CVE-2025-38463: tcp: Correct signedness in skb remaining space calculation

Vulnerability summary

CVE-2025-38463 is a high (CVSS 7.8) Integer Underflow (Wrap or Wraparound) vulnerability in linux. EPSS estimates a 0.19% chance of exploitation in the next 30 days, higher than 8.4% of scored vulnerabilities.

Timeline

1. 25-07-2025
   PublishedVulnerability first disclosed.
2. 05-08-2026
   Last modifiedVulnerability information updated.

Description

In the Linux kernel, the following vulnerability has been resolved: tcp: Correct signedness in skb remaining space calculation Syzkaller reported a bug [1] where sk->sk_forward_alloc can overflow. When we send data, if an skb exists at the tail of the write queue, the kernel will attempt to append the new data to that skb. However, the code that checks for available space in the skb is flawed: ''' copy = size_goal - skb->len ''' The types of the variables involved are: ''' copy: ssize_t (s64 on 64-bit systems) size_goal: int skb->len: unsigned int ''' Due to C's type promotion rules, the signed size_goal is converted to an unsigned int to match skb->len before the subtraction. The result is an unsigned int. When this unsigned int result is then assigned to the s64 copy variable, it is zero-extended, preserving its non-negative value. Consequently, copy is always >= 0. Assume we are sending 2GB of data and size_goal has been adjusted to a value smaller than skb->len. The subtraction will result in copy holding a very large positive integer. In the subsequent logic, this large value is used to update sk->sk_forward_alloc, which can easily cause it to overflow. The syzkaller reproducer uses TCP_REPAIR to reliably create this condition. However, this can also occur in real-world scenarios. The tcp_bound_to_half_wnd() function can also reduce size_goal to a small value. This would cause the subsequent tcp_wmem_schedule() to set sk->sk_forward_alloc to a value close to INT_MAX. Further memory allocation requests would then cause sk_forward_alloc to wrap around and become negative. [1]: https://syzkaller.appspot.com/bug?extid=de6565462ab540f50e47

• CWE-191Integer Underflow (Wrap or Wraparound)The product subtracts one value from another, such that the result is less than the minimum allowable integer value, which produces a value that is not equal to the correct result.