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[PATCH 0/2] setgid hardening

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  [PATCH 0/2] setgid hardening Andy Lutomirski <luto@kernel.org> - 2017-01-25 22:10 +0100
    Re: [PATCH 0/2] setgid hardening Willy Tarreau <w@1wt.eu> - 2017-01-26 01:10 +0100

#1566954 — [PATCH 0/2] setgid hardening

The kernel has some dangerous behavior involving the creation and
modification of setgid executables.  These issues aren't kernel
security bugs per se, but they have been used to turn various
filesystem permission oddities into reliably privilege escalation
exploits.

See http://www.halfdog.net/Security/2015/SetgidDirectoryPrivilegeEscalation/
for a nice writeup.

Let's fix them for real.

Andy Lutomirski (2):
  fs: Check f_cred instead of current's creds in should_remove_suid()
  fs: Harden against open(..., O_CREAT, 02777) in a setgid directory

 fs/inode.c         | 37 ++++++++++++++++++++++++++++++-------
 fs/internal.h      |  2 +-
 fs/ocfs2/file.c    |  4 ++--
 fs/open.c          |  2 +-
 include/linux/fs.h |  2 +-
 5 files changed, 35 insertions(+), 12 deletions(-)

-- 
2.9.3

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#1567051

On Wed, Jan 25, 2017 at 01:06:50PM -0800, Andy Lutomirski wrote:
> The kernel has some dangerous behavior involving the creation and
> modification of setgid executables.  These issues aren't kernel
> security bugs per se, but they have been used to turn various
> filesystem permission oddities into reliably privilege escalation
> exploits.
> 
> See http://www.halfdog.net/Security/2015/SetgidDirectoryPrivilegeEscalation/
> for a nice writeup.
> 
> Let's fix them for real.

BTW I like this. I vaguely remember having played with this when I
was a student 2 decades ago on a system where /var/spool/mail was
3777 (yes, setgid+sticky) and the mail files were 660. You could
deposit a shell there, then execute it with mail's permissions and
access any mailbox. That was quite odd as a design choice. The
impacts are often limited unless you find other ways to escalate
but generally it's not really clean.

Willy

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