From: David Howells <dhowells@redhat.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: dhowells@redhat.com,
syzbot <syzbot+6455648abc28dbdd1e7f@syzkaller.appspotmail.com>,
aou@eecs.berkeley.edu,
Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
James Morris James Morris <jmorris@namei.org>,
keyrings@vger.kernel.org,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
linux-riscv@lists.infradead.org,
LSM List <linux-security-module@vger.kernel.org>,
Palmer Dabbelt <palmer@sifive.com>,
"Serge E. Hallyn" <serge@hallyn.com>,
syzkaller-bugs <syzkaller-bugs@googlegroups.com>
Subject: Re: WARNING: refcount bug in find_key_to_update
Date: Fri, 18 Oct 2019 17:38:05 +0100 [thread overview]
Message-ID: <31180.1571416685@warthog.procyon.org.uk> (raw)
In-Reply-To: <CAHk-=wjFozfjV34_qy3_Z155uz_Z7qFVfE8h=_9ceGU-SVk9hA@mail.gmail.com>
Linus Torvalds <torvalds@linux-foundation.org> wrote:
> 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.
find_key_to_update() must be called with the keyring-to-be-searched locked, as
stated in the comment on that function.
If a key-to-be-updated can be found in that keyring, then the keyring must be
holding a ref on that key already, so it's refcount must be > 0, so it
shouldn't be necessary to use refcount_inc_not_zero().
There shouldn't be a race with key_link(), key_unlink(), key_move(),
keyring_clear() or keyring_gc() (garbage collection) as all of those take a
write-lock on the keyring.
> But I haven't followed the key locking rules, so who knows. That "put
> without lock" scenario would explain the crash, though.
That shouldn't explain it. When key_put() reduces the refcount to 0, it just
schedules the garbage collector. It doesn't touch the key again directly.
I would guess that something incorrectly put a ref when it shouldn't have. Do
we know which type of key is involved? Looking at the syzkaller reproducer,
it's adding an encrypted key and a user key to the process keyring -
presumably repeating the procedure within the same process, hence how it finds
something to update.
David
next prev parent reply other threads:[~2019-10-18 16:38 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-17 1:42 WARNING: refcount bug in find_key_to_update syzbot
2019-10-17 2:42 ` syzbot
2019-10-17 15:53 ` Linus Torvalds
2019-10-17 16:00 ` Eric Biggers
2019-10-18 10:54 ` Tetsuo Handa
2019-10-18 16:45 ` David Howells
2019-10-18 16:38 ` David Howells [this message]
2019-10-22 10:35 ` David Howells
2019-10-22 13:17 ` David Howells
2019-10-21 15:59 ` David Howells
2019-10-21 16:05 ` Dmitry Vyukov
[not found] <20191017092428.7336-1-hdanton@sina.com>
2019-10-18 16:46 ` David Howells
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