WARNING: refcount bug in find_key_to_update

[Eric Biggers]

On Thu, Oct 17, 2019 at 08:53:06AM -0700, Linus Torvalds wrote:
> On Wed, Oct 16, 2019 at 7:42 PM syzbot
> <syzbot+6455648abc28dbdd1e7f at syzkaller.appspotmail.com> wrote:
> >
> > syzbot has bisected this bug to 0570bc8b7c9b ("Merge tag
> >  'riscv/for-v5.3-rc1' ...")
> 
> Yeah, that looks unlikely. The only non-riscv changes are from
> documentation updates and moving a config variable around.
> 
> Looks like the crash is quite unlikely, and only happens in one out of
> ten runs for the ones it has happened to.
> 
> The backtrace looks simple enough, though:
> 
>   RIP: 0010:refcount_inc_checked+0x2b/0x30 lib/refcount.c:156
>    __key_get include/linux/key.h:281 [inline]
>    find_key_to_update+0x67/0x80 security/keys/keyring.c:1127
>    key_create_or_update+0x4e5/0xb20 security/keys/key.c:905
>    __do_sys_add_key security/keys/keyctl.c:132 [inline]
>    __se_sys_add_key security/keys/keyctl.c:72 [inline]
>    __x64_sys_add_key+0x219/0x3f0 security/keys/keyctl.c:72
>    do_syscall_64+0xd0/0x540 arch/x86/entry/common.c:296
>    entry_SYSCALL_64_after_hwframe+0x49/0xbe
> 
> which to me implies that there's some locking bug, and somebody
> released the key without holding a lock.
> 
> That code looks a bit confused to me. Releasing a key without holding
> a lock looks permitted, but if that's the case then __key_get() is
> complete garbage. It would need to use 'refcount_inc_not_zero()' and
> failure would require failing the caller.
> 
> But I haven't followed the key locking rules, so who knows. That "put
> without lock" scenario would explain the crash, though.
> 
> David?
> 

Yes this is a bogus bisection.

The key is supposed to have refcount >= 1 since it's in a keyring.
So some bug is causing it to have refcount 0.  Perhaps some place calling
key_put() too many times.

Unfortunately I can't get the reproducer to work locally.

Note that there are 2 other syzbot reports that look related.
No reproducers for them, though:

Title:              KASAN: use-after-free Read in key_put
Last occurred:      1 day ago
Reported:           28 days ago
Branches:           Mainline
Dashboard link:     https://syzkaller.appspot.com/bug?id=f13750b1124e01191250cf930086dcc40740fa30
Original thread:    https://lore.kernel.org/lkml/0000000000008c3e590592cf4b7f@google.com/T/#u

Title:              KASAN: use-after-free Read in keyring_compare_object
Last occurred:      49 days ago
Reported:           84 days ago
Branches:           Mainline
Dashboard link:     https://syzkaller.appspot.com/bug?id=529ab6a98286c2a97c445988a62760a58d4a1d4b
Original thread:    https://lore.kernel.org/lkml/000000000000038ef6058e6f3592@google.com/T/#u

- Eric