| From b5e683d5cab8cd433b06ae178621f083cabd4f63 Mon Sep 17 00:00:00 2001 |
| From: Jens Axboe <axboe@kernel.dk> |
| Date: Sun, 2 Feb 2020 08:23:03 -0700 |
| Subject: eventfd: track eventfd_signal() recursion depth |
| |
| From: Jens Axboe <axboe@kernel.dk> |
| |
| commit b5e683d5cab8cd433b06ae178621f083cabd4f63 upstream. |
| |
| eventfd use cases from aio and io_uring can deadlock due to circular |
| or resursive calling, when eventfd_signal() tries to grab the waitqueue |
| lock. On top of that, it's also possible to construct notification |
| chains that are deep enough that we could blow the stack. |
| |
| Add a percpu counter that tracks the percpu recursion depth, warn if we |
| exceed it. The counter is also exposed so that users of eventfd_signal() |
| can do the right thing if it's non-zero in the context where it is |
| called. |
| |
| Cc: stable@vger.kernel.org # 4.19+ |
| Signed-off-by: Jens Axboe <axboe@kernel.dk> |
| Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> |
| |
| --- |
| fs/eventfd.c | 15 +++++++++++++++ |
| include/linux/eventfd.h | 14 ++++++++++++++ |
| 2 files changed, 29 insertions(+) |
| |
| --- a/fs/eventfd.c |
| +++ b/fs/eventfd.c |
| @@ -22,6 +22,8 @@ |
| #include <linux/proc_fs.h> |
| #include <linux/seq_file.h> |
| |
| +DEFINE_PER_CPU(int, eventfd_wake_count); |
| + |
| struct eventfd_ctx { |
| struct kref kref; |
| wait_queue_head_t wqh; |
| @@ -55,12 +57,25 @@ __u64 eventfd_signal(struct eventfd_ctx |
| { |
| unsigned long flags; |
| |
| + /* |
| + * Deadlock or stack overflow issues can happen if we recurse here |
| + * through waitqueue wakeup handlers. If the caller users potentially |
| + * nested waitqueues with custom wakeup handlers, then it should |
| + * check eventfd_signal_count() before calling this function. If |
| + * it returns true, the eventfd_signal() call should be deferred to a |
| + * safe context. |
| + */ |
| + if (WARN_ON_ONCE(this_cpu_read(eventfd_wake_count))) |
| + return 0; |
| + |
| spin_lock_irqsave(&ctx->wqh.lock, flags); |
| + this_cpu_inc(eventfd_wake_count); |
| if (ULLONG_MAX - ctx->count < n) |
| n = ULLONG_MAX - ctx->count; |
| ctx->count += n; |
| if (waitqueue_active(&ctx->wqh)) |
| wake_up_locked_poll(&ctx->wqh, EPOLLIN); |
| + this_cpu_dec(eventfd_wake_count); |
| spin_unlock_irqrestore(&ctx->wqh.lock, flags); |
| |
| return n; |
| --- a/include/linux/eventfd.h |
| +++ b/include/linux/eventfd.h |
| @@ -12,6 +12,8 @@ |
| #include <linux/fcntl.h> |
| #include <linux/wait.h> |
| #include <linux/err.h> |
| +#include <linux/percpu-defs.h> |
| +#include <linux/percpu.h> |
| |
| /* |
| * CAREFUL: Check include/uapi/asm-generic/fcntl.h when defining |
| @@ -40,6 +42,13 @@ __u64 eventfd_signal(struct eventfd_ctx |
| int eventfd_ctx_remove_wait_queue(struct eventfd_ctx *ctx, wait_queue_entry_t *wait, |
| __u64 *cnt); |
| |
| +DECLARE_PER_CPU(int, eventfd_wake_count); |
| + |
| +static inline bool eventfd_signal_count(void) |
| +{ |
| + return this_cpu_read(eventfd_wake_count); |
| +} |
| + |
| #else /* CONFIG_EVENTFD */ |
| |
| /* |
| @@ -68,6 +77,11 @@ static inline int eventfd_ctx_remove_wai |
| return -ENOSYS; |
| } |
| |
| +static inline bool eventfd_signal_count(void) |
| +{ |
| + return false; |
| +} |
| + |
| #endif |
| |
| #endif /* _LINUX_EVENTFD_H */ |