[PATCH v2 8/9] nfsd: cap decoded POSIX ACL count to bound sort cost

[Jeff Layton]

From: Chris Mason <clm@xxxxxxxx>

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_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: 5fc51dfc2eb1 ("NFSD: Add support for XDR decoding POSIX draft ACLs")
Assisted-by: kres:claude-opus-4-7
Reported-by: Chris Mason <clm@xxxxxxxx>
Signed-off-by: Chris Mason <clm@xxxxxxxx>
---
 fs/nfsd/nfs4xdr.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/fs/nfsd/nfs4xdr.c b/fs/nfsd/nfs4xdr.c
index c6c50c376b23..508f6986842f 100644
--- a/fs/nfsd/nfs4xdr.c
+++ b/fs/nfsd/nfs4xdr.c
@@ -449,9 +449,18 @@ nfsd4_decode_posixacl(struct nfsd4_compoundargs *argp, struct posix_acl **acl)
 	if (xdr_stream_decode_u32(argp->xdr, &count) < 0)
 		return nfserr_bad_xdr;
 
+	/*
+	 * The NFSv4 POSIX ACL draft doesn't define a max number of ACE's, but
+	 * the NFSACL spec does. For NFSv4, cap the number of entries to the v3
+	 * limit, as we want to ensure that ACLs set via NFSv4 POSIX ACL
+	 * extensions are retrievable via NFSACL.
+	 */
+	if (count > NFS_ACL_MAX_ENTRIES)
+		return nfserr_inval;
+
 	*acl = posix_acl_alloc(count, GFP_KERNEL);
 	if (*acl == NULL)
-		return nfserr_resource;
+		return nfserr_jukebox;
 
 	(*acl)->a_count = count;
 	for (ace = (*acl)->a_entries; ace < (*acl)->a_entries + count; ace++) {

-- 
2.54.0