Re: [RFC PATCH] fs: allow open(dir, O_TMPFILE|..., 0) with mode 0

[Andy Lutomirski]

On Wed, Nov 5, 2014 at 7:21 AM, Eric Rannaud <e@xxxxxxxxxxxxxxxx> wrote:
> On Wed, Nov 5, 2014 at 12:27 AM, Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
>> On Mon, Nov 03, 2014 at 11:04:27AM -0800, Linus Torvalds wrote:
>>> Oh, so you don't actually need any file contents at all?
>>>
>>> If that is actually a real usage, then maybe we should just say that
>>> "O_TMPFILE|O_RDONLY" is fine, and remove the check that it has to be
>>> writable.
>>
>> Wasn't this disallowed to prevent problems on old kernels that don't use
>> O_TMPFILE?  In that case we'd ignore the flag and would just get a file
>> handle for the directory instead.
>
> Yes, that was the idea at first, as discussed at
> http://article.gmane.org/gmane.linux.file-systems/76273, in commit
> bb458c644. But soon after that, O_WRONLY was allowed (ba57ea64cb1),
> and O_RDWR was removed from O_TMPFILE. Now only remain the explicit
> checks in build_open_flags() that Linus mentioned.
>
> If we allow
>     fd = open("/tmp", O_TMPFILE|O_RDONLY, 0600)
> it would be seen by an old kernel as
>     fd = open("/tmp", O_DIRECTORY|O_RDONLY, 0600)
> which will succeed.
>
> But unlike the other cases the creative definition of O_TMPFILE was
> meant to prevent, this does not create a security risk for anyone
> implementing a secure tmpfile, as they would be asking for a writable
> fd.

I have no idea whether it's a security risk, but it's a serious "wtf?!?" risk.

>
> To implement an atomic open() with O_TMPFILE+flink, if neither
> O_WRONLY nor O_RDWR is in flags, you would have to manually check with
> fstat that fd is indeed a regular file and not a directory. At least
> if you need to run on old kernels.
>
> If such a changes goes in, the man page for open(2) should talk about
> what happens on old kernels (it already has an explanation for the
> writable case).

With my occasional-API-reviewer hat on, NAK NAK NAK.  This is even
more insane than the rest of the O_TMPFILE interface.  If you want to
support this kind of use case (which seems entirely reasonable to me),
then just add a syscall, please.

Also, you can, mostly, downgrade from O_RDWR to O_RDONLY.  We could
easily add a way to downgrade all the way, I think.

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC
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