linux-kernel - Re: GPF in keyring

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Date:	Mon, 19 Oct 2015 10:26:21 +0200
From:	Dmitry Vyukov <dvyukov@...gle.com>
To:	David Howells <dhowells@...hat.com>
Cc:	james.l.morris@...cle.com, serge@...lyn.com,
	keyrings@...r.kernel.org, linux-security-module@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>, syzkaller@...glegroups.com,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Andrey Konovalov <andreyknvl@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Julien Tinnes <jln@...gle.com>, Kees Cook <keescook@...gle.com>
Subject: Re: GPF in keyring_destroy

On Thu, Oct 15, 2015 at 9:21 PM, David Howells <dhowells@...hat.com> wrote:
> Does the attached patch fix it for you?

Yes, it fixes the crash for me.


> David
> ---
> commit a7609e0bb3973d6ee3c9f1ecd0b6a382d99d6248
> Author: David Howells <dhowells@...hat.com>
> Date:   Thu Oct 15 17:21:37 2015 +0100
>
>     KEYS: Fix crash when attempt to garbage collect an uninstantiated keyring
>
>     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.
>
>     Reported-by: Dmitry Vyukov <dvyukov@...gle.com>
>     Signed-off-by: David Howells <dhowells@...hat.com>
>
> diff --git a/security/keys/gc.c b/security/keys/gc.c
> index 39eac1fd5706..addf060399e0 100644
> --- a/security/keys/gc.c
> +++ b/security/keys/gc.c
> @@ -134,8 +134,10 @@ static noinline void key_gc_unused_keys(struct list_head *keys)
>                 kdebug("- %u", key->serial);
>                 key_check(key);
>
> -               /* Throw away the key data */
> -               if (key->type->destroy)
> +               /* Throw away the key data if the key is instantiated */
> +               if (test_bit(KEY_FLAG_INSTANTIATED, &key->flags) &&
> +                   !test_bit(KEY_FLAG_NEGATIVE, &key->flags) &&
> +                   key->type->destroy)
>                         key->type->destroy(key);
>
>                 security_key_free(key);
--
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