syzbot reports a slab-use-after-free read in clear_tfile_check_list() during EPOLL_CTL_ADD when ep_insert() takes an error path that calls ep_remove() before do_epoll_ctl_file() drains ctx->tfile_check_list. ep_remove_file() decides whether to kmem_cache_free() a struct epitems_head by testing v->next == NULL, on the convention that NULL means "this head is not linked on any check list". list_file() pushes a head onto ctx->tfile_check_list by storing the previous list head into head->next; when the list is empty that store is NULL, so the head that ends up at the tail of the check list also has next == NULL. ep_remove_file() then misreads the tail as "not linked" and frees it. clear_tfile_check_list() later walks the list and dereferences head->next on the freed object: BUG: KASAN: slab-use-after-free in clear_tfile_check_list+0x114/0x380 fs/eventpoll.c:2443 Allocated by task 5985: ep_attach_file fs/eventpoll.c:1751 [inline] ep_register_epitem fs/eventpoll.c:1833 [inline] ep_insert+0x512/0x1820 fs/eventpoll.c:1876 Freed by task 5985: ep_remove+0x155/0x2a0 fs/eventpoll.c:1135 ep_insert+0x1372/0x1820 Terminate ctx->tfile_check_list with EP_UNACTIVE_PTR (the existing non-NULL sentinel used elsewhere in this file for inactive list slots) so that next == NULL unambiguously means "not on any check list". list_file() stores the sentinel when the list is empty, reverse_path_check() and clear_tfile_check_list() stop the walk on the sentinel, and do_epoll_ctl_file() seeds ctx->tfile_check_list with it. The guard in ep_remove_file() then correctly refuses to free a head while it remains on the check list. Reported-by: syzbot+69a3d7738ad3aa175caf@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=69a3d7738ad3aa175caf Link: https://lore.kernel.org/all/20260523091107.61880-1-kartikey406@gmail.com/T/ [v1] Link: https://lore.kernel.org/all/20260529042507.87593-1-kartikey406@gmail.com/T/ [v2] Signed-off-by: Deepanshu Kartikey --- v3: Drop the (struct epitems_head *) casts on EP_UNACTIVE_PTR. void * converts implicitly to typed pointers in C; the casts were noise. v2: Reuse the existing EP_UNACTIVE_PTR sentinel instead of adding a new EP_TFILE_LIST_END define, per review feedback. --- fs/eventpoll.c | 15 +++++++++++---- 1 file changed, 11 insertions(+), 4 deletions(-) diff --git a/fs/eventpoll.c b/fs/eventpoll.c index a569e98d4a99..1756c5f489a0 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -472,7 +472,13 @@ static void list_file(struct file *file, struct ep_ctl_ctx *ctx) head = container_of(file->f_ep, struct epitems_head, epitems); if (!head->next) { - head->next = ctx->tfile_check_list; + /* + * Terminate the check list with EP_UNACTIVE_PTR (non-NULL) + * so that head->next == NULL unambiguously means "not on + * any check list". ep_remove_file() relies on that + * invariant to decide whether the head is safe to free. + */ + head->next = ctx->tfile_check_list ? : EP_UNACTIVE_PTR; ctx->tfile_check_list = head; } } @@ -1685,7 +1691,7 @@ 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(); @@ -2438,11 +2444,12 @@ 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); } + ctx->tfile_check_list = EP_UNACTIVE_PTR; rcu_read_unlock(); } @@ -2601,7 +2608,7 @@ 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