[RFC,0/4] coredump: mitigate privilege escalation of process coredump

Message

Wander Lairson Costa Dec. 27, 2021, 10:34 p.m. UTC

A set-uid executable might be a vector to privilege escalation if the
system configures the coredump file name pattern as a relative
directory destiny. The full description of the vulnerability and
a demonstration of how we can exploit it can be found at [1].

This patch series adds a PF_SUID flag to the process in execve if it is
set-[ug]id binary and elevates the new image's privileges.

In the do_coredump function, we check if:

1) We have the SUID_FLAG set
2) We have CAP_SYS_ADMIN (the process might have decreased its
   privileges)
3) The current directory is owned by root (the current code already
   checks for core_pattern being a relative path).
4) non-privileged users don't have permission to write to the current
   directory.

If all four conditions match, we set the need_suid_safe flag.

An alternative implementation (and more elegant IMO) would be saving
the fsuid and fsgid of the process in the task_struct before loading the
new image to the memory. But this approach would add eight bytes to all
task_struct instances where only a tiny fraction of the processes need
it and under a configuration that not all (most?) distributions don't
adopt by default.

Wander Lairson Costa (4):
  exec: add a flag indicating if an exec file is a suid/sgid
  process: add the PF_SUID flag
  coredump: mitigate privilege escalation of process coredump
  exec: only set the suid flag if the current proc isn't root

 fs/coredump.c           | 15 +++++++++++++++
 fs/exec.c               | 10 ++++++++++
 include/linux/binfmts.h |  6 +++++-
 include/linux/sched.h   |  1 +
 kernel/fork.c           |  2 ++
 5 files changed, 33 insertions(+), 1 deletion(-)