linux-kernel - Re: [RFC 2/2] fs,proc: Respect FMODE

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CANq1E4R4PSuDD599n=0Xff+rfH01CTRtzLSkFDWSdnXMggVKRA@mail.gmail.com>
Date:	Tue, 22 Apr 2014 23:31:11 +0200
From:	David Herrmann <dh.herrmann@...il.com>
To:	Pavel Machek <pavel@....cz>
Cc:	Andy Lutomirski <luto@...capital.net>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	"Theodore Ts'o" <tytso@....edu>
Subject: Re: [RFC 2/2] fs,proc: Respect FMODE_WRITE when opening /proc/pid/fd/N

Hi

On Tue, Apr 22, 2014 at 8:58 PM, Pavel Machek <pavel@....cz> 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())

Thanks
David
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/