[ 16/82] cifs: ensure that cifs_get_root() only traverses directories

Ben Hutchings
2013-03-18T04:31:08Z

From: Jeff Layton <jlayton@redhat.com>

commit ce2ac52105aa663056dfc17966ebed1bf93e6e64 upstream.

Kjell Braden reported this oops:

[ 833.211970] BUG: unable to handle kernel NULL pointer dereference at (null)
[ 833.212816] IP: [< (null)>] (null)
[ 833.213280] PGD 1b9b2067 PUD e9f7067 PMD 0
[ 833.213874] Oops: 0010 [#1] SMP
[ 833.214344] CPU 0
[ 833.214458] Modules linked in: des_generic md4 nls_utf8 cifs vboxvideo drm snd_intel8x0 snd_ac97_codec ac97_bus snd_pcm snd_seq_midi snd_rawmidi snd_seq_midi_event snd_seq bnep rfcomm snd_timer bluetooth snd_seq_device ppdev snd vboxguest parport_pc joydev mac_hid soundcore snd_page_alloc psmouse i2c_piix4 serio_raw lp parport usbhid hid e1000
[ 833.215629]
[ 833.215629] Pid: 1752, comm: mount.cifs Not tainted 3.0.0-rc7-bisectcifs-fec11dd9a0+ #18 innotek GmbH VirtualBox/VirtualBox
[ 833.215629] RIP: 0010:[<0000000000000000>] [< (null)>] (null)
[ 833.215629] RSP: 0018:ffff8800119c9c50 EFLAGS: 00010282
[ 833.215629] RAX: ffffffffa02186c0 RBX: ffff88000c427780 RCX: 0000000000000000
[ 833.215629] RDX: 0000000000000000 RSI: ffff88000c427780 RDI: ffff88000c4362e8
[ 833.215629] RBP: ffff8800119c9c88 R08: ffff88001fc15e30 R09: 00000000d69515c7
[ 833.215629] R10: ffffffffa0201972 R11: ffff88000e8f6a28 R12: ffff88000c4362e8
[ 833.215629] R13: 0000000000000000 R14: 0000000000000000 R15: ffff88001181aaa6
[ 833.215629] FS: 00007f2986171700(0000) GS:ffff88001fc00000(0000) knlGS:0000000000000000
[ 833.215629] CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[ 833.215629] CR2: 0000000000000000 CR3: 000000001b982000 CR4: 00000000000006f0
[ 833.215629] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 833.215629] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[ 833.215629] Process mount.cifs (pid: 1752, threadinfo ffff8800119c8000, task ffff88001c1c16f0)
[ 833.215629] Stack:
[ 833.215629] ffffffff8116a9b5 ffff8800119c9c88 ffffffff81178075 0000000000000286
[ 833.215629] 0000000000000000 ffff88000c4276c0 ffff8800119c9ce8 ffff8800119c9cc8
[ 833.215629] ffffffff8116b06e ffff88001bc6fc00 ffff88000c4276c0 ffff88000c4276c0
[ 833.215629] Call Trace:
[ 833.215629] [<ffffffff8116a9b5>] ? d_alloc_and_lookup+0x45/0x90
[ 833.215629] [<ffffffff81178075>] ? d_lookup+0x35/0x60
[ 833.215629] [<ffffffff8116b06e>] __lookup_hash.part.14+0x9e/0xc0
[ 833.215629] [<ffffffff8116b1d6>] lookup_one_len+0x146/0x1e0
[ 833.215629] [<ffffffff815e4f7e>] ? _raw_spin_lock+0xe/0x20
[ 833.215629] [<ffffffffa01eef0d>] cifs_do_mount+0x26d/0x500 [cifs]
[ 833.215629] [<ffffffff81163bd3>] mount_fs+0x43/0x1b0
[ 833.215629] [<ffffffff8117d41a>] vfs_kern_mount+0x6a/0xd0
[ 833.215629] [<ffffffff8117e584>] do_kern_mount+0x54/0x110
[ 833.215629] [<ffffffff8117fdc2>] do_mount+0x262/0x840
[ 833.215629] [<ffffffff81108a0e>] ? __get_free_pages+0xe/0x50
[ 833.215629] [<ffffffff8117f9ca>] ? copy_mount_options+0x3a/0x180
[ 833.215629] [<ffffffff8118075d>] sys_mount+0x8d/0xe0
[ 833.215629] [<ffffffff815ece82>] system_call_fastpath+0x16/0x1b
[ 833.215629] Code: Bad RIP value.
[ 833.215629] RIP [< (null)>] (null)
[ 833.215629] RSP <ffff8800119c9c50>
[ 833.215629] CR2: 0000000000000000
[ 833.238525] ---[ end trace ec00758b8d44f529 ]---

When walking down the path on the server, it's possible to hit a
symlink. The path walking code assumes that the caller will handle that
situation properly, but cifs_get_root() isn't set up for it. This patch
prevents the oops by simply returning an error.

A better solution would be to try and chase the symlinks here, but that's
fairly complicated to handle.

Fixes:

https://bugzilla.kernel.org/show_bug.cgi?id=53221

Reported-and-tested-by: Kjell Braden <afflux@pentabarf.de>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
fs/cifs/cifsfs.c | 5 +++++
1 file changed, 5 insertions(+)

--- a/fs/cifs/cifsfs.c
+++ b/fs/cifs/cifsfs.c
@@ -561,6 +561,11 @@ cifs_get_root(struct smb_vol *vol, struc
dentry = ERR_PTR(-ENOENT);
break;
}
+ if (!S_ISDIR(dir->i_mode)) {
+ dput(dentry);
+ dentry = ERR_PTR(-ENOTDIR);
+ break;
+ }

/* skip separators */
while (*s == sep)