[PATCH] Fix prctl privilege escalation and suid_dumpable (CVE-2006-2451)

Based on a patch from Ernie Petrides

During security research, Red Hat discovered a behavioral flaw in core
dump handling. A local user could create a program that would cause a
core file to be dumped into a directory they would not normally have
permissions to write to. This could lead to a denial of service (disk
consumption), or allow the local user to gain root privileges.

The prctl() system call should never allow to set "dumpable" to the
value 2. Especially not for non-privileged users.

This can be split into three cases:

  1) running as root -- then core dumps will already be done as root,
     and so prctl(PR_SET_DUMPABLE, 2) is not useful

  2) running as non-root w/setuid-to-root -- this is the debatable case

  3) running as non-root w/setuid-to-non-root -- then you definitely
     do NOT want "dumpable" to get set to 2 because you have the
     privilege escalation vulnerability

With case #2, the only potential usefulness is for a program that has
designed to run with higher privilege (than the user invoking it) that
wants to be able to create root-owned root-validated core dumps. This
might be useful as a debugging aid, but would only be safe if the program
had done a chdir() to a safe directory.

There is no benefit to a production setuid-to-root utility, because it
shouldn't be dumping core in the first place. If this is true, then the
same debugging aid could also be accomplished with the "suid_dumpable"
sysctl.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>