[PATCH v2 9/9] nfsd: validate symlink target length in NFSv4 CREATE

nfsd4_decode_create() accepts an unbounded cr_datalen from the wire for
NF4LNK symlink targets, allowing a client to force a kmalloc of up to
the RPC-max size (~1 MiB) per COMPOUND op that persists until compound
teardown.  The VFS rejects oversized targets with ENAMETOOLONG, but the
allocation has already occurred.

Reject cr_datalen == 0 early with nfserr_inval and
cr_datalen >= PATH_MAX with nfserr_nametoolong to bound the
allocation.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Assisted-by: kres:claude-opus-4-7
Reported-by: Chris Mason <[email protected]>
Signed-off-by: Jeff Layton <[email protected]>
---
 fs/nfsd/nfs4xdr.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/nfsd/nfs4xdr.c b/fs/nfsd/nfs4xdr.c
index 508f6986842f..a5cfce95d2d7 100644
--- a/fs/nfsd/nfs4xdr.c
+++ b/fs/nfsd/nfs4xdr.c
@@ -964,6 +964,10 @@ nfsd4_decode_create(struct nfsd4_compoundargs *argp, union nfsd4_op_u *u)
 	case NF4LNK:
 		if (xdr_stream_decode_u32(argp->xdr, &create->cr_datalen) < 0)
 			return nfserr_bad_xdr;
+		if (create->cr_datalen == 0)
+			return nfserr_inval;
+		if (create->cr_datalen > NFS4_MAXPATHLEN)
+			return nfserr_nametoolong;
 		p = xdr_inline_decode(argp->xdr, create->cr_datalen);
 		if (!p)
 			return nfserr_bad_xdr;

-- 
2.54.0