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Date: Sat, 21 Feb 2009 19:37:20 +0100
Message-ID: <19f34abd0902211037w2293af16t561444d11cc834b8@mail.gmail.com>
Subject: Re: [PATCH] mm: fix lazy vmap purging (use-after-free error)
From: Vegard Nossum <vegard.nossum@gmail.com>
To: paulmck@linux.vnet.ibm.com
Cc: Ingo Molnar <mingo@elte.hu>, stable@kernel.org,
       Andrew Morton <akpm@linux-foundation.org>,
       Nick Piggin <npiggin@suse.de>, Pekka Enberg <penberg@cs.helsinki.fi>,
       linux-kernel@vger.kernel.org
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2009/2/21 Vegard Nossum <vegard.nossum@gmail.com>:
> Here's the disassembly (I hope it won't wrap):
>
> 0xc1073ec0 <rcu_check_callbacks+0>:     push   %ebp
> 0xc1073ec1 <rcu_check_callbacks+1>:     test   %edx,%edx
> 0xc1073ec3 <rcu_check_callbacks+3>:     mov    %esp,%ebp
> 0xc1073ec5 <rcu_check_callbacks+5>:     push   %ebx
> 0xc1073ec6 <rcu_check_callbacks+6>:     mov    %eax,%ebx
> 0xc1073ec8 <rcu_check_callbacks+8>:     je     0xc1073f08
> <rcu_check_callbacks+72>
> 0xc1073eca <rcu_qsctr_inc+0>:   mov    $0xc1771320,%eax
> 0xc1073ecf <rcu_qsctr_inc+5>:   add    -0x3e8fa900(,%ebx,4),%eax
> 0xc1073ed6 <rcu_qsctr_inc+12>:  mov    (%eax),%edx
> 0xc1073ed8 <rcu_qsctr_inc+14>:  movb   $0x1,0xc(%eax)
> 0xc1073edc <rcu_qsctr_inc+18>:  mov    %edx,0x8(%eax)
> 0xc1073edf <rcu_bh_qsctr_inc+0>:        mov    $0xc1771380,%eax
> 0xc1073ee4 <rcu_bh_qsctr_inc+5>:        add    -0x3e8fa900(,%ebx,4),%eax
> 0xc1073eeb <rcu_bh_qsctr_inc+12>:       mov    (%eax),%edx
> 0xc1073eed <rcu_bh_qsctr_inc+14>:       movb   $0x1,0xc(%eax)
> 0xc1073ef1 <rcu_bh_qsctr_inc+18>:       mov    %edx,0x8(%eax)
> 0xc1073ef4 <rcu_check_callbacks+52>:    mov    $0x8,%eax
>
> Seems to be rcu_qsctr_inc() that reloads %edx. If I'd guess, I'd say
> x86's per_cpu macros. But it seems so strange that the corruption
> would not manifest in other ways too.
>

Okay, I don't really think it's an error. The if (user) test happens
at the very beginning and gcc decides to reuse %edx. GDB doesn't know
this, so it thinks the parameter changed, but at this point the
parameter simply won't be used anymore.

So you're right: The value can't be trusted (after entry, anyway).


Vegard

-- 
"The animistic metaphor of the bug that maliciously sneaked in while
the programmer was not looking is intellectually dishonest as it
disguises that the error is the programmer's own creation."
	-- E. W. Dijkstra, EWD1036