netdev - Re: use-after-free in sctp_do

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Date:	Thu, 3 Dec 2015 16:55:44 +0100
From:	Dmitry Vyukov <dvyukov@...gle.com>
To:	Eric Dumazet <edumazet@...gle.com>
Cc:	syzkaller <syzkaller@...glegroups.com>,
	Vladislav Yasevich <vyasevich@...il.com>,
	linux-sctp@...r.kernel.org, netdev <netdev@...r.kernel.org>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>
Subject: Re: use-after-free in sctp_do_sm

On Thu, Dec 3, 2015 at 3:48 PM, Eric Dumazet <edumazet@...gle.com> wrote:
>>
>> No, I don't. But pr_debug always computes its arguments. See no_printk
>> in printk.h. So this use-after-free happens for all users.
>
> Hmm.
>
> pr_debug() should be a nop unless either DEBUG or CONFIG_DYNAMIC_DEBUG are set
>
> On our production kernels, pr_debug() is a nop.
>
> Can you double check ? Thanks !


Why should it be nop? no_printk thing in printk.h pretty much
explicitly makes it not a nop...

Double-checked: debug_post_sfx leads to some generated code:

        debug_post_sfx();
ffffffff8229f256:       48 8b 85 58 fe ff ff    mov    -0x1a8(%rbp),%rax
ffffffff8229f25d:       48 85 c0                test   %rax,%rax
ffffffff8229f260:       74 24                   je
ffffffff8229f286 <sctp_do_sm+0x176>
ffffffff8229f262:       8b b0 a8 00 00 00       mov    0xa8(%rax),%esi
ffffffff8229f268:       48 8b 85 60 fe ff ff    mov    -0x1a0(%rbp),%rax
ffffffff8229f26f:       44 89 85 74 fe ff ff    mov    %r8d,-0x18c(%rbp)
ffffffff8229f276:       48 8b 78 20             mov    0x20(%rax),%rdi
ffffffff8229f27a:       e8 71 28 01 00          callq
ffffffff822b1af0 <sctp_id2assoc>
ffffffff8229f27f:       44 8b 85 74 fe ff ff    mov    -0x18c(%rbp),%r8d

        return error;
}
ffffffff8229f286:       48 81 c4 a0 01 00 00    add    $0x1a0,%rsp
ffffffff8229f28d:       44 89 c0                mov    %r8d,%eax
ffffffff8229f290:       5b                      pop    %rbx
ffffffff8229f291:       41 5c                   pop    %r12
ffffffff8229f293:       41 5d                   pop    %r13
ffffffff8229f295:       41 5e                   pop    %r14
ffffffff8229f297:       41 5f                   pop    %r15
ffffffff8229f299:       5d                      pop    %rbp
ffffffff8229f29a:       c3                      retq
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