tcp/udp: Fix memleaks of sk and zerocopy skbs with TX timestamp
CVE: CVE-2023-53489
Component: kernel
CWE: CWE-772
Severity: Low
CVSS v3: 5.6/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
Release date: 2023-01-01T00:00:00+00:00

In the Linux kernel, the following vulnerability has been resolved:

tcp/udp: Fix memleaks of sk and zerocopy skbs with TX timestamp.

syzkaller reported [0] memory leaks of an UDP socket and ZEROCOPY
skbs.  We can reproduce the problem with these sequences:

  sk = socket(AF_INET, SOCK_DGRAM, 0)
  sk.setsockopt(SOL_SOCKET, SO_TIMESTAMPING, SOF_TIMESTAMPING_TX_SOFTWARE)
  sk.setsockopt(SOL_SOCKET, SO_ZEROCOPY, 1)
  sk.sendto(b'', MSG_ZEROCOPY, ('127.0.0.1', 53))
  sk.close()

sendmsg() calls msg_zerocopy_alloc(), which allocates a skb, sets
skb->cb->ubuf.refcnt to 1, and calls sock_hold().  Here, struct
ubuf_info_msgzc indirectly holds a refcnt of the socket.  When the
skb is sent, __skb_tstamp_tx() clones it and puts the clone into
the socket's error queue with the TX timestamp.

When the original skb is received locally, skb_copy_ubufs() calls
skb_unclone(), and pskb_expand_head() increments skb->cb->ubuf.refcnt.
This additional count is decremented while freeing the skb, but struct
ubuf_info_msgzc still has a refcnt, so __msg_zerocopy_callback() is
not called.

The last refcnt is not released unless we retrieve the TX timestamped
skb by recvmsg().  Since we clear the error queue in inet_sock_destruct()
after the socket's refcnt reaches 0, there is a circular dependency.
If we close() the socket holding such skbs, we never call sock_put()
and leak the count, sk, and skb.

TCP has the same problem, and commit e0c8bccd40fc ("net: stream:
purge sk_error_queue in sk_stream_kill_queues()") tried to fix it
by calling skb_queue_purge() during close().  However, there is a
small chance that skb queued in a qdisc or device could be put
into the error queue after the skb_queue_purge() call.

In __skb_tstamp_tx(), the cloned skb should not have a reference
to the ubuf to remove the circular dependency, but skb_clone() does
not call skb_copy_ubufs() for zerocopy skb.  So, we need to call
skb_orphan_frags_rx() for the cloned skb to call skb_copy_ubufs().

[0]:
BUG: memory leak
unreferenced object 0xffff88800c6d2d00 (size 1152):
  comm "syz-executor392", pid 264, jiffies 4294785440 (age 13.044s)
  hex dump (first 32 bytes):
    00 00 00 00 00 00 00 00 cd af e8 81 00 00 00 00  ................
    02 00 07 40 00 00 00 00 00 00 00 00 00 00 00 00  ...@............
  backtrace:
    [<0000000055636812>] sk_prot_alloc+0x64/0x2a0 net/core/sock.c:2024
    [<0000000054d77b7a>] sk_alloc+0x3b/0x800 net/core/sock.c:2083
    [<0000000066f3c7e0>] inet_create net/ipv4/af_inet.c:319 [inline]
    [<0000000066f3c7e0>] inet_create+0x31e/0xe40 net/ipv4/af_inet.c:245
    [<000000009b83af97>] __sock_create+0x2ab/0x550 net/socket.c:1515
    [<00000000b9b11231>] sock_create net/socket.c:1566 [inline]
    [<00000000b9b11231>] __sys_socket_create net/socket.c:1603 [inline]
    [<00000000b9b11231>] __sys_socket_create net/socket.c:1588 [inline]
    [<00000000b9b11231>] __sys_socket+0x138/0x250 net/socket.c:1636
    [<000000004fb45142>] __do_sys_socket net/socket.c:1649 [inline]
    [<000000004fb45142>] __se_sys_socket net/socket.c:1647 [inline]
    [<000000004fb45142>] __x64_sys_socket+0x73/0xb0 net/socket.c:1647
    [<0000000066999e0e>] do_syscall_x64 arch/x86/entry/common.c:50 [inline]
    [<0000000066999e0e>] do_syscall_64+0x38/0x90 arch/x86/entry/common.c:80
    [<0000000017f238c1>] entry_SYSCALL_64_after_hwframe+0x63/0xcd

BUG: memory leak
unreferenced object 0xffff888017633a00 (size 240):
  comm "syz-executor392", pid 264, jiffies 4294785440 (age 13.044s)
  hex dump (first 32 bytes):
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
    00 00 00 00 00 00 00 00 00 2d 6d 0c 80 88 ff ff  .........-m.....
  backtrace:
    [<000000002b1c4368>] __alloc_skb+0x229/0x320 net/core/skbuff.c:497
    [<00000000143579a6>] alloc_skb include/linux/skbuff.h:1265 [inline]
    [<00000000143579a6>] sock_omalloc+0xaa/0x190 net/core/sock.c:2596
    [<00000000be626478>] msg_zerocopy_alloc net/core/skbuff.c:1294 [inline]
    [<00000000be626478>]
---truncated---


Remediation:
For details on how to apply this update, which includes the changes described in this advisory, refer to:

https://access.redhat.com/articles/11258

The system must be rebooted for this update to take effect.