linux-kernel - Re: [PATCH v3 3/3] exec: do unshare_files after de

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Message-ID: <87efdttmjm.fsf@xmission.com>
Date:   Sun, 16 Sep 2018 18:49:33 +0200
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Jeff Layton <jlayton@...nel.org>, viro@...iv.linux.org.uk,
        berrange@...hat.com, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v3 3/3] exec: do unshare_files after de_thread

Oleg Nesterov <oleg@...hat.com> writes:

> On 09/14, Jeff Layton wrote:
>>
>> POSIX mandates that open fds and their associated file locks should be
>> preserved across an execve. This works, unless the process is
>> multithreaded at the time that execve is called.
>>
>> In that case, we'll end up unsharing the files_struct but the locks will
>> still have their fl_owner set to the address of the old one. Eventually,
>> when the other threads die and the last reference to the old
>> files_struct is put, any POSIX locks get torn down since it looks like
>> a close occurred on them.
>>
>> The result is that all of your open files will be intact with none of
>> the locks you held before execve. The simple answer to this is "use OFD
>> locks", but this is a nasty surprise and it violates the spec.
>>
>> Fix this by doing unshare_files later during exec,
>
> See my reply to 1/3... if we can forget about the races with get_files_struct()
> we can probably make a much simpler patch, plus we do not need 2/2, afaics.
>
> What I really can't understand is why we need to _change_ current->files
> early in do_execve().
>
> IOW. Lets ignore do_close_on_exec(), lets ignore the fact that unshare_fd()
> can fail and thus it makes sense to call it before point-of-no-return.
>
> Any other reason why we can't simply call unshare_files() at the end of
> __do_execve_file() on success?

The reason we call we call unshare_files is in case the files are shared
with another process.  AKA old style linux threads, or someone being
clever.  In that case we need a private copy of files for close on exec
because we should not close the files of the other process that has not
called exec.

The only reason for calling unshare_files before the point of no return
is so that we can get a good error message to the calling process if
unshare_files fails.

Given that "files->count > 1" should only exist in rare and crazy cases.
I expect we can legitimately have exec fail hard if we -ENOMEM in that
case and kill the calling process.

AKA it would be reasonable to move unshare_files to just above
do_close_on_exec in flush_old_exec.  We could further make the
unshare_files not return displaced and just drop it.

Thinking about Jeff's version already by necessity places unshare_files
after de_thread.  So it is already after the point of no return.  So
there really is no point in getting trying hard with displaced files.

Eric