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

[David Herrmann]

Hi

On Tue, Apr 22, 2014 at 3:49 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> Anyone who opens a file read-only and sends it over SCM_RIGHTS is
> likely broken.  They may think that it's read-only, so it can't be
> written, but this /proc/fd issue means that whoever receives it can
> reopen it.
>
> It's true that, if the inode doesn't allow the recipient write access,
> then the recipient can't reopen, but there are lots of cases where the
> inode can't reliably be expected not to allow write.  For example, the
> inode could be unlinked, an O_TMPFILE file, a memfd handle, or in a
> non-world-executable directory, and the file mode should be respected.

I think it's safe to assume that any object you create is never
world-accessible. So the worst you can get is 0600. So if we now take
your example, your patch doesn't fix the problem at all. Imagine two
processes, $sender and $receiver. If the receiver runs as a different
user as the sender, it cannot open /proc/self/fd/ writable due to
0600. So the only problematic case is if both run as the same user.
However, in that case, the receiver can _always_ access
/proc/$sender/fd/ and thus still gain writable access to the object,
even if its own fd is read-only and your patch was applied. (ignoring
the fact that they can kill() and ptrace each other..)

Protecting world-accessible objects by hiding them is imho wrong. And
protecting users against themselves is even worse.

>>
>> fd = open("/run", O_RDWR | O_TMPFILE);
>
> Did you mean fd = open("/run", O_RDWR | O_TMPFILE, 0666)?  0600?

Sorry, I meant S_IWUSR | S_IRUSR (0600).

Thanks
David
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