[PATCH] net: nfs: fix buffer overflow in nfs_readlink_reply()

[Sebastian Alba Vives <sebasjosue84@gmail.com>]

From: Sebastian Josue Alba Vives <sebasjosue84@gmail.com>

nfs_readlink_reply() validates rlen only against the incoming packet
length (inherited from CVE-2019-14195), but not against the destination
buffer nfs_path_buff[2048]. A malicious NFS server can send a valid
READLINK reply where pathlen + rlen exceeds sizeof(nfs_path_buff),
overflowing the BSS buffer into adjacent memory.

The recent fix in fd6e3d34097f addressed the same overflow class in
net/lwip/nfs.c but left the legacy path in net/nfs-common.c unpatched.

Add bounds checks before both memcpy calls in nfs_readlink_reply():
- relative path branch: reject if pathlen + rlen >= sizeof(nfs_path_buff)
- absolute path branch: reject if rlen >= sizeof(nfs_path_buff)

Fixes: cf3a4f1e86 ("net: nfs: Fix CVE-2019-14195")
Cc: stable@vger.kernel.org
Signed-off-by: Sebastian Alba Vives <sebasjosue84@gmail.com>
---
 net/nfs-common.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/nfs-common.c b/net/nfs-common.c
index 4fbde67a..72d8fd82 100644
--- a/net/nfs-common.c
+++ b/net/nfs-common.c
@@ -674,11 +674,15 @@ static int nfs_readlink_reply(uchar *pkt, unsigned int len)
 
 		strcat(nfs_path, "/");
 		pathlen = strlen(nfs_path);
+		if (pathlen + rlen >= sizeof(nfs_path_buff))
+			return -NFS_RPC_DROP;
 		memcpy(nfs_path + pathlen,
 		       (uchar *)&rpc_pkt.u.reply.data[2 + nfsv3_data_offset],
 		       rlen);
 		nfs_path[pathlen + rlen] = 0;
 	} else {
+		if (rlen >= sizeof(nfs_path_buff))
+			return -NFS_RPC_DROP;
 		memcpy(nfs_path,
 		       (uchar *)&rpc_pkt.u.reply.data[2 + nfsv3_data_offset],
 		       rlen);
-- 
2.43.0