stack buffer overflow in pam_env

pam_env reads ~/.pam_environment by default. The routine that parses this file does not correctly validate the size of leading whitespace, and can overflow a character array on the stack. This is currently caught by the stack protections on Ubuntu, but looks to be a more serious problem on Debian which, prior to current unstable, doesn't have pam built with stack protection.

Since this is a bug in a shared library, this will crash whatever is running the code. Most pam-using applications use a separate process for these calls, so the effects should be minimal on Ubuntu, but there could be applications that don't deal well with the pam libraries suddenly exploding.

To reproduce:

perl -e 'print " " x 256, "\\";' >> ~/.pam_environment
perl -e 'print " " x 256, "\\";' >> ~/.pam_environment
perl -e 'print " " x 256, "\\";' >> ~/.pam_environment
perl -e 'print " " x 256, "\\";' >> ~/.pam_environment
perl -e 'print "A" x 256;' >> ~/.pam_environment

Logging in will be violently disabled:
*** stack smashing detected ***: sshd: kees [priv] terminated