[PATCH] security/commoncap: don't assume "setid" if all ids are identical

[Max Kellermann <max.kellermann@ionos.com>]

If a program enables `NO_NEW_PRIVS` and sets up
differing real/effective/saved/fs ids, the effective ids are
downgraded during exec because the kernel believes it should "get no
more than they had, and maybe less".

I believe it is safe to keep differing ids even if `NO_NEW_PRIVS` is
set.  The newly executed program doesn't get any more, but there's no
reason to give it less.

This is different from "set[ug]id/setpcap" execution where privileges
may be raised; here, the assumption that it's "set[ug]id" if
effective!=real is too broad.

If we verify that all user/group ids remain as they were, we can
safely allow the new program to keep them.

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
---
 security/commoncap.c | 23 ++++++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)

diff --git a/security/commoncap.c b/security/commoncap.c
index 58a0c1c3e409..057a7400ef7d 100644
--- a/security/commoncap.c
+++ b/security/commoncap.c
@@ -861,6 +861,26 @@ static inline bool __is_setuid(struct cred *new, const struct cred *old)
 static inline bool __is_setgid(struct cred *new, const struct cred *old)
 { return !gid_eq(new->egid, old->gid); }
 
+/**
+ * Are all user/group ids in both cred instances identical?
+ *
+ * It can be used after __is_setuid() / __is_setgid() to check whether
+ * this is really a set*id operation or whether both processes just
+ * have differing real/effective ids.  It is safe to keep differing
+ * real/effective ids in "unsafe" program execution.
+ */
+static bool has_identical_uids_gids(const struct cred *a, const struct cred *b)
+{
+	return uid_eq(a->uid, b->uid) &&
+		gid_eq(a->gid, b->gid) &&
+		uid_eq(a->suid, b->suid) &&
+		gid_eq(a->sgid, b->sgid) &&
+		uid_eq(a->euid, b->euid) &&
+		gid_eq(a->egid, b->egid) &&
+		uid_eq(a->fsuid, b->fsuid) &&
+		gid_eq(a->fsgid, b->fsgid);
+}
+
 /*
  * 1) Audit candidate if current->cap_effective is set
  *
@@ -940,7 +960,8 @@ int cap_bprm_creds_from_file(struct linux_binprm *bprm, const struct file *file)
 	 *
 	 * In addition, if NO_NEW_PRIVS, then ensure we get no new privs.
 	 */
-	is_setid = __is_setuid(new, old) || __is_setgid(new, old);
+	is_setid = (__is_setuid(new, old) || __is_setgid(new, old)) &&
+		!has_identical_uids_gids(new, old);
 
 	if ((is_setid || __cap_gained(permitted, new, old)) &&
 	    ((bprm->unsafe & ~LSM_UNSAFE_PTRACE) ||
-- 
2.47.2