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 a non-NULL sentinel (EP_TFILE_LIST_END) 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 Signed-off-by: Deepanshu Kartikey --- fs/eventpoll.c | 18 ++++++++++++++---- 1 file changed, 14 insertions(+), 4 deletions(-) diff --git a/fs/eventpoll.c b/fs/eventpoll.c index a569e98d4a99..349d79e8ddc8 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -222,6 +222,15 @@ #define EP_UNACTIVE_PTR ((void *) -1L) +/* Non-NULL sentinel terminating ctx->tfile_check_list, so that + * "head->next == NULL" unambiguously means "this head is not on any + * check list" -- the invariant ep_remove_file() and list_file() rely + * on. Without this, the tail of the check list aliases the + * "not linked" state and ep_remove_file() may free a head that + * clear_tfile_check_list() still references. + */ +#define EP_TFILE_LIST_END ((struct epitems_head *)EP_UNACTIVE_PTR) + #define EP_ITEM_COST (sizeof(struct epitem) + sizeof(struct eppoll_entry)) /* Wait structure used by the poll hooks */ @@ -472,7 +481,7 @@ 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; + head->next = ctx->tfile_check_list ? : EP_TFILE_LIST_END; ctx->tfile_check_list = head; } } @@ -1685,7 +1694,7 @@ 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_TFILE_LIST_END; p = p->next) { int error; path_count_init(ctx); rcu_read_lock(); @@ -2438,11 +2447,12 @@ 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_TFILE_LIST_END) { struct epitems_head *head = ctx->tfile_check_list; ctx->tfile_check_list = head->next; unlist_file(head); } + ctx->tfile_check_list = NULL; rcu_read_unlock(); } @@ -2601,7 +2611,7 @@ 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_TFILE_LIST_END }; /* The target file descriptor must support poll */ if (!file_can_poll(tf->file)) -- 2.43.0