CVE-2026-89551 | In the Linux kernel, the following vulnerability has been resolved: SUNRPC: xdr_buf_trim: clamp buf->len to avoid underflow xdr_buf_trim() trims `len` bytes from the tail of an xdr_buf by walking the tail, pages, and head iovecs. Each per-section step uses min_t() so it never removes more bytes than that section holds, but the final accounting at the fix_len label subtracts the total bytes actually consumed from buf->len without any clamp: fix_len:
In the Linux kernel, the following vulnerability has been resolved:
SUNRPC: xdr_buf_trim: clamp buf->len to avoid underflow
xdr_buf_trim() trims `len` bytes from the tail of an xdr_buf by
walking the tail, pages, and head iovecs. Each per-section step
uses min_t() so it never removes more bytes than that section
holds, but the final accounting at the fix_len label subtracts the
total bytes actually consumed from buf->len without any clamp:
fix_len:
buf->len -= (len - trim);
When the caller has set buf->len to a value smaller than the sum
of the iov_lens, (len - trim) can exceed buf->len and the unsigned
subtraction wraps to near UINT_MAX. gss_krb5_unwrap_v2() reaches
xdr_buf_trim() in exactly that state:
buf->head[0].iov_len -= GSS_KRB5_TOK_HDR_LEN + headskip;
buf->len = len - (GSS_KRB5_TOK_HDR_LEN + headskip);
xdr_buf_trim(buf, ec + GSS_KRB5_TOK_HDR_LEN + tailskip);
buf->len is a small wire-derived value while the iov_lens are at
page scale, so the per-section loops legitimately consume far more
bytes than buf->len records. The wrapped buf->len then propagates
as the authoritative stream bound into every downstream XDR
decoder.
Fix by clamping the decrement so buf->len bottoms out at zero:
buf->len -= min_t(unsigned int, buf->len, len - trim);
On the normal path where the iov_lens sum to buf->len, (len - trim)
is always len and the result is identical to before. No
callers change behavior outside the underflow case.
- 🔗 git.kernel.org/stable/c/a3d77bcd974b8625d16bddf448cb3a1b…
- 🔗 git.kernel.org/stable/c/fa16bbe987b47e771e52eeb1b4540f18…
- 🔗 git.kernel.org/stable/c/a924ac4c78afab71bf82641afa3b62e0…
- 🔗 git.kernel.org/stable/c/4bf59cb0ea5b0ddfbc46a1dc2fa78fc8…
- 🔗 git.kernel.org/stable/c/e6267cccd7b05cc514e57f2160aa8db8…
- 🔗 git.kernel.org/stable/c/ad0cce80d4af2f74674e8b635d97aa38…
- 🔗 git.kernel.org/stable/c/85e9602650e9df07190abe817cee3b4d…
- 🔗 git.kernel.org/stable/c/3f491306dcb673ff5e78e1044ba450c5…
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