[PATCH 5.2 04/85] isdn/capi: check message length in capi_write()

Greg Kroah-Hartman
2019-09-18T06:25:44Z

From: Eric Biggers <ebiggers@google.com>

[ Upstream commit fe163e534e5eecdfd7b5920b0dfd24c458ee85d6 ]

syzbot reported:

BUG: KMSAN: uninit-value in capi_write+0x791/0xa90 drivers/isdn/capi/capi.c:700
CPU: 0 PID: 10025 Comm: syz-executor379 Not tainted 4.20.0-rc7+ #2
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
__dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0x173/0x1d0 lib/dump_stack.c:113
kmsan_report+0x12e/0x2a0 mm/kmsan/kmsan.c:613
__msan_warning+0x82/0xf0 mm/kmsan/kmsan_instr.c:313
capi_write+0x791/0xa90 drivers/isdn/capi/capi.c:700
do_loop_readv_writev fs/read_write.c:703 [inline]
do_iter_write+0x83e/0xd80 fs/read_write.c:961
vfs_writev fs/read_write.c:1004 [inline]
do_writev+0x397/0x840 fs/read_write.c:1039
__do_sys_writev fs/read_write.c:1112 [inline]
__se_sys_writev+0x9b/0xb0 fs/read_write.c:1109
__x64_sys_writev+0x4a/0x70 fs/read_write.c:1109
do_syscall_64+0xbc/0xf0 arch/x86/entry/common.c:291
entry_SYSCALL_64_after_hwframe+0x63/0xe7
[...]

The problem is that capi_write() is reading past the end of the message.
Fix it by checking the message's length in the needed places.

Reported-and-tested-by: syzbot+0849c524d9c634f5ae66@syzkaller.appspotmail.com
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/isdn/capi/capi.c | 10 +++++++++-
include/uapi/linux/isdn/capicmd.h | 1 +
2 files changed, 10 insertions(+), 1 deletion(-)

--- a/drivers/isdn/capi/capi.c
+++ b/drivers/isdn/capi/capi.c
@@ -688,6 +688,9 @@ capi_write(struct file *file, const char
if (!cdev->ap.applid)
return -ENODEV;

+ if (count < CAPIMSG_BASELEN)
+ return -EINVAL;
+
skb = alloc_skb(count, GFP_USER);
if (!skb)
return -ENOMEM;
@@ -698,7 +701,8 @@ capi_write(struct file *file, const char
}
mlen = CAPIMSG_LEN(skb->data);
if (CAPIMSG_CMD(skb->data) == CAPI_DATA_B3_REQ) {
- if ((size_t)(mlen + CAPIMSG_DATALEN(skb->data)) != count) {
+ if (count < CAPI_DATA_B3_REQ_LEN ||
+ (size_t)(mlen + CAPIMSG_DATALEN(skb->data)) != count) {
kfree_skb(skb);
return -EINVAL;
}
@@ -711,6 +715,10 @@ capi_write(struct file *file, const char
CAPIMSG_SETAPPID(skb->data, cdev->ap.applid);

if (CAPIMSG_CMD(skb->data) == CAPI_DISCONNECT_B3_RESP) {
+ if (count < CAPI_DISCONNECT_B3_RESP_LEN) {
+ kfree_skb(skb);
+ return -EINVAL;
+ }
mutex_lock(&cdev->lock);
capincci_free(cdev, CAPIMSG_NCCI(skb->data));
mutex_unlock(&cdev->lock);
--- a/include/uapi/linux/isdn/capicmd.h
+++ b/include/uapi/linux/isdn/capicmd.h
@@ -16,6 +16,7 @@
#define CAPI_MSG_BASELEN 8
#define CAPI_DATA_B3_REQ_LEN (CAPI_MSG_BASELEN+4+4+2+2+2)
#define CAPI_DATA_B3_RESP_LEN (CAPI_MSG_BASELEN+4+2)
+#define CAPI_DISCONNECT_B3_RESP_LEN (CAPI_MSG_BASELEN+4)

/*----- CAPI commands -----*/
#define CAPI_ALERT 0x01