[PATCH] eventpoll: restore EP_UNACTIVE_PTR sentinel for ctx->tfile_check_list

[Zhan Wei <zhanwei919@gmail.com>]

Commit e09c77d94003 ("eventpoll: hoist CTL_ADD scratch state into
struct ep_ctl_ctx") moved tfile_check_list from a file-scope global into
the stack-allocated struct ep_ctl_ctx, and in doing so replaced the
EP_UNACTIVE_PTR sentinel with NULL on the grounds that "NULL is the
obvious 'empty' value and zero-init handles it for free", describing the
change as "No functional change". It is not.

epitems_head->next is overloaded with two roles:

  1. the "next" pointer that threads a head onto ctx->tfile_check_list;
  2. a membership flag: ep_remove_file() uses
     !smp_load_acquire(&v->next) to mean "this head is not on any
     pending ctx->tfile_check_list and is therefore safe to free".

Before that change the EP_UNACTIVE_PTR sentinel kept the two roles
disjoint: a head on the list always had a non-NULL ->next (another head,
or the sentinel at the tail), so ->next == NULL was equivalent to "never
listed". With the sentinel gone the list is NULL-terminated, so the tail
head's ->next is NULL as well. ep_remove_file()'s gate can no longer
distinguish "never listed" from "listed at the tail", and misfires on
the tail head.

The reader (reverse_path_check_proc) holds epnested_mutex +
rcu_read_lock; the freer (ep_remove_file) holds ep->mtx + file->f_lock.
The two sides share no mutex -- the sentinel was the invariant the gate
relied on to know it could skip the read side. With it gone,
ep_remove_file() frees the tail head while reverse_path_check_proc() is
still walking it, producing the slab-use-after-free read. The syzbot
reproducer hits this within seconds on a multi-CPU VM.

Restore the sentinel: initialize ctx.tfile_check_list to EP_UNACTIVE_PTR
in do_epoll_ctl_file(), and terminate the walk on "!= EP_UNACTIVE_PTR"
in reverse_path_check() and clear_tfile_check_list(). The tail head's
->next becomes the sentinel again rather than NULL, so
ep_remove_file()'s gate regains its exclusivity and stops misfiring on
the tail. ep_remove_file() itself is unchanged.

This restores the invariant the file-scope tfile_check_list relied on
before that change while preserving the ctx packaging it introduced.

Reported-by: syzbot+e70e1b6cba8714543f7c@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=e70e1b6cba8714543f7c
Fixes: e09c77d94003 ("eventpoll: hoist CTL_ADD scratch state into struct ep_ctl_ctx")
Suggested-by: Christian Brauner <brauner@kernel.org>
Link: https://lore.kernel.org/all/20260528-rotwild-summt-kuhhandel-7276ef4c33b7@brauner.io/
Signed-off-by: Zhan Wei <zhanwei919@gmail.com>
---
 fs/eventpoll.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index a569e98d4a99..abef3bc48cc4 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -429,7 +429,11 @@ struct ep_ctl_ctx {
 	/*
 	 * Singly-linked list of epitems_head objects collected during
 	 * ep_loop_check_proc(), then walked by reverse_path_check().
-	 * NULL means empty.
+	 * Terminated by EP_UNACTIVE_PTR, not NULL: epitems_head->next
+	 * doubles as a membership flag (a NULL ->next means "not on this
+	 * list", see ep_remove_file()), so the list uses a non-NULL
+	 * sentinel to keep the tail head distinguishable from an unlisted
+	 * one.
 	 */
 	struct epitems_head *tfile_check_list;
 
@@ -1685,7 +1689,7 @@ static int reverse_path_check(struct ep_ctl_ctx *ctx)
 {
 	struct epitems_head *p;
 
-	for (p = ctx->tfile_check_list; p; p = p->next) {
+	for (p = ctx->tfile_check_list; p != EP_UNACTIVE_PTR; p = p->next) {
 		int error;
 		path_count_init(ctx);
 		rcu_read_lock();
@@ -2438,7 +2442,7 @@ static int ep_loop_check(struct ep_ctl_ctx *ctx, struct eventpoll *ep,
 static void clear_tfile_check_list(struct ep_ctl_ctx *ctx)
 {
 	rcu_read_lock();
-	while (ctx->tfile_check_list) {
+	while (ctx->tfile_check_list != EP_UNACTIVE_PTR) {
 		struct epitems_head *head = ctx->tfile_check_list;
 		ctx->tfile_check_list = head->next;
 		unlist_file(head);
@@ -2601,7 +2605,9 @@ int do_epoll_ctl_file(struct file *f, int op, struct epoll_key *tf,
 	int full_check;
 	struct eventpoll *ep;
 	struct epitem *epi;
-	struct ep_ctl_ctx ctx = { };
+	struct ep_ctl_ctx ctx = {
+		.tfile_check_list = EP_UNACTIVE_PTR,
+	};
 
 	/* The target file descriptor must support poll */
 	if (!file_can_poll(tf->file))
-- 
2.43.0