[PATCH 09/10] nfsd: cap decoded POSIX ACL count to bound sort cost

[Jeff Layton <jlayton@kernel.org>]

From: Chris Mason <clm@meta.com>

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
5fc51dfc2eb1 ("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_resource, before any allocation, so the sort is bounded by
NFS_ACL_MAX_ENTRIES^2 comparisons.

Fixes: 5fc51dfc2eb1 ("NFSD: Add support for XDR decoding POSIX draft ACLs")
Assisted-by: kres:claude-opus-4-7
Signed-off-by: Chris Mason <clm@meta.com>
---
 fs/nfsd/nfs4xdr.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/nfsd/nfs4xdr.c b/fs/nfsd/nfs4xdr.c
index c6c50c376b23..5469c6c207ba 100644
--- a/fs/nfsd/nfs4xdr.c
+++ b/fs/nfsd/nfs4xdr.c
@@ -448,6 +448,8 @@ nfsd4_decode_posixacl(struct nfsd4_compoundargs *argp, struct posix_acl **acl)
 
 	if (xdr_stream_decode_u32(argp->xdr, &count) < 0)
 		return nfserr_bad_xdr;
+	if (count > NFS_ACL_MAX_ENTRIES)
+		return nfserr_resource;
 
 	*acl = posix_acl_alloc(count, GFP_KERNEL);
 	if (*acl == NULL)

-- 
2.54.0