Description

In the Linux kernel, the following vulnerability has been resolved: udp: Fix memory accounting leak. Matt Dowling reported a weird UDP memory usage issue. Under normal operation, the UDP memory usage reported in /proc/net/sockstat remains close to zero. However, it occasionally spiked to 524,288 pages and never dropped. Moreover, the value doubled when the application was terminated. Finally, it caused intermittent packet drops. We can reproduce the issue with the script below [0]: 1. /proc/net/sockstat reports 0 pages # cat /proc/net/sockstat | grep UDP: UDP: inuse 1 mem 0 2. Run the script till the report reaches 524,288 # python3 test.py & sleep 5 # cat /proc/net/sockstat | grep UDP: UDP: inuse 3 mem 524288 <-- (INT_MAX + 1) >> PAGE_SHIFT 3. Kill the socket and confirm the number never drops # pkill python3 && sleep 5 # cat /proc/net/sockstat | grep UDP: UDP: inuse 1 mem 524288 4. (necessary since v6.0) Trigger proto_memory_pcpu_drain() # python3 test.py & sleep 1 && pkill python3 5. The number doubles # cat /proc/net/sockstat | grep UDP: UDP: inuse 1 mem 1048577 The application set INT_MAX to SO_RCVBUF, which triggered an integer overflow in udp_rmem_release(). When a socket is close()d, udp_destruct_common() purges its receive queue and sums up skb->truesize in the queue. This total is calculated and stored in a local unsigned integer variable. The total size is then passed to udp_rmem_release() to adjust memory accounting. However, because the function takes a signed integer argument, the total size can wrap around, causing an overflow. Then, the released amount is calculated as follows: 1) Add size to sk->sk_forward_alloc. 2) Round down sk->sk_forward_alloc to the nearest lower multiple of PAGE_SIZE and assign it to amount. 3) Subtract amount from sk->sk_forward_alloc. 4) Pass amount >> PAGE_SHIFT to __sk_mem_reduce_allocated(). When the issue occurred, the total in udp_destruct_common() was 2147484480 (INT_MAX + 833), which was cast to -2147482816 in udp_rmem_release(). At 1) sk->sk_forward_alloc is changed from 3264 to -2147479552, and 2) sets -2147479552 to amount. 3) reverts the wraparound, so we don't see a warning in inet_sock_destruct(). However, udp_memory_allocated ends up doubling at 4). Since commit 3cd3399dd7a8 ("net: implement per-cpu reserves for memory_allocated"), memory usage no longer doubles immediately after a socket is close()d because __sk_mem_reduce_allocated() caches the amount in udp_memory_per_cpu_fw_alloc. However, the next time a UDP socket receives a packet, the subtraction takes effect, causing UDP memory usage to double. This issue makes further memory allocation fail once the socket's sk->sk_rmem_alloc exceeds net.ipv4.udp_rmem_min, resulting in packet drops. To prevent this issue, let's use unsigned int for the calculation and call sk_forward_alloc_add() only once for the small delta. Note that first_packet_length() also potentially has the same problem. [0]: from socket import * SO_RCVBUFFORCE = 33 INT_MAX = (2 ** 31) - 1 s = socket(AF_INET, SOCK_DGRAM) s.bind(('', 0)) s.setsockopt(SOL_SOCKET, SO_RCVBUFFORCE, INT_MAX) c = socket(AF_INET, SOCK_DGRAM) c.connect(s.getsockname()) data = b'a' * 100 while True: c.send(data)

Metrics

 

CVSS Version 4.0 CVSS Version 3.x CVSS Version 2.0

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CVSS 4.0 Severity and Vector Strings:

NIST: NVD

N/A

NVD assessment not yet provided.

CVSS 3.x Severity and Vector Strings:

NIST: NVD

Base Score:  N/A

NVD assessment not yet provided.

CVSS 2.0 Severity and Vector Strings:

NIST: NVD

Base Score: N/A

NVD assessment not yet provided.

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

udp: Fix memory accounting leak.

Matt Dowling reported a weird UDP memory usage issue.

Under normal operation, the UDP memory usage reported in /proc/net/sockstat
remains close to zero.  However, it occasionally spiked to 524,288 pages
and never dropped.  Moreover, the value doubled when the application was
terminated.  Finally, it caused intermittent packet drops.

We can reproduce the issue with the script below [0]:

  1. /proc/net/sockstat reports 0 pages

    # cat /proc/net/sockstat | grep UDP:
    UDP: inuse 1 mem 0

  2. Run the script till the report reaches 524,288

    # python3 test.py & sleep 5
    # cat /proc/net/sockstat | grep UDP:
    UDP: inuse 3 mem 524288  <-- (INT_MAX + 1) >> PAGE_SHIFT

  3. Kill the socket and confirm the number never drops

    # pkill python3 && sleep 5
    # cat /proc/net/sockstat | grep UDP:
    UDP: inuse 1 mem 524288

  4. (necessary since v6.0) Trigger proto_memory_pcpu_drain()

    # python3 test.py & sleep 1 && pkill python3

  5. The number doubles

    # cat /proc/net/sockstat | grep UDP:
    UDP: inuse 1 mem 1048577

The application set INT_MAX to SO_RCVBUF, which triggered an integer
overflow in udp_rmem_release().

When a socket is close()d, udp_destruct_common() purges its receive
queue and sums up skb->truesize in the queue.  This total is calculated
and stored in a local unsigned integer variable.

The total size is then passed to udp_rmem_release() to adjust memory
accounting.  However, because the function takes a signed integer
argument, the total size can wrap around, causing an overflow.

Then, the released amount is calculated as follows:

  1) Add size to sk->sk_forward_alloc.
  2) Round down sk->sk_forward_alloc to the nearest lower multiple of
      PAGE_SIZE and assign it to amount.
  3) Subtract amount from sk->sk_forward_alloc.
  4) Pass amount >> PAGE_SHIFT to __sk_mem_reduce_allocated().

When the issue occurred, the total in udp_destruct_common() was 2147484480
(INT_MAX + 

https://git.kernel.org/stable/c/3836029448e76c1e6f77cc5fe0adc09b018b5fa8

https://git.kernel.org/stable/c/9122fec396950cc866137af7154b1d0d989be52e

https://git.kernel.org/stable/c/a116b271bf3cb72c8155b6b7f39083c1b80dcd00

https://git.kernel.org/stable/c/aeef6456692c6f11ae53d278df64f1316a2a405a

https://git.kernel.org/stable/c/c4bac6c398118fba79e32b1cd01db22dbfe29fbf

https://git.kernel.org/stable/c/df207de9d9e7a4d92f8567e2c539d9c8c12fd99d