Re: [RFC 2/2] fs,proc: Respect FMODE_WRITE when opening /proc/pid/fd/N

[Andy Lutomirski]

On Tue, Apr 22, 2014 at 2:31 PM, David Herrmann <dh.herrmann@xxxxxxxxx> wrote:
> Hi
>
> On Tue, Apr 22, 2014 at 8:58 PM, Pavel Machek <pavel@xxxxxx> wrote:
>> I don't think openat helps you. This is what we are talking about, it
>> is easy to reproduce. Can you reproduce it without /proc mounted?
>>
>> I think that chmod 700 . should stop you. Openat seems no worse than
>> just placing cwd there...
>
> Example1:
> $ mkdir -p subdir/next
> $ chmod 000 subdir
> $ touch subdir/next/test
> => EACCES
> $ cd subdir
> => EACCES
>
> Example2:
> $ mkdir -p subdir/next
> $ cd subdir/next
> $ chmod 000 ..
> $ touch test
> => SUCCESS
>
> This is the exact same situation. The filesystem tree is exactly the
> same in both situations, but in the first example CWD is outside of
> "subdir", in the second example CWD is inside of "subdir". Thus, they
> can create files in that directory, even though they have no access to
> _any_ absolute path to that directory.
>
> This is the exact same race that you describe via /proc/self/fd/. But
> instead of keeping a ref to the dir via CWD, in your example you keep
> the ref via a FD in that exact same directory and access it via /proc.
>
> (Hint: instead of using CWD, you can also keep an FD via open(O_PATH)
> and pass it to openat())

So what?

It's well-known that the execute bit is only checked when you look
something up, and it's not checked all the way back to the root.  It's
not well known, nor is it POSIX, that you can fudge it via proc.

--Andy
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