CVE-2016-7117: Data Processing Errors (CWE-19)
value: Use-after-free vulnerability in the __sys_recvmmsg function in net/socket.c in the Linux kernel before 4.5.2 allows remote attackers to execute arbitrary code via vectors involving a recvmmsg system call that is mishandled during error processing.
CWE description: Weaknesses in this category are typically found in functionality that processes data. Data processing is the manipulation of input to retrieve or save information.
CVSS v3 base score: 9.8
Published: 2016-10-10T11:00Z
Repository: https://github.com/torvalds/linux
Language: C
Files changed: net/socket.c
Commit date: 2016-03-14 12:41:49 -0400
Commit message: net: Fix use after free in the recvmmsg exit path

The syzkaller fuzzer hit the following use-after-free:

  Call Trace:
   [<ffffffff8175ea0e>] __asan_report_load8_noabort+0x3e/0x40 mm/kasan/report.c:295
   [<ffffffff851cc31a>] __sys_recvmmsg+0x6fa/0x7f0 net/socket.c:2261
   [<     inline     >] SYSC_recvmmsg net/socket.c:2281
   [<ffffffff851cc57f>] SyS_recvmmsg+0x16f/0x180 net/socket.c:2270
   [<ffffffff86332bb6>] entry_SYSCALL_64_fastpath+0x16/0x7a
  arch/x86/entry/entry_64.S:185

And, as Dmitry rightly assessed, that is because we can drop the
reference and then touch it when the underlying recvmsg calls return
some packets and then hit an error, which will make recvmmsg to set
sock->sk->sk_err, oops, fix it.

Reported-and-Tested-by: Dmitry Vyukov <dvyukov@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Kostya Serebryany <kcc@google.com>
Cc: Sasha Levin <sasha.levin@oracle.com>
Fixes: a2e2725541fa ("net: Introduce recvmmsg socket syscall")
http://lkml.kernel.org/r/20160122211644.GC2470@redhat.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Diff: diff --git a/net/socket.c b/net/socket.c
index c5ddc52cf2b29..5f77a8e93830b 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -2242,35 +2242,35 @@ int __sys_recvmmsg(int fd, struct mmsghdr __user *mmsg, unsigned int vlen,
 		if (msg_sys.msg_flags & MSG_OOB)
 			break;
 		cond_resched();
 	}
 
-out_put:
-	fput_light(sock->file, fput_needed);
-
 	if (err == 0)
-		return datagrams;
+		goto out_put;
 
-	if (datagrams != 0) {
+	if (datagrams == 0) {
+		datagrams = err;
+		goto out_put;
+	}
+
+	/*
+	 * We may return less entries than requested (vlen) if the
+	 * sock is non block and there aren't enough datagrams...
+	 */
+	if (err != -EAGAIN) {
 		/*
-		 * We may return less entries than requested (vlen) if the
-		 * sock is non block and there aren't enough datagrams...
+		 * ... or  if recvmsg returns an error after we
+		 * received some datagrams, where we record the
+		 * error to return on the next call or if the
+		 * app asks about it using getsockopt(SO_ERROR).
 		 */
-		if (err != -EAGAIN) {
-			/*
-			 * ... or  if recvmsg returns an error after we
-			 * received some datagrams, where we record the
-			 * error to return on the next call or if the
-			 * app asks about it using getsockopt(SO_ERROR).
-			 */
-			sock->sk->sk_err = -err;
-		}
-
-		return datagrams;
+		sock->sk->sk_err = -err;
 	}
+out_put:
+	fput_light(sock->file, fput_needed);
 
-	return err;
+	return datagrams;
 }
 
 SYSCALL_DEFINE5(recvmmsg, int, fd, struct mmsghdr __user *, mmsg,
 		unsigned int, vlen, unsigned int, flags,
 		struct timespec __user *, timeout)

Vulnerable code: out_put:
	fput_light(sock->file, fput_needed);

		return datagrams;
	if (datagrams != 0) {
		 * We may return less entries than requested (vlen) if the
		 * sock is non block and there aren't enough datagrams...
		if (err != -EAGAIN) {
			/*
			 * ... or  if recvmsg returns an error after we
			 * received some datagrams, where we record the
			 * error to return on the next call or if the
			 * app asks about it using getsockopt(SO_ERROR).
			 */
			sock->sk->sk_err = -err;
		}

		return datagrams;
	return err;
Fixed code: 		goto out_put;
	if (datagrams == 0) {
		datagrams = err;
		goto out_put;
	}

	/*
	 * We may return less entries than requested (vlen) if the
	 * sock is non block and there aren't enough datagrams...
	 */
	if (err != -EAGAIN) {
		 * ... or  if recvmsg returns an error after we
		 * received some datagrams, where we record the
		 * error to return on the next call or if the
		 * app asks about it using getsockopt(SO_ERROR).
		sock->sk->sk_err = -err;
out_put:
	fput_light(sock->file, fput_needed);
	return datagrams;