nfsd: fix XDR length calculation in nfsd4_ff_encode_layoutget

Overview

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

nfsd: fix XDR length calculation in nfsd4_ff_encode_layoutget

The XDR buffer size calculation in nfsd4_ff_encode_layoutget() has multiple errors that can result in either an out-of-bounds write or leaking uninitialized kernel memory to the client:

• fh_len doesn't account for XDR padding on the file handle data
• uid and gid lengths use "8 + len" but xdr_encode_opaque() actually writes "4 + xdr_align_size(len)" bytes
• ds_len omits the flags and stats_collect_hint fields (8 bytes), while len's header constant overestimates by 8 bytes -- these partially cancel but leave a net mismatch

The worst case occurs with short strings (e.g. uid=0, gid=0 with an odd-sized file handle), where the function writes up to 5 bytes past the reserved XDR buffer. Conversely, when string lengths happen to be 4-byte aligned, the reservation is too large and stale buffer content is sent to the client.

Fix this by breaking out every encoded field explicitly in the ds_len calculation, using xdr_align_size() for all variable-length opaque fields, and correcting the header constants.

Affected

• Linux >= 9b9960a0ca4773e21c4b153ed355583946346b25 < 3a7fd224df0fbb42167eb1b77be45d72fe0b1098
• Linux >= 9b9960a0ca4773e21c4b153ed355583946346b25 < 29e4478e2ed5a227e8f0c33cacb91bf227cedd47
• Linux >= 9b9960a0ca4773e21c4b153ed355583946346b25 < 65a72b721943618eeb3a41c8b36e915591f3f83f
• Linux >= 9b9960a0ca4773e21c4b153ed355583946346b25 < bee826c00ac900473f91306f1f3e5a5a81fd4a74
• Linux >= 9b9960a0ca4773e21c4b153ed355583946346b25 < e7d9d23ecd9172f05b09bb678ff22db8e361c428
• Linux >= 9b9960a0ca4773e21c4b153ed355583946346b25 < 0380129b1373c437eb35401a174671c8888f4b80
• Linux >= 9b9960a0ca4773e21c4b153ed355583946346b25 < c81cef6a805dec266c10fc4f83c93d6fcf1a2b43
• Linux >= 9b9960a0ca4773e21c4b153ed355583946346b25 < f9868174af49d207fbaf0c5e055d088a983684af
• Linux 4.8

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.

Vendor advisories

• Red Hat VEX · Moderate · updated 2026-09-16 · vex