[PATCH 2/4] NFS: bound multipath address count in file-layout GETDEVICEINFO

nfs4_fl_alloc_deviceid_node() bounds the number of data-server multipath
lists, but not the number of addresses in each list. The latter count is
supplied by the server and can be as large as U32_MAX.

Once the XDR stream is exhausted, nfs4_decode_mp_ds_addr() returns NULL
without consuming input. The enclosing loop can therefore spin for
billions of iterations in kernel context when decoding a malicious
GETDEVICEINFO response.

A netaddr4 contains at least the two XDR length words for its netid and
universal-address strings. Reject a count when that minimum representation
cannot fit in the remaining XDR stream. This makes the loop bound
proportional to the reply length without imposing an arbitrary protocol
limit.

Keep the existing file-layout multipath-list limit separate because that
limit is tied to the u8 stripe-index representation.

Fixes: 14f9a6076f53 ("NFS: Parse and store all multipath DS addresses")
Cc: [email protected]
Signed-off-by: Prabhakar Pujeri <[email protected]>
---
 fs/nfs/filelayout/filelayoutdev.c | 5 +++++
 fs/nfs/pnfs.h                     | 8 ++++++++
 2 files changed, 13 insertions(+)

diff --git a/fs/nfs/filelayout/filelayoutdev.c b/fs/nfs/filelayout/filelayoutdev.c
index d06d303fdcc3..d44baac25d43 100644
--- a/fs/nfs/filelayout/filelayoutdev.c
+++ b/fs/nfs/filelayout/filelayoutdev.c
@@ -159,6 +159,11 @@ nfs4_fl_alloc_deviceid_node(struct nfs_server *server, struct pnfs_device *pdev,
 			goto out_err_free_deviceid;
 
 		mp_count = be32_to_cpup(p); /* multipath count */
+		if (!nfs4_pnfs_ds_addr_count_valid(&stream, mp_count)) {
+			pr_warn_ratelimited("NFS: %s: multipath address count %u exceeds XDR capacity\n",
+					    __func__, mp_count);
+			goto out_err_drain_dsaddrs;
+		}
 		for (j = 0; j < mp_count; j++) {
 			da = nfs4_decode_mp_ds_addr(net, &stream, gfp_flags);
 			if (da)
diff --git a/fs/nfs/pnfs.h b/fs/nfs/pnfs.h
index bab81f769636..38df700e6a0c 100644
--- a/fs/nfs/pnfs.h
+++ b/fs/nfs/pnfs.h
@@ -33,11 +33,19 @@
 #include <linux/refcount.h>
 #include <linux/nfs_fs.h>
 #include <linux/nfs_page.h>
+#include <linux/sunrpc/xdr.h>
 #include <linux/workqueue.h>
 
 struct nfs4_exception;
 struct nfs4_opendata;
 
+/* A netaddr4 contains at least the length words of its two XDR strings. */
+static inline bool
+nfs4_pnfs_ds_addr_count_valid(const struct xdr_stream *xdr, u32 count)
+{
+	return count <= xdr_stream_remaining(xdr) / (2 * sizeof(__be32));
+}
+
 enum {
 	NFS_LSEG_VALID = 0,	/* cleared when lseg is recalled/returned */
 	NFS_LSEG_ROC,		/* roc bit received from server */
-- 
2.54.0