[PATCH 05/11] exec: In bprm_fill_uid use CAP_SETGID to see if a gid change is safe

From: Eric W. Biederman
Date: Thu May 28 2020 - 11:48:37 EST

If the task has CAP_SETGID and a shared fs struct or is being ptraced
than it is clear that nothing new is being introduced when the gid
changes, and so it is safe to honor a setgid executable.

However if all we know is that the task has CAP_SETUID things are less

This bug looks like it was introduced in v2.1.100 when !suser was
replaced by !capable(CAP_SETUID). It appears to have been an oversight
at that time.

Fixing this 22 years later seems weird but even now it still looks
worth fixing. As conceptually what is happening is testing to see if
the process already had the potential to make a gid change or if the
trancer needs permissions in addition to the permissions needed to
trace the process to trace the process through a gid change.

Fixes: v2.1.100
Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
fs/exec.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/exec.c b/fs/exec.c
index 956ee3a0d824..bac8db14f30d 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1654,7 +1654,7 @@ static void bprm_fill_uid(struct linux_binprm *bprm)
new->euid = new->uid;
if (need_cap && !gid_eq(new->egid, new->gid) &&
- (!ns_capable(new->user_ns, CAP_SETUID) ||
+ (!ns_capable(new->user_ns, CAP_SETGID) ||
(bprm->unsafe & LSM_UNSAFE_NO_NEW_PRIVS))) {
new->egid = new->gid;