Commit b5e683d5 authored by Jens Axboe's avatar Jens Axboe
Browse files

eventfd: track eventfd_signal() recursion depth 



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: default avatarJens Axboe <axboe@kernel.dk>
parent d7f62e82

fs/eventfd.c

+15 −0
Original line number Diff line number Diff line
@@ -24,6 +24,8 @@ 
#include <linux/seq_file.h>

#include <linux/idr.h>



DEFINE_PER_CPU(int, eventfd_wake_count);



static DEFINE_IDA(eventfd_ida);



struct eventfd_ctx {
@@ -60,12 +62,25 @@ __u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n) 
{

	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;

include/linux/eventfd.h

+14 −0
Original line number Diff line number Diff line
@@ -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 *ctx, __u64 n); 
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_wait_queue(struct eventfd_ctx *ctx, 
	return -ENOSYS;

}



static inline bool eventfd_signal_count(void)

{

	return false;

}



#endif



#endif /* _LINUX_EVENTFD_H */

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