From: Jann Horn <jannh@google.com> mainline inclusion from mainline-v6.16-rc1 commit f2e467a48287c868818085aa35389a224d226732 category: bugfix bugzilla: https://gitee.com/src-openeuler/kernel/issues/ICU73P CVE: CVE-2025-38614 Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?i... -------------------------------- Ensure that epoll instances can never form a graph deeper than EP_MAX_NESTS+1 links. Currently, ep_loop_check_proc() ensures that the graph is loop-free and does some recursion depth checks, but those recursion depth checks don't limit the depth of the resulting tree for two reasons: - They don't look upwards in the tree. - If there are multiple downwards paths of different lengths, only one of the paths is actually considered for the depth check since commit 28d82dc1c4ed ("epoll: limit paths"). Essentially, the current recursion depth check in ep_loop_check_proc() just serves to prevent it from recursing too deeply while checking for loops. A more thorough check is done in reverse_path_check() after the new graph edge has already been created; this checks, among other things, that no paths going upwards from any non-epoll file with a length of more than 5 edges exist. However, this check does not apply to non-epoll files. As a result, it is possible to recurse to a depth of at least roughly 500, tested on v6.15. (I am unsure if deeper recursion is possible; and this may have changed with commit 8c44dac8add7 ("eventpoll: Fix priority inversion problem").) To fix it: 1. In ep_loop_check_proc(), note the subtree depth of each visited node, and use subtree depths for the total depth calculation even when a subtree has already been visited. 2. Add ep_get_upwards_depth_proc() for similarly determining the maximum depth of an upwards walk. 3. In ep_loop_check(), use these values to limit the total path length between epoll nodes to EP_MAX_NESTS edges. Fixes: 22bacca48a17 ("epoll: prevent creating circular epoll structures") Cc: stable@vger.kernel.org Signed-off-by: Jann Horn <jannh@google.com> Link: https://lore.kernel.org/20250711-epoll-recursion-fix-v1-1-fb2457c33292@googl... Signed-off-by: Christian Brauner <brauner@kernel.org> Signed-off-by: Long Li <leo.lilong@huawei.com> --- fs/eventpoll.c | 60 ++++++++++++++++++++++++++++++++++++++------------ 1 file changed, 46 insertions(+), 14 deletions(-) diff --git a/fs/eventpoll.c b/fs/eventpoll.c index 31b32d9e7bbc..6b2d655c1cef 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -217,6 +217,7 @@ struct eventpoll { /* used to optimize loop detection check */ u64 gen; struct hlist_head refs; + u8 loop_check_depth; /* * usage count, used together with epitem->dying to @@ -1986,23 +1987,24 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events, } /** - * ep_loop_check_proc - verify that adding an epoll file inside another - * epoll structure does not violate the constraints, in - * terms of closed loops, or too deep chains (which can - * result in excessive stack usage). + * ep_loop_check_proc - verify that adding an epoll file @ep inside another + * epoll file does not create closed loops, and + * determine the depth of the subtree starting at @ep * * @ep: the &struct eventpoll to be currently checked. * @depth: Current depth of the path being checked. * - * Return: %zero if adding the epoll @file inside current epoll - * structure @ep does not violate the constraints, or %-1 otherwise. + * Return: depth of the subtree, or INT_MAX if we found a loop or went too deep. */ static int ep_loop_check_proc(struct eventpoll *ep, int depth) { - int error = 0; + int result = 0; struct rb_node *rbp; struct epitem *epi; + if (ep->gen == loop_check_gen) + return ep->loop_check_depth; + mutex_lock_nested(&ep->mtx, depth + 1); ep->gen = loop_check_gen; for (rbp = rb_first_cached(&ep->rbr); rbp; rbp = rb_next(rbp)) { @@ -2010,13 +2012,11 @@ static int ep_loop_check_proc(struct eventpoll *ep, int depth) if (unlikely(is_file_epoll(epi->ffd.file))) { struct eventpoll *ep_tovisit; ep_tovisit = epi->ffd.file->private_data; - if (ep_tovisit->gen == loop_check_gen) - continue; if (ep_tovisit == inserting_into || depth > EP_MAX_NESTS) - error = -1; + result = INT_MAX; else - error = ep_loop_check_proc(ep_tovisit, depth + 1); - if (error != 0) + result = max(result, ep_loop_check_proc(ep_tovisit, depth + 1) + 1); + if (result > EP_MAX_NESTS) break; } else { /* @@ -2030,9 +2030,27 @@ static int ep_loop_check_proc(struct eventpoll *ep, int depth) list_file(epi->ffd.file); } } + ep->loop_check_depth = result; mutex_unlock(&ep->mtx); - return error; + return result; +} + +/** + * ep_get_upwards_depth_proc - determine depth of @ep when traversed upwards + */ +static int ep_get_upwards_depth_proc(struct eventpoll *ep, int depth) +{ + int result = 0; + struct epitem *epi; + + if (ep->gen == loop_check_gen) + return ep->loop_check_depth; + hlist_for_each_entry_rcu(epi, &ep->refs, fllink) + result = max(result, ep_get_upwards_depth_proc(epi->ep, depth + 1) + 1); + ep->gen = loop_check_gen; + ep->loop_check_depth = result; + return result; } /** @@ -2048,8 +2066,22 @@ static int ep_loop_check_proc(struct eventpoll *ep, int depth) */ static int ep_loop_check(struct eventpoll *ep, struct eventpoll *to) { + int depth, upwards_depth; + inserting_into = ep; - return ep_loop_check_proc(to, 0); + /* + * Check how deep down we can get from @to, and whether it is possible + * to loop up to @ep. + */ + depth = ep_loop_check_proc(to, 0); + if (depth > EP_MAX_NESTS) + return -1; + /* Check how far up we can go from @ep. */ + rcu_read_lock(); + upwards_depth = ep_get_upwards_depth_proc(ep, 0); + rcu_read_unlock(); + + return (depth+1+upwards_depth > EP_MAX_NESTS) ? -1 : 0; } static void clear_tfile_check_list(void) -- 2.39.2