Re: [PATCH 4.1.y] KEYS: Fix crash when attempt to garbage collect an uninstantiated keyring

[Greg KH <greg@kroah.com>]

On Tue, Dec 01, 2015 at 05:46:40PM -0500, Chas Williams wrote:
> From: David Howells <dhowells@redhat.com>
> 
> Commit f05819df10d7b09f6d1eb6f8534a8f68e5a4fe61 upstream.
> 
> 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.
> 
> Fixes CVE-2015-7872.
> 
> Reported-by: Dmitry Vyukov <dvyukov@google.com>
> Signed-off-by: David Howells <dhowells@redhat.com>
> Tested-by: Dmitry Vyukov <dvyukov@google.com>
> [3chas3@gmail.com: backported to 4.1 due to context changes]

I took an earlier keys patch as well, which didn't require this context
change, but thanks for the backport.

greg k-h