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.
