[PATCH net] isdn/capi: check message length in capi_write()

[Eric Biggers]

From: Eric Biggers <ebigg...@google.com>

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+0849c524d9c634f5a...@syzkaller.appspotmail.com
Signed-off-by: Eric Biggers <ebigg...@google.com>
---
 drivers/isdn/capi/capi.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/drivers/isdn/capi/capi.c b/drivers/isdn/capi/capi.c
index 3c3ad42f22bf..a016d8c3c410 100644
--- a/drivers/isdn/capi/capi.c
+++ b/drivers/isdn/capi/capi.c
@@ -688,6 +688,9 @@ capi_write(struct file *file, const char __user *buf, 
size_t count, loff_t *ppos
        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 __user *buf, 
size_t count, loff_t *ppos
        }
        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 < 18 ||
+                   (size_t)(mlen + CAPIMSG_DATALEN(skb->data)) != count) {
                        kfree_skb(skb);
                        return -EINVAL;
                }
@@ -711,6 +715,10 @@ capi_write(struct file *file, const char __user *buf, 
size_t count, loff_t *ppos
        CAPIMSG_SETAPPID(skb->data, cdev->ap.applid);
 
        if (CAPIMSG_CMD(skb->data) == CAPI_DISCONNECT_B3_RESP) {
+               if (count < 12) {
+                       kfree_skb(skb);
+                       return -EINVAL;
+               }
                mutex_lock(&cdev->lock);
                capincci_free(cdev, CAPIMSG_NCCI(skb->data));
                mutex_unlock(&cdev->lock);
-- 
2.23.0