CVE request: crash when attempt to garbage collect an uninstantiated keyring

[Adam Maris <amaris () redhat com>]

Hello,

I'd like to request CVE for the following issue.

------------------------------------------------------------

The following sequence of commands:

    i=`keyctl add user a a @s`
    keyctl request2 keyring foo bar @t
    keyctl unlink $i @s

tries to invoke an upcall to instantiate a keyring if one doesn't already
exist by that name within the user's keyring set. However, if the upcall
fails, the code sets keyring->type_data.reject_error to -ENOKEY or some
other error code.  When the key is garbage collected, the key destroy
function is called unconditionally and keyring_destroy() uses list_empty()
on keyring->type_data.link - which is in a union with reject_error.
Subsequently, the kernel tries to unlink the keyring from the keyring names
list - which oopses like this:

        BUG: unable to handle kernel paging request at 00000000ffffff8a
        IP: [<ffffffff8126e051>] keyring_destroy+0x3d/0x88
        ...
        Workqueue: events key_garbage_collector
        ...
        RIP: 0010:[<ffffffff8126e051>] keyring_destroy+0x3d/0x88
        RSP: 0018:ffff88003e2f3d30  EFLAGS: 00010203
        RAX: 00000000ffffff82 RBX: ffff88003bf1a900 RCX: 0000000000000000
        RDX: 0000000000000000 RSI: 000000003bfc6901 RDI: ffffffff81a73a40
        RBP: ffff88003e2f3d38 R08: 0000000000000152 R09: 0000000000000000
        R10: ffff88003e2f3c18 R11: 000000000000865b R12: ffff88003bf1a900
        R13: 0000000000000000 R14: ffff88003bf1a908 R15: ffff88003e2f4000
        ...
        CR2: 00000000ffffff8a CR3: 000000003e3ec000 CR4: 00000000000006f0
        ...
        Call Trace:
         [<ffffffff8126c756>] key_gc_unused_keys.constprop.1+0x5d/0x10f
         [<ffffffff8126ca71>] key_garbage_collector+0x1fa/0x351
         [<ffffffff8105ec9b>] process_one_work+0x28e/0x547
         [<ffffffff8105fd17>] worker_thread+0x26e/0x361
         [<ffffffff8105faa9>] ? rescuer_thread+0x2a8/0x2a8
         [<ffffffff810648ad>] kthread+0xf3/0xfb
         [<ffffffff810647ba>] ? kthread_create_on_node+0x1c2/0x1c2
         [<ffffffff815f2ccf>] ret_from_fork+0x3f/0x70
         [<ffffffff810647ba>] ? kthread_create_on_node+0x1c2/0x1c2
Note the value in RAX. This is a 32-bit representation of -ENOKEY.

The solution is to only call ->destroy() if the key was successfully
instantiated.

Upstream patch:

http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=ce1fad2740c648a4340f6f6c391a8a83769d2e8c

Reported by: Dmitry Vyukov
Fixed by: David Howells

References:

https://bugzilla.redhat.com/show_bug.cgi?id=1272371
https://bugzilla.redhat.com/show_bug.cgi?id=1272172

------------------------------------------------------
Note that provided commit contains two fixes, although the second one is 
not a security fix.

Thanks!

--
Adam Mariš / Red Hat Product Security