CVE-2013-2207, BZ #15755: Disable pt_chown.

The helper binary pt_chown tricked into granting access to another
user's pseudo-terminal.

Pre-conditions for the attack:

 * Attacker with local user account
 * Kernel with FUSE support
 * "user_allow_other" in /etc/fuse.conf
 * Victim with allocated slave in /dev/pts

Using the setuid installed pt_chown and a weak check on whether a file
descriptor is a tty, an attacker could fake a pty check using FUSE and
trick pt_chown to grant ownership of a pty descriptor that the current
user does not own.  It cannot access /dev/pts/ptmx however.

In most modern distributions pt_chown is not needed because devpts
is enabled by default. The fix for this CVE is to disable building
and using pt_chown by default. We still provide a configure option
to enable hte use of pt_chown but distributions do so at their own
risk.

1 files changed, 12 insertions, 0 deletions

diff --git a/INSTALL b/INSTALL
index 721a7fc..2c61704 100644
--- a/INSTALL
+++ b/INSTALL
@@ -136,6 +136,18 @@ will be used, and CFLAGS sets optimization options for the compiler.
 `--enable-lock-elision=yes'
      Enable lock elision for pthread mutexes by default.
 
+`--enable-pt_chown'
+     The file `pt_chown' is a helper binary for `grantpt' (*note
+     Pseudo-Terminals: Allocation.) that is installed setuid root to
+     fix up pseudo-terminal ownership.  It is not built by default
+     because systems using the Linux kernel are commonly built with the
+     `devpts' filesystem enabled and mounted at `/dev/pts', which
+     manages pseudo-terminal ownership automatically.  By using
+     `--enable-pt_chown', you may build `pt_chown' and install it
+     setuid and owned by `root'.  The use of `pt_chown' introduces
+     additional security risks to the system and you should enable it
+     only if you understand and accept those risks.
+
 `--build=BUILD-SYSTEM'
 `--host=HOST-SYSTEM'
      These options are for cross-compiling.  If you specify both