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Re: /proc filesystem allows bypassing directory permissions on Linux
From: Marco Verschuur <marco () osp nl>
Date: Fri, 30 Oct 2009 18:22:21 +0100

Jim,

Your assumption that the same file descriptor is being re-opened is wrong! The file descriptor retrieved via /proc is a new one. It is not the same as the
initial read-only.

Do a strace on your test and you will see that the 'file descriptor' in /proc will be accessed as an ordinairy file. After checking the directory permissions of that particular file an open will be performed on '/proc/self/fd/0' and a new
O_WRONLY file descriptor is being created.

As Martin Rex already explained yesterday, /proc is all virtual.
The item referred as fd in /proc is not a real file descriptor and as
of that, that 'not-tfor-real file descriptor' is also not re-opend and so does
not become read-write.

The entire discussion about the file descriptor behavior the past days,
including your statement below is all based on false assumptions...
I'll show you a snip out of my strace of the original scenario, being performed by Pavel. But the same mechanism is being performed by you, Jim, in the following step:
                 # su nobody -c 'echo "hacked" >/proc/self/fd/0' < /dir/file.txt

All you do is just open the FILE via the path of /proc, not via the assumed path via /tmp (or /dir in your example) nor access the assumed read- only fd being presented via /proc. Therefor it's totally of no influence what you do with the original directory permission. File access has nothing to do with directory permissions...!

Imagen:
- a house surrounded with a fence with all doors unlocked (file with perm 0666) - a drive-way leads to the gate in the fence and the gate is unlocked (dir with perms 777) - next we put a lock on the gate and don't give guest the key (dir with perms 700)
- guest cannot access the house because he can't pass the gate
- now we take an airplane and parachute guest straight into the perimeter of the fence (/proc access) - guest can access the house (write the file), because the house has all doors unlocked

There is no such mechanism that gives guest the ability to break the lock / steel or duplicate the key of the lock on the gate (circumvent the original directory permissions or upgrade the read-only fd to be read-write), because guest never takes that path via the drive-way and gate
if he accesses the file via /proc

Please do your own strace of the mentioned scenario's, so you'll see it with your own eyes. People tent to debate mechanisms, based on the assumption that a certain mechanisme is being involved here. But the assumed mechanisms are not in play in this particular case.

Below you'll find the parts of the strace output that proves it all;

7649  stat64("/proc", {st_mode=S_IFDIR|0555, st_size=0, ...}) = 0
7649  stat64("/proc/self", {st_mode=S_IFDIR|0555, st_size=0, ...}) = 0
7649 stat64("/proc/self/fd", {st_mode=S_IFDIR|0500, st_size=0, ...}) = 0 7649 stat64("/proc/self/fd/3", {st_mode=S_IFREG|0666, st_size=35, ...}) = 0
<snip>
7649 rt_sigaction(SIGINT, {0x8085067, [], 0}, {0x8085067, [], 0}, 8) = 0
7649  time(NULL)                        = 1256668021
7649 open("/proc/self/fd/3", O_WRONLY|O_CREAT|O_TRUNC|O_LARGEFILE, 0666) = 4
7649  fcntl64(1, F_GETFD)               = 0
7649  fcntl64(1, F_DUPFD, 10)           = 10
7649  fcntl64(1, F_GETFD)               = 0
7649  fcntl64(10, F_SETFD, FD_CLOEXEC)  = 0
7649  close(1)                          = 0
7649  dup2(4, 1)                        = 1
7649  close(4)                          = 0
7649  write(1, "got you\n", 8)          = 8
7649  close(1)                          = 0


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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