linux-kernel - Re: [PATCH 1/2] vfs: implement fchmodat2() syscall

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Date:   Tue, 11 Apr 2017 13:07:02 -0500
From:   Eric Blake <eblake@...hat.com>
To:     Colin Walters <walters@...bum.org>, Greg Kurz <groug@...d.org>,
        viro@...iv.linux.org.uk
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] vfs: implement fchmodat2() syscall

On 04/11/2017 12:55 PM, Eric Blake wrote:
> On 04/11/2017 12:52 PM, Colin Walters wrote:
>>
>>
>> On Tue, Feb 28, 2017, at 02:23 PM, Eric Blake wrote:
>>
>>> Might also be worth mentioning that this patch is required in order to
>>> solve CVE-2016-9602, per discussion at
>>> https://lists.gnu.org/archive/html/qemu-devel/2017-02/msg06089.html
>>
>> I only briefly looked at this, but can't `open(..., O_PATH)` be used to solve
>> this today?
> 
> O_PATH was the fallback that qemu used

Hmm - actually, qemu used O_PATH for the directory portion of *at
traversals:
git.qemu-project.org/?p=qemu.git;a=commitdiff;h=918112c

but did not use O_PATH for its chmod() fallback:
git.qemu-project.org/?p=qemu.git;a=commitdiff;h=e3187a4

A good idea on the surface. But reading the man page of openat(), the
section on O_PATH says:
   The  file
              itself  is not opened, and other file operations (e.g.,
read(2),
              write(2), fchmod(2), fchown(2), fgetxattr(2), mmap(2))
fail with
              the error EBADF.

> - but that's non-POSIX, which
> means we have to have a different solution for POSIX systems than for
> Linux systems, while waiting for Linux to catch up to POSIX.

But even if using open(O_PATH)/fchmod() works, it is not immediately
obvious whether it can catch all the same cases that chmodat(O_NOFOLLOW)
would cover, as there are cases where you have permissions to change
mode bits but not open() the file for reading or writing.  And even if
it gets rid of a TOCTTOU race, it still is a 2-syscall hit rather than
an atomic single syscall.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org



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