[PATCH] NFS: fs_context: validate UDP retrans to prevent shift out-of-bounds

[Randy Dunlap <rdunlap@infradead.org>]

Fix shift out-of-bounds in xprt_calc_majortimeo(). This is caused
by a garbage timeout (retrans) mount option being passed to nfs mount,
in this case from syzkaller.

If the protocol is XPRT_TRANSPORT_UDP, then 'retrans' is a shift
value for a 64-bit long integer, so 'retrans' cannot be >= 64.
If it is >= 64, fail the mount and return an error.

Fixes: 9954bf92c0cd ("NFS: Move mount parameterisation bits into their own file")
Reported-by: syzbot+ba2e91df8f74809417fa@syzkaller.appspotmail.com
Reported-by: syzbot+f3a0fa110fd630ab56c8@syzkaller.appspotmail.com
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Trond Myklebust <trond.myklebust@hammerspace.com>
Cc: Anna Schumaker <anna.schumaker@netapp.com>
Cc: linux-nfs@vger.kernel.org
Cc: David Howells <dhowells@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: stable@vger.kernel.org
---
 fs/nfs/fs_context.c |   12 ++++++++++++
 1 file changed, 12 insertions(+)

--- lnx-512-rc1.orig/fs/nfs/fs_context.c
+++ lnx-512-rc1/fs/nfs/fs_context.c
@@ -974,6 +974,15 @@ static int nfs23_parse_monolithic(struct
 			       sizeof(mntfh->data) - mntfh->size);
 
 		/*
+		 * for proto == XPRT_TRANSPORT_UDP, which is what uses
+		 * to_exponential, implying shift: limit the shift value
+		 * to BITS_PER_LONG (majortimeo is unsigned long)
+		 */
+		if (!(data->flags & NFS_MOUNT_TCP)) /* this will be UDP */
+			if (data->retrans >= 64) /* shift value is too large */
+				goto out_invalid_data;
+
+		/*
 		 * Translate to nfs_fs_context, which nfs_fill_super
 		 * can deal with.
 		 */
@@ -1073,6 +1082,9 @@ out_no_address:
 
 out_invalid_fh:
 	return nfs_invalf(fc, "NFS: invalid root filehandle");
+
+out_invalid_data:
+	return nfs_invalf(fc, "NFS: invalid binary mount data");
 }
 
 #if IS_ENABLED(CONFIG_NFS_V4)