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nfsd: cap decoded POSIX ACL count to bound sort cost

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nfsd4_decode_posixacl() reads a u32 entry count off the wire and passes
it straight to posix_acl_alloc() and sort_pacl_range(). The latter is
an O(n^2) bubble sort, so a client-chosen count drives unbounded CPU in
the server's compound processing path.

    nfsd4_decode_posixacl()
      xdr_stream_decode_u32(&count)       /* uncapped u32 */
      posix_acl_alloc(count, GFP_KERNEL)
      sort_pacl_range(*acl, 0, count - 1) /* O(n^2) bubble sort */

The encoder side in the same file already rejects ACLs whose a_count
exceeds NFS_ACL_MAX_ENTRIES, but the decoder introduced in commit
5fc51dfc2e ("NFSD: Add support for XDR decoding POSIX draft ACLs")
omitted the symmetric check.

Fix by rejecting a wire count greater than NFS_ACL_MAX_ENTRIES with
nfserr_inval, before any allocation, so the sort is bounded by
NFS_ACL_MAX_ENTRIES^2 comparisons.

While we're in here, also fix the nfserr_resource return if
posix_acl_alloc() fails. That's not a legal error code for v4.1+. Change
it to return nfserr_jukebox as that's more appropriate for memory
allocation failures.

Fixes: 5fc51dfc2e ("NFSD: Add support for XDR decoding POSIX draft ACLs")
Cc: stable@vger.kernel.org
Assisted-by: kres:claude-opus-4-7
Reported-by: Chris Mason <clm@meta.com>
Signed-off-by: Chris Mason <clm@meta.com>
Link: https://patch.msgid.link/20260530-nfsd-fixes-v2-8-f27e8eb4d974@kernel.org
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
This commit is contained in:
Chris Mason 2026-05-30 09:19:24 -04:00 • committed by Chuck Lever
commit 4bc1108e87
1 changed files with 10 additions and 1 deletions
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