[PATCH net] RDS: Check cmsg_len before dereferencing CMSG_DATA

From: Avinash Repaka
Date: Thu Dec 21 2017 - 23:19:23 EST

RDS currently doesn't check if the length of the control message is
large enough to hold the required data, before dereferencing the control
message data. This results in following crash:

BUG: KASAN: stack-out-of-bounds in rds_rdma_bytes net/rds/send.c:1013
BUG: KASAN: stack-out-of-bounds in rds_sendmsg+0x1f02/0x1f90
Read of size 8 at addr ffff8801c928fb70 by task syzkaller455006/3157

CPU: 0 PID: 3157 Comm: syzkaller455006 Not tainted 4.15.0-rc3+ #161
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
Call Trace:
__dump_stack lib/dump_stack.c:17 [inline]
dump_stack+0x194/0x257 lib/dump_stack.c:53
print_address_description+0x73/0x250 mm/kasan/report.c:252
kasan_report_error mm/kasan/report.c:351 [inline]
kasan_report+0x25b/0x340 mm/kasan/report.c:409
__asan_report_load8_noabort+0x14/0x20 mm/kasan/report.c:430
rds_rdma_bytes net/rds/send.c:1013 [inline]
rds_sendmsg+0x1f02/0x1f90 net/rds/send.c:1066
sock_sendmsg_nosec net/socket.c:628 [inline]
sock_sendmsg+0xca/0x110 net/socket.c:638
___sys_sendmsg+0x320/0x8b0 net/socket.c:2018
__sys_sendmmsg+0x1ee/0x620 net/socket.c:2108
SYSC_sendmmsg net/socket.c:2139 [inline]
SyS_sendmmsg+0x35/0x60 net/socket.c:2134
RIP: 0033:0x43fe49
RSP: 002b:00007fffbe244ad8 EFLAGS: 00000217 ORIG_RAX: 0000000000000133
RAX: ffffffffffffffda RBX: 00000000004002c8 RCX: 000000000043fe49
RDX: 0000000000000001 RSI: 000000002020c000 RDI: 0000000000000003
RBP: 00000000006ca018 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000217 R12: 00000000004017b0
R13: 0000000000401840 R14: 0000000000000000 R15: 0000000000000000

To fix this, we verify that the cmsg_len is large enough to hold the
data to be read, before proceeding further.

Reported-by: syzbot <syzkaller-bugs@xxxxxxxxxxxxxxxx>
Signed-off-by: Avinash Repaka <avinash.repaka@xxxxxxxxxx>
net/rds/send.c | 3 +++
1 file changed, 3 insertions(+)

diff --git a/net/rds/send.c b/net/rds/send.c
index b52cdc8..f72466c 100644
--- a/net/rds/send.c
+++ b/net/rds/send.c
@@ -1009,6 +1009,9 @@ static int rds_rdma_bytes(struct msghdr *msg, size_t *rdma_bytes)

if (cmsg->cmsg_type == RDS_CMSG_RDMA_ARGS) {
+ if (cmsg->cmsg_len <
+ CMSG_LEN(sizeof(struct rds_rdma_args)))
+ return -EINVAL;
args = CMSG_DATA(cmsg);
*rdma_bytes += args->remote_vec.bytes;