linux-kernel - Re: [PATCH] fork: fix incorrect fput of ->exe

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Message-ID: <20170824132041.GA22882@redhat.com>
Date:   Thu, 24 Aug 2017 15:20:41 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Eric Biggers <ebiggers3@...il.com>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, Dmitry Vyukov <dvyukov@...gle.com>,
        Ingo Molnar <mingo@...nel.org>,
        Konstantin Khlebnikov <koct9i@...il.com>,
        Michal Hocko <mhocko@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>, stable@...r.kernel.org,
        Eric Biggers <ebiggers@...gle.com>
Subject: Re: [PATCH] fork: fix incorrect fput of ->exe_file causing
 use-after-free

On 08/23, Eric Biggers wrote:
>
> From: Eric Biggers <ebiggers@...gle.com>
>
> Commit 7c051267931a ("mm, fork: make dup_mmap wait for mmap_sem for
> write killable") made it possible to kill a forking task while it is
> waiting to acquire its ->mmap_sem for write, in dup_mmap().  However, it
> was overlooked that this introduced an new error path before a reference
> is taken on the mm_struct's ->exe_file.

Hmm. Unless I am totally confused, the same problem with mm->exol_area?
I'll recheck....

> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -806,6 +806,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
>  	mm_init_cpumask(mm);
>  	mm_init_aio(mm);
>  	mm_init_owner(mm, p);
> +	RCU_INIT_POINTER(mm->exe_file, NULL);

Can't we simply move

	RCU_INIT_POINTER(mm->exe_file, get_mm_exe_file(oldmm));

from dup_mmap() here? Afaics this doesn't need mmap_sem.

Good catch!

Oleg.