exec: Correct the permission check for unsafe exec

Max Kellerman recently experienced a problem[1] when calling exec with
differing uid and euid's and he triggered the logic that is supposed
to only handle setuid executables.

When exec isn't changing anything in struct cred it doesn't make sense
to go into the code that is there to handle the case when the
credentials change.

When looking into the history of the code I discovered that this issue
was not present in Linux-2.4.0-test12 and was introduced in
Linux-2.4.0-prerelease when the logic for handling this case was moved
from prepare_binprm to compute_creds in fs/exec.c.

The bug introdused was to comparing euid in the new credentials with
uid instead of euid in the old credentials, when testing if setuid
had changed the euid.

Since triggering the keep ptrace limping along case for setuid
executables makes no sense when it was not a setuid exec revert back
to the logic present in Linux-2.4.0-test12.

This removes the confusingly named and subtlety incorrect helpers
is_setuid and is_setgid, that helped this bug to persist.

The varaiable is_setid is renamed to id_changed (it's Linux-2.4.0-test12)
as the old name describes what matters rather than it's cause.

The code removed in Linux-2.4.0-prerelease was:
- /* Set-uid? */
- if (mode & S_ISUID) {
- bprm->e_uid = inode->i_uid;
- if (bprm->e_uid != current->euid)
- id_change = 1;
- }
-
- /* Set-gid? */
- /*
- * If setgid is set but no group execute bit then this
- * is a candidate for mandatory locking, not a setgid
- * executable.
- */
- if ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP)) {
- bprm->e_gid = inode->i_gid;
- if (!in_group_p(bprm->e_gid))
- id_change = 1;

Linux-2.4.0-prerelease added the current logic as:
+ if (bprm->e_uid != current->uid || bprm->e_gid != current->gid ||
+ !cap_issubset(new_permitted, current->cap_permitted)) {
+ current->dumpable = 0;
+
+ lock_kernel();
+ if (must_not_trace_exec(current)
+ || atomic_read(&current->fs->count) > 1
+ || atomic_read(&current->files->count) > 1
+ || atomic_read(&current->sig->count) > 1) {
+ if(!capable(CAP_SETUID)) {
+ bprm->e_uid = current->uid;
+ bprm->e_gid = current->gid;
+ }
+ if(!capable(CAP_SETPCAP)) {
+ new_permitted = cap_intersect(new_permitted,
+ current->cap_permitted);
+ }
+ }
+ do_unlock = 1;
+ }

I have condenced the logic from Linux-2.4.0-test12 to just:
id_changed = !uid_eq(new->euid, old->euid) || !in_group_p(new->egid);

This change is userspace visible, but I don't expect anyone to care.

For the bug that is being fixed to trigger bprm->unsafe has to be set.
The variable bprm->unsafe is set when ptracing an executable, when
sharing a working directory, or when no_new_privs is set. Properly
testing for cases that are safe even in those conditions and doing
nothing special should not affect anyone. Especially if they were
previously ok with their credentials getting munged

To minimize behavioural changes the code continues to set secureexec
when euid != uid or when egid != gid.

[1] https://lkml.kernel.org/r/20250306082615.174777-1-max.kellermann@ionos.com

Reported-by: Max Kellermann <max.kellermann@ionos.com>
Fixes: 64444d3d0d7f ("Linux version 2.4.0-prerelease")
v1: https://lkml.kernel.org/r/878qmxsuy8.fsf@email.froward.int.ebiederm.org
Reviewed-by: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Reviewed-by: Jann Horn <jannh@google.com>
Acked-by: Kees Cook <kees@kernel.org>