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a1e9718b40Commite09c77d940("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:e09c77d940("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> Link: https://patch.msgid.link/20260529142533.23696-1-zhanwei919@gmail.com Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
3012 lines
82 KiB
3012 lines
82 KiB