hwpmc: Fix the execve handler

When a process execve()s, pmc_process_exec() is supposed to evaluate
whether the new image is setuid/setgid and if so, whether to detach
PMCs.  This was handled by pmc_can_attach(), which is effectively an
open-coded copy of cr_xids_subset().

Unfortunately, the test of the result of this function was inverted,
with the result that we'd detach PMCs only if the predicate said it was
okay to do so.  It appears the bug has always been there; it seems the
intent was to return 0 on "success", i.e., it is okay to attach the
PMCs, much like p_candebug().  Commits 1c3c698ba4c4 and 1c40b15971f0
obscured this a bit.

I think this check is trying to be too clever.  Let's make it simpler:
simply do not attach PMCs unless the owner is privileged.  This is how,
e.g., ktrace works.  I do not think it's worth trying to be more
sophisticated than this unless we can generalize the policy in a way
that's applicable to other subsystems.

Also fix a bug at the end of pmc_process_exec():
pmc_detach_one_process() will call pmc_remove_process_descriptor() for
us.

Approved by:	so
Security:	FreeBSD-SA-26:56.hwpmc
Security:	CVE-2026-58089
Reported by:	netchild
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D59102

2 files changed, 13 insertions, 61 deletions

diff --git a/sys/dev/hwpmc/hwpmc_mod.c b/sys/dev/hwpmc/hwpmc_mod.c
index 70c974857f5c..98ef14885c10 100644
--- a/sys/dev/hwpmc/hwpmc_mod.c
+++ b/sys/dev/hwpmc/hwpmc_mod.c
@@ -210,7 +210,6 @@ static int	pmc_attach_one_process(struct proc *p, struct pmc *pm);
 static bool	pmc_can_allocate_row(int ri, enum pmc_mode mode);
 static bool	pmc_can_allocate_rowindex(struct proc *p, unsigned int ri,
     int cpu);
-static bool	pmc_can_attach(struct pmc *pm, struct proc *p);
 static void	pmc_capture_user_callchain(int cpu, int soft,
     struct trapframe *tf);
 static void	pmc_cleanup(void);
@@ -1026,60 +1025,6 @@ pmc_unlink_target_process(struct pmc *pm, struct pmc_process *pp)
 }
 
 /*
- * Check if PMC 'pm' may be attached to target process 't'.
- */
-
-static bool
-pmc_can_attach(struct pmc *pm, struct proc *t)
-{
-	struct proc *o;		/* pmc owner */
-	struct ucred *oc, *tc;	/* owner, target credentials */
-	bool decline_attach;
-
-	/*
-	 * A PMC's owner can always attach that PMC to itself.
-	 */
-
-	if ((o = pm->pm_owner->po_owner) == t)
-		return (true);
-
-	PROC_LOCK(o);
-	oc = o->p_ucred;
-	crhold(oc);
-	PROC_UNLOCK(o);
-
-	PROC_LOCK(t);
-	tc = t->p_ucred;
-	crhold(tc);
-	PROC_UNLOCK(t);
-
-	/*
-	 * The effective uid of the PMC owner should match at least one
-	 * of the {effective,real,saved} uids of the target process.
-	 */
-
-	decline_attach = oc->cr_uid != tc->cr_uid &&
-	    oc->cr_uid != tc->cr_svuid &&
-	    oc->cr_uid != tc->cr_ruid;
-
-	/*
-	 * Every one of the target's group ids, must be in the owner's
-	 * group list.
-	 */
-	for (int i = 0; !decline_attach && i < tc->cr_ngroups; i++)
-		decline_attach = !groupmember(tc->cr_groups[i], oc);
-	if (!decline_attach)
-		decline_attach = !groupmember(tc->cr_gid, oc) ||
-		    !groupmember(tc->cr_rgid, oc) ||
-		    !groupmember(tc->cr_svgid, oc);
-
-	crfree(tc);
-	crfree(oc);
-
-	return (!decline_attach);
-}
-
-/*
  * Attach a process to a PMC.
  */
 static int
@@ -1442,10 +1387,19 @@ pmc_process_exec(struct thread *td, struct pmckern_procexec *pk)
 	 */
 	for (ri = 0; ri < md->pmd_npmc; ri++) {
 		if ((pm = pp->pp_pmcs[ri].pp_pmc) != NULL) {
-			if (pmc_can_attach(pm, td->td_proc)) {
+			struct proc *owner;
+			struct ucred *cred;
+
+			owner = pm->pm_owner->po_owner;
+			PROC_LOCK(owner);
+			cred = crhold(owner->p_ucred);
+			PROC_UNLOCK(owner);
+
+			if (priv_check_cred(cred, PRIV_DEBUG_DIFFCRED) != 0)
 				pmc_detach_one_process(td->td_proc, pm,
 				    PMC_FLAG_NONE);
-			}
+
+			crfree(cred);
 		}
 	}
 
@@ -1458,10 +1412,8 @@ pmc_process_exec(struct thread *td, struct pmckern_procexec *pk)
 	 * PMCs, we can remove the process entry and free
 	 * up space.
 	 */
-	if (pp->pp_refcnt == 0) {
-		pmc_remove_process_descriptor(pp);
+	if (pp->pp_refcnt == 0)
 		pmc_destroy_process_descriptor(pp);
-	}
 }
 
 /*
diff --git a/sys/kern/kern_exec.c b/sys/kern/kern_exec.c
index 5f7adae8df03..a109871d7673 100644
--- a/sys/kern/kern_exec.c
+++ b/sys/kern/kern_exec.c
@@ -1037,7 +1037,7 @@ interpret:
 	 */
 	if (PMC_SYSTEM_SAMPLING_ACTIVE() || PMC_PROC_IS_USING_PMCS(p)) {
 		VOP_UNLOCK(imgp->vp);
-		pe.pm_credentialschanged = credential_changing;
+		pe.pm_credentialschanged = imgp->credential_setid;
 		pe.pm_baseaddr = imgp->reloc_base;
 		pe.pm_dynaddr = imgp->et_dyn_addr;