RE: [patch added to 3.12-stable] net: sctp, forbid negative length

[David Laight]

From: Jiri Slaby
> Sent: 24 November 2016 09:18
> This patch has been added to the 3.12 stable tree. If you have any
> objections, please let us know.
> 
> ===
> 
> [ Upstream commit a4b8e71b05c27bae6bad3bdecddbc6b68a3ad8cf ]
> 
> Most of getsockopt handlers in net/sctp/socket.c check len against
> sizeof some structure like:
> if (len < sizeof(int))
> return -EINVAL;
> 
> On the first look, the check seems to be correct. But since len is int
> and sizeof returns size_t, int gets promoted to unsigned size_t too. So
> the test returns false for negative lengths. Yes, (-1 < sizeof(long)) is
> false.

Would it be worth adding the check in the generic setsockopt/getsockopt system
call code instead of in each and every protocol?
(Clearly for net-next, not stable.)

David

[patch added to 3.12-stable] net: sctp, forbid negative length

[Jiri Slaby]

This patch has been added to the 3.12 stable tree. If you have any
objections, please let us know.

===

[ Upstream commit a4b8e71b05c27bae6bad3bdecddbc6b68a3ad8cf ]

Most of getsockopt handlers in net/sctp/socket.c check len against
sizeof some structure like:
if (len < sizeof(int))
return -EINVAL;

On the first look, the check seems to be correct. But since len is int
and sizeof returns size_t, int gets promoted to unsigned size_t too. So
the test returns false for negative lengths. Yes, (-1 < sizeof(long)) is
false.

Fix this in sctp by explicitly checking len < 0 before any getsockopt
handler is called.

Note that sctp_getsockopt_events already handled the negative case.
Since we added the < 0 check elsewhere, this one can be removed.

If not checked, this is the result:
UBSAN: Undefined behaviour in ../mm/page_alloc.c:2722:19
shift exponent 52 is too large for 32-bit type 'int'
CPU: 1 PID: 24535 Comm: syz-executor Not tainted 4.8.1-0-syzkaller #1
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 
rel-1.9.1-0-gb3ef39f-prebuilt.qemu-project.org 04/01/2014
  88006d99f2a8 b2f7bdea 41b58ab3
 b4363c14 b2f7bcde 88006d99f2d0 88006d99f270
   0034 b5096422
Call Trace:
 [] ? __ubsan_handle_shift_out_of_bounds+0x29c/0x300
...
 [] ? kmalloc_order+0x24/0x90
 [] ? kmalloc_order_trace+0x24/0x220
 [] ? __kmalloc+0x330/0x540
 [] ? sctp_getsockopt_local_addrs+0x174/0xca0 [sctp]
 [] ? sctp_getsockopt+0x10d/0x1b0 [sctp]
 [] ? sock_common_getsockopt+0xb9/0x150
 [] ? SyS_getsockopt+0x1a5/0x270

Signed-off-by: Jiri Slaby 
Cc: Vlad Yasevich 
Cc: Neil Horman 
Cc: "David S. Miller" 
Cc: linux-s...@vger.kernel.org
Cc: netdev@vger.kernel.org
Acked-by: Neil Horman 
Signed-off-by: David S. Miller 
Signed-off-by: Jiri Slaby 
---
 net/sctp/socket.c | 5 -
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/sctp/socket.c b/net/sctp/socket.c
index ead3a8adca08..98cd6606f4a4 100644
--- a/net/sctp/socket.c
+++ b/net/sctp/socket.c
@@ -4247,7 +4247,7 @@ static int sctp_getsockopt_disable_fragments(struct sock 
*sk, int len,
 static int sctp_getsockopt_events(struct sock *sk, int len, char __user 
*optval,
  int __user *optlen)
 {
-   if (len <= 0)
+   if (len == 0)
return -EINVAL;
if (len > sizeof(struct sctp_event_subscribe))
len = sizeof(struct sctp_event_subscribe);
@@ -5758,6 +5758,9 @@ static int sctp_getsockopt(struct sock *sk, int level, 
int optname,
if (get_user(len, optlen))
return -EFAULT;
 
+   if (len < 0)
+   return -EINVAL;
+
sctp_lock_sock(sk);
 
switch (optname) {
-- 
2.10.2