nfsd: fix XDR length calculation in nfsd4_ff_encode_layoutget

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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.

Fixes: 9b9960a0ca ("nfsd: Add a super simple flex file server")
Cc: stable@vger.kernel.org
Assisted-by: kres:claude-opus-4-7
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260528-pnfs-fixes-v1-1-8a1255ae2f16@kernel.org
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

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This commit is contained in:

Jeff Layton 2026-05-28 10:38:15 -04:00 • committed by Chuck Lever

parent 8b989aaec8

commit f9868174af

1 changed files with 11 additions and 6 deletions

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17

fs/nfsd/flexfilelayoutxdr.c

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