procfs gives read/write access to RO/WO pipes

[Frank Heckenbach <f.heckenbach@fh-soft.de>]

Using /proc/*/fd, you can get read/write access to a pipe that you
have read-only or write-only access to. The program below
demonstrates this. It reads and writes through the "0" fd.
Tested with 2.6.34, i686.

At first sight, it doesn't look too serious if a program can write
to its own readable pipe, or read from its writeable pipe, but
perhaps I'm just not creative enough to see an exploit. Also, I
don't know it it might affect other file types where it might be a
real problem. At least, it does seem to violate the permissions.

#include <unistd.h>
#include <stdio.h>
#include <fcntl.h>

int main ()
{
  char a, n[64];
  int p[2], f, r;
  pipe (p);
  sprintf (n, "/proc/self/fd/%i", p[0]);
  f = open (n, O_RDWR);
  write (f, "x", 1);
  r = read (f, &a, 1);
  fprintf (stderr, "%i %c\n", r, a);
  return 0;
}