(const void __user *src, size_t len);
+extern int store_msg(void __user *dest, struct msg_msg *msg, size_t len);

 extern void recompute_msgmni(struct ipc_namespace *);


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