usbfs: Fix oops related to user namespace conversion.

When running the Point Grey "flycap" program for their USB 3.0 camera
(which was running as a USB 2.0 device for some reason), I trigger this
oops whenever I try to open a video stream:

Dec 15 16:48:34 puck kernel: [ 1798.715559] BUG: unable to handle kernel NULL pointer dereference at           (null)
Dec 15 16:48:34 puck kernel: [ 1798.719153] IP: [<ffffffff8147841e>] free_async+0x1e/0x70
Dec 15 16:48:34 puck kernel: [ 1798.720991] PGD 6f833067 PUD 6fc56067 PMD 0
Dec 15 16:48:34 puck kernel: [ 1798.722815] Oops: 0002 [#1] SMP
Dec 15 16:48:34 puck kernel: [ 1798.724627] CPU 0
Dec 15 16:48:34 puck kernel: [ 1798.724636] Modules linked in: ecryptfs encrypted_keys sha1_generic trusted binfmt_misc sha256_generic aesni_intel cryptd aes_x86_64 aes_generic parport_pc dm_crypt ppdev joydev snd_hda_codec_hdmi snd_hda_codec_conexant arc4 iwlwifi snd_hda_intel snd_hda_codec snd_hwdep snd_pcm thinkpad_acpi mac80211 snd_seq_midi snd_rawmidi snd_seq_midi_event snd_seq snd_timer btusb uvcvideo snd_seq_device bluetooth videodev psmouse snd v4l2_compat_ioctl32 serio_raw tpm_tis cfg80211 tpm tpm_bios nvram soundcore snd_page_alloc lp parport i915 xhci_hcd ahci libahci drm_kms_helper drm sdhci_pci sdhci e1000e i2c_algo_bit video
Dec 15 16:48:34 puck kernel: [ 1798.734212]
Dec 15 16:48:34 puck kernel: [ 1798.736162] Pid: 2713, comm: FlyCap2 Not tainted 3.2.0-rc5+ #28 LENOVO 4286CTO/4286CTO
Dec 15 16:48:34 puck kernel: [ 1798.738148] RIP: 0010:[<ffffffff8147841e>]  [<ffffffff8147841e>] free_async+0x1e/0x70
Dec 15 16:48:34 puck kernel: [ 1798.740134] RSP: 0018:ffff88005715fd78  EFLAGS: 00010296
Dec 15 16:48:34 puck kernel: [ 1798.742118] RAX: 00000000fffffff4 RBX: ffff88006fe8f900 RCX: 0000000000004118
Dec 15 16:48:34 puck kernel: [ 1798.744116] RDX: 0000000001000000 RSI: 0000000000016390 RDI: 0000000000000000
Dec 15 16:48:34 puck kernel: [ 1798.746087] RBP: ffff88005715fd88 R08: 0000000000000000 R09: ffffffff8146f22e
Dec 15 16:48:34 puck kernel: [ 1798.748018] R10: ffff88006e520ac0 R11: 0000000000000001 R12: ffff88005715fe28
Dec 15 16:48:34 puck kernel: [ 1798.749916] R13: ffff88005d31df00 R14: ffff88006fe8f900 R15: 00007f688c995cb8
Dec 15 16:48:34 puck kernel: [ 1798.751785] FS:  00007f68a366da40(0000) GS:ffff880100200000(0000) knlGS:0000000000000000
Dec 15 16:48:34 puck kernel: [ 1798.753659] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
Dec 15 16:48:34 puck kernel: [ 1798.755509] CR2: 0000000000000000 CR3: 00000000706bb000 CR4: 00000000000406f0
Dec 15 16:48:34 puck kernel: [ 1798.757334] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
Dec 15 16:48:34 puck kernel: [ 1798.759124] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Dec 15 16:48:34 puck kernel: [ 1798.760871] Process FlyCap2 (pid: 2713, threadinfo ffff88005715e000, task ffff88006c675b80)
Dec 15 16:48:34 puck kernel: [ 1798.762605] Stack:
Dec 15 16:48:34 puck kernel: [ 1798.764297]  ffff88005715fe28 0000000000000000 ffff88005715fe08 ffffffff81479058
Dec 15 16:48:34 puck kernel: [ 1798.766020]  0000000000000000 ffffea0000004000 ffff880000004118 0000000000000000
Dec 15 16:48:34 puck kernel: [ 1798.767750]  ffff880000000001 ffff88006e520ac0 fffffff46fd81180 0000000000000000
Dec 15 16:48:34 puck kernel: [ 1798.769472] Call Trace:
Dec 15 16:48:34 puck kernel: [ 1798.771147]  [<ffffffff81479058>] proc_do_submiturb+0x778/0xa00
Dec 15 16:48:34 puck kernel: [ 1798.772798]  [<ffffffff8147a5fd>] usbdev_do_ioctl+0x24d/0x1200
Dec 15 16:48:34 puck kernel: [ 1798.774410]  [<ffffffff8147b5de>] usbdev_ioctl+0xe/0x20
Dec 15 16:48:34 puck kernel: [ 1798.775975]  [<ffffffff81189259>] do_vfs_ioctl+0x99/0x600
Dec 15 16:48:34 puck kernel: [ 1798.777534]  [<ffffffff81189851>] sys_ioctl+0x91/0xa0
Dec 15 16:48:34 puck kernel: [ 1798.779088]  [<ffffffff816247c2>] system_call_fastpath+0x16/0x1b
ec 15 16:48:34 puck kernel: [ 1798.780634] Code: 51 ff ff ff e9 29 ff ff ff 0f 1f 40 00 55 48 89 e5 53 48 83 ec 08 66 66 66 66 90 48 89 fb 48 8b 7f 18 e8 a6 ea c0 ff 4
8 8b 7b 20 <f0> ff 0f 0f 94 c0 84 c0 74 05 e8 d3 99 c1 ff 48 8b 43 40 48 8b
Dec 15 16:48:34 puck kernel: [ 1798.783970] RIP  [<ffffffff8147841e>] free_async+0x1e/0x70
Dec 15 16:48:34 puck kernel: [ 1798.785630]  RSP <ffff88005715fd78>
Dec 15 16:48:34 puck kernel: [ 1798.787274] CR2: 0000000000000000
Dec 15 16:48:34 puck kernel: [ 1798.794728] ---[ end trace 52894d3355f88d19 ]---

markup_oops.pl says the oops is in put_cred:

 ffffffff81478401:      48 89 e5                mov    %rsp,%rbp
 ffffffff81478404:      53                      push   %rbx
 ffffffff81478405:      48 83 ec 08             sub    $0x8,%rsp
 ffffffff81478409:      e8 f2 c0 1a 00          callq  ffffffff81624500 <mcount>
 ffffffff8147840e:      48 89 fb                mov    %rdi,%rbx   |  %ebx => ffff88006fe8f900
        put_pid(as->pid);
 ffffffff81478411:      48 8b 7f 18             mov    0x18(%rdi),%rdi
 ffffffff81478415:      e8 a6 ea c0 ff          callq  ffffffff81086ec0 <put_pid>
        put_cred(as->cred);
 ffffffff8147841a:      48 8b 7b 20             mov    0x20(%rbx),%rdi |  %edi => 0  %ebx = ffff88006fe8f900
  */
 static inline int atomic_dec_and_test(atomic_t *v)
 {
        unsigned char c;

        asm volatile(LOCK_PREFIX "decl %0; sete %1"
*ffffffff8147841e:      f0 ff 0f                lock decl (%rdi)   |  %edi = 0 <--- faulting instruction
 ffffffff81478421:      0f 94 c0                sete   %al
 static inline void put_cred(const struct cred *_cred)
 {
        struct cred *cred = (struct cred *) _cred;

        validate_creds(cred);
        if (atomic_dec_and_test(&(cred)->usage))
 ffffffff81478424:      84 c0                   test   %al,%al
 ffffffff81478426:      74 05                   je     ffffffff8147842d <free_async+0x2d>
                __put_cred(cred);
 ffffffff81478428:      e8 d3 99 c1 ff          callq  ffffffff81091e00 <__put_cred>
        kfree(as->urb->transfer_buffer);
 ffffffff8147842d:      48 8b 43 40             mov    0x40(%rbx),%rax
 ffffffff81478431:      48 8b 78 68             mov    0x68(%rax),%rdi
 ffffffff81478435:      e8 a6 e1 ce ff          callq  ffffffff811665e0 <kfree>
        kfree(as->urb->setup_packet);
 ffffffff8147843a:      48 8b 43 40             mov    0x40(%rbx),%rax
 ffffffff8147843e:      48 8b b8 90 00 00 00    mov    0x90(%rax),%rdi
 ffffffff81478445:      e8 96 e1 ce ff          callq  ffffffff811665e0 <kfree>
        usb_free_urb(as->urb);
 ffffffff8147844a:      48 8b 7b 40             mov    0x40(%rbx),%rdi
 ffffffff8147844e:      e8 0d 6b ff ff          callq  ffffffff8146ef60 <usb_free_urb>

This bug seems to have been introduced by commit
d178bc3a "user namespace: usb: make usb
urbs user namespace aware (v2)"

I'm not sure if this is right fix, but it does stop the oops.

Unfortunately, the Point Grey software still refuses to work, but it's a
closed source app, so I can't fix it.
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/core/devio.c

@@ -286,7 +286,8 @@ static struct async *alloc_async(unsigned int numisoframes)
 static void free_async(struct async *as)
 {
 	put_pid(as->pid);
-	put_cred(as->cred);
+	if (as->cred)
+		put_cred(as->cred);
 	kfree(as->urb->transfer_buffer);
 	kfree(as->urb->setup_packet);
 	usb_free_urb(as->urb);