bpf: Fix effective prog array index with BPF_F_PREORDER

replace_effective_prog() and purge_effective_progs() located the slot in
the effective array by walking the program hlist and counting entries
linearly. That count does not match the array layout: compute_effective_
progs() places BPF_F_PREORDER programs at the front (ancestor cgroup
first, attach order within a cgroup) and the rest after them (descendant
cgroup first). So when a preorder program is present, the linear hlist
position no longer equals the program's index in the effective array.

For replace_effective_prog() (bpf_link_update()) this overwrote the
wrong slot, corrupting the effective order. For purge_effective_progs(),
it could dummy out a slot belonging to a different program and leave the
detached program in the array while bpf_prog_put() drops its reference,
i.e. a use-after-free.

Fix both by replaying compute_effective_progs()'s placement (including
the per-cgroup preorder reversal) in a shared effective_prog_pos()
helper. Identify the entry by its struct bpf_prog_list pointer rather
than by (prog, link) value, so the lookup resolves to exactly the
attachment the syscall selected even when the same bpf_prog is attached
to several cgroups in the hierarchy.

Fixes: 4b82b181a26c ("bpf: Allow pre-ordering for bpf cgroup progs")
Signed-off-by: Amery Hung <ameryhung@gmail.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20260619063520.2690547-2-ameryhung@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 62 insertions, 46 deletions

diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
index 83ce66296ac1..4355ccb78a9c 100644
--- a/kernel/bpf/cgroup.c
+++ b/kernel/bpf/cgroup.c
@@ -939,19 +939,65 @@ static int cgroup_bpf_attach(struct cgroup *cgrp,
 	return ret;
 }
 
+static int effective_prog_pos(struct cgroup *cgrp,
+			      enum cgroup_bpf_attach_type atype,
+			      struct bpf_prog_list *target_pl)
+{
+	int cnt = 0, preorder_cnt = 0, fstart, bstart, init_bstart, pos = -1;
+	struct bpf_prog_list *pl;
+	struct cgroup *p = cgrp;
+
+	/* count effective programs to find where the preorder region ends */
+	do {
+		if (cnt == 0 || (p->bpf.flags[atype] & BPF_F_ALLOW_MULTI))
+			cnt += prog_list_length(&p->bpf.progs[atype], &preorder_cnt);
+		p = cgroup_parent(p);
+	} while (p);
+
+	/* replay compute_effective_progs() placement and record target's slot */
+	cnt = 0;
+	p = cgrp;
+	fstart = preorder_cnt;
+	bstart = preorder_cnt - 1;
+	do {
+		if (cnt > 0 && !(p->bpf.flags[atype] & BPF_F_ALLOW_MULTI))
+			continue;
+
+		init_bstart = bstart;
+		hlist_for_each_entry(pl, &p->bpf.progs[atype], node) {
+			if (!prog_list_prog(pl))
+				continue;
+
+			if (pl->flags & BPF_F_PREORDER) {
+				if (pl == target_pl)
+					pos = bstart;
+				bstart--;
+			} else {
+				if (pl == target_pl)
+					pos = fstart;
+				fstart++;
+			}
+			cnt++;
+		}
+
+		/* reverse pre-ordering progs at this cgroup level */
+		if (pos >= bstart + 1 && pos <= init_bstart)
+			pos = bstart + 1 + init_bstart - pos;
+	} while ((p = cgroup_parent(p)));
+
+	return pos;
+}
+
 /* Swap updated BPF program for given link in effective program arrays across
  * all descendant cgroups. This function is guaranteed to succeed.
  */
 static void replace_effective_prog(struct cgroup *cgrp,
 				   enum cgroup_bpf_attach_type atype,
-				   struct bpf_cgroup_link *link)
+				   struct bpf_prog_list *pl)
 {
 	struct bpf_prog_array_item *item;
 	struct cgroup_subsys_state *css;
 	struct bpf_prog_array *progs;
-	struct bpf_prog_list *pl;
-	struct hlist_head *head;
-	struct cgroup *cg;
 	int pos;
 
 	css_for_each_descendant_pre(css, &cgrp->self) {
@@ -960,27 +1006,15 @@ static void replace_effective_prog(struct cgroup *cgrp,
 		if (percpu_ref_is_zero(&desc->bpf.refcnt))
 			continue;
 
-		/* find position of link in effective progs array */
-		for (pos = 0, cg = desc; cg; cg = cgroup_parent(cg)) {
-			if (pos && !(cg->bpf.flags[atype] & BPF_F_ALLOW_MULTI))
-				continue;
+		pos = effective_prog_pos(desc, atype, pl);
+		if (WARN_ON_ONCE(pos < 0))
+			continue;
 
-			head = &cg->bpf.progs[atype];
-			hlist_for_each_entry(pl, head, node) {
-				if (!prog_list_prog(pl))
-					continue;
-				if (pl->link == link)
-					goto found;
-				pos++;
-			}
-		}
-found:
-		BUG_ON(!cg);
 		progs = rcu_dereference_protected(
 				desc->bpf.effective[atype],
 				lockdep_is_held(&cgroup_mutex));
 		item = &progs->items[pos];
-		WRITE_ONCE(item->prog, link->link.prog);
+		WRITE_ONCE(item->prog, pl->link->link.prog);
 	}
 }
 
@@ -1024,7 +1058,7 @@ static int __cgroup_bpf_replace(struct cgroup *cgrp,
 
 	cgrp->bpf.revisions[atype] += 1;
 	old_prog = xchg(&link->link.prog, new_prog);
-	replace_effective_prog(cgrp, atype, link);
+	replace_effective_prog(cgrp, atype, pl);
 	bpf_prog_put(old_prog);
 	return 0;
 }
@@ -1091,19 +1125,14 @@ static struct bpf_prog_list *find_detach_entry(struct hlist_head *progs,
  *                           recomputing the array in place.
  *
  * @cgrp: The cgroup which descendants to travers
- * @prog: A program to detach or NULL
- * @link: A link to detach or NULL
+ * @pl: The prog_list entry being detached
  * @atype: Type of detach operation
  */
-static void purge_effective_progs(struct cgroup *cgrp, struct bpf_prog *prog,
-				  struct bpf_cgroup_link *link,
+static void purge_effective_progs(struct cgroup *cgrp, struct bpf_prog_list *pl,
 				  enum cgroup_bpf_attach_type atype)
 {
 	struct cgroup_subsys_state *css;
 	struct bpf_prog_array *progs;
-	struct bpf_prog_list *pl;
-	struct hlist_head *head;
-	struct cgroup *cg;
 	int pos;
 
 	/* recompute effective prog array in place */
@@ -1113,24 +1142,11 @@ static void purge_effective_progs(struct cgroup *cgrp, struct bpf_prog *prog,
 		if (percpu_ref_is_zero(&desc->bpf.refcnt))
 			continue;
 
-		/* find position of link or prog in effective progs array */
-		for (pos = 0, cg = desc; cg; cg = cgroup_parent(cg)) {
-			if (pos && !(cg->bpf.flags[atype] & BPF_F_ALLOW_MULTI))
-				continue;
-
-			head = &cg->bpf.progs[atype];
-			hlist_for_each_entry(pl, head, node) {
-				if (!prog_list_prog(pl))
-					continue;
-				if (pl->prog == prog && pl->link == link)
-					goto found;
-				pos++;
-			}
-		}
-
+		pos = effective_prog_pos(desc, atype, pl);
 		/* no link or prog match, skip the cgroup of this layer */
-		continue;
-found:
+		if (pos < 0)
+			continue;
+
 		progs = rcu_dereference_protected(
 				desc->bpf.effective[atype],
 				lockdep_is_held(&cgroup_mutex));
@@ -1196,7 +1212,7 @@ static int __cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog,
 		/* if update effective array failed replace the prog with a dummy prog*/
 		pl->prog = old_prog;
 		pl->link = link;
-		purge_effective_progs(cgrp, old_prog, link, atype);
+		purge_effective_progs(cgrp, pl, atype);
 	}
 
 	/* now can actually delete it from this cgroup list */