linux-kernel - Re: Kernel crash in free_pipe

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Message-ID: <20171031050000.GK21978@ZenIV.linux.org.uk>
Date:   Tue, 31 Oct 2017 05:00:00 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Cong Wang <xiyou.wangcong@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: Kernel crash in free_pipe_info()

On Mon, Oct 30, 2017 at 08:06:23PM -0700, Linus Torvalds wrote:

> We do that "free_pipe_info(inode->i_pipe);", but we never actually
> clear inode->i_pipe, so now we have an inode that looks like a pipe
> inode, and has a stale pointer to a pipe_inode_info.
> 
> It all looks technically correct. It's fine to use put_filp(), because
> the file pointer has never really been used. And the inode should
> never get re-used anyway without going through the whole reinit in
> inode_init_always().
> 
> So I don't see anything *wrong*, but I see a lot that is just unusual,

FWIW, it's really brittle - consider
        if ((mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ)
                i_readcount_inc(path->dentry->d_inode);
in alloc_file().  It's not the source of trouble in this case, but only
because it's the second call that gets FMODE_READ; reorder them in
create_pipe_files() and you've got a bug.

I considered using fput() there, but that would've required manually
decrementing pipe->files first, which made it rather unappealing...

I don't see anything relevant there, but that's not saying much - flu
and debugging do not mix well, and lack of sleep also doesn't help ;-/