mirror of
https://mirrors.bfsu.edu.cn/git/linux.git
synced 2024-12-17 07:54:54 +08:00
758b492047
For eventfd with flag EFD_SEMAPHORE, when its ctx->count is 0, calling
eventfd_ctx_do_read will cause ctx->count to overflow to ULLONG_MAX.
An underflow can happen with EFD_SEMAPHORE eventfds in at least the
following three subsystems:
(1) virt/kvm/eventfd.c
(2) drivers/vfio/virqfd.c
(3) drivers/virt/acrn/irqfd.c
where (2) and (3) are just modeled after (1). An eventfd must be
specified for use with the KVM_IRQFD ioctl(). This can also be an
EFD_SEMAPHORE eventfd. When the eventfd count is zero or has been
decremented to zero an underflow can be triggered when the irqfd is shut
down by raising the KVM_IRQFD_FLAG_DEASSIGN flag in the KVM_IRQFD
ioctl():
// ctx->count == 0
kvm_vm_ioctl()
-> kvm_irqfd()
-> kvm_irqfd_deassign()
-> irqfd_deactivate()
-> irqfd_shutdown()
-> eventfd_ctx_remove_wait_queue(&cnt)
-> eventfd_ctx_do_read(&cnt)
Userspace polling on the eventfd wouldn't notice the underflow because 1
is always returned as the value from eventfd_read() while ctx->count
would've underflowed. It's not a huge deal because this should only be
happening when the irqfd is shutdown but we should still fix it and
avoid the spurious wakeup.
Fixes: cb289d6244
("eventfd - allow atomic read and waitqueue remove")
Signed-off-by: Wen Yang <wenyang.linux@foxmail.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Dylan Yudaken <dylany@fb.com>
Cc: David Woodhouse <dwmw@amazon.co.uk>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Message-Id: <tencent_7588DFD1F365950A757310D764517A14B306@qq.com>
[brauner: rewrite commit message and add explanation how this underflow can happen]
Signed-off-by: Christian Brauner <brauner@kernel.org>
443 lines
12 KiB
C
443 lines
12 KiB
C
// SPDX-License-Identifier: GPL-2.0-only
|
|
/*
|
|
* fs/eventfd.c
|
|
*
|
|
* Copyright (C) 2007 Davide Libenzi <davidel@xmailserver.org>
|
|
*
|
|
*/
|
|
|
|
#include <linux/file.h>
|
|
#include <linux/poll.h>
|
|
#include <linux/init.h>
|
|
#include <linux/fs.h>
|
|
#include <linux/sched/signal.h>
|
|
#include <linux/kernel.h>
|
|
#include <linux/slab.h>
|
|
#include <linux/list.h>
|
|
#include <linux/spinlock.h>
|
|
#include <linux/anon_inodes.h>
|
|
#include <linux/syscalls.h>
|
|
#include <linux/export.h>
|
|
#include <linux/kref.h>
|
|
#include <linux/eventfd.h>
|
|
#include <linux/proc_fs.h>
|
|
#include <linux/seq_file.h>
|
|
#include <linux/idr.h>
|
|
#include <linux/uio.h>
|
|
|
|
static DEFINE_IDA(eventfd_ida);
|
|
|
|
struct eventfd_ctx {
|
|
struct kref kref;
|
|
wait_queue_head_t wqh;
|
|
/*
|
|
* Every time that a write(2) is performed on an eventfd, the
|
|
* value of the __u64 being written is added to "count" and a
|
|
* wakeup is performed on "wqh". If EFD_SEMAPHORE flag was not
|
|
* specified, a read(2) will return the "count" value to userspace,
|
|
* and will reset "count" to zero. The kernel side eventfd_signal()
|
|
* also, adds to the "count" counter and issue a wakeup.
|
|
*/
|
|
__u64 count;
|
|
unsigned int flags;
|
|
int id;
|
|
};
|
|
|
|
__u64 eventfd_signal_mask(struct eventfd_ctx *ctx, __u64 n, __poll_t mask)
|
|
{
|
|
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_allowed() before calling this function. If
|
|
* it returns false, the eventfd_signal() call should be deferred to a
|
|
* safe context.
|
|
*/
|
|
if (WARN_ON_ONCE(current->in_eventfd))
|
|
return 0;
|
|
|
|
spin_lock_irqsave(&ctx->wqh.lock, flags);
|
|
current->in_eventfd = 1;
|
|
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 | mask);
|
|
current->in_eventfd = 0;
|
|
spin_unlock_irqrestore(&ctx->wqh.lock, flags);
|
|
|
|
return n;
|
|
}
|
|
|
|
/**
|
|
* eventfd_signal - Adds @n to the eventfd counter.
|
|
* @ctx: [in] Pointer to the eventfd context.
|
|
* @n: [in] Value of the counter to be added to the eventfd internal counter.
|
|
* The value cannot be negative.
|
|
*
|
|
* This function is supposed to be called by the kernel in paths that do not
|
|
* allow sleeping. In this function we allow the counter to reach the ULLONG_MAX
|
|
* value, and we signal this as overflow condition by returning a EPOLLERR
|
|
* to poll(2).
|
|
*
|
|
* Returns the amount by which the counter was incremented. This will be less
|
|
* than @n if the counter has overflowed.
|
|
*/
|
|
__u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n)
|
|
{
|
|
return eventfd_signal_mask(ctx, n, 0);
|
|
}
|
|
EXPORT_SYMBOL_GPL(eventfd_signal);
|
|
|
|
static void eventfd_free_ctx(struct eventfd_ctx *ctx)
|
|
{
|
|
if (ctx->id >= 0)
|
|
ida_simple_remove(&eventfd_ida, ctx->id);
|
|
kfree(ctx);
|
|
}
|
|
|
|
static void eventfd_free(struct kref *kref)
|
|
{
|
|
struct eventfd_ctx *ctx = container_of(kref, struct eventfd_ctx, kref);
|
|
|
|
eventfd_free_ctx(ctx);
|
|
}
|
|
|
|
/**
|
|
* eventfd_ctx_put - Releases a reference to the internal eventfd context.
|
|
* @ctx: [in] Pointer to eventfd context.
|
|
*
|
|
* The eventfd context reference must have been previously acquired either
|
|
* with eventfd_ctx_fdget() or eventfd_ctx_fileget().
|
|
*/
|
|
void eventfd_ctx_put(struct eventfd_ctx *ctx)
|
|
{
|
|
kref_put(&ctx->kref, eventfd_free);
|
|
}
|
|
EXPORT_SYMBOL_GPL(eventfd_ctx_put);
|
|
|
|
static int eventfd_release(struct inode *inode, struct file *file)
|
|
{
|
|
struct eventfd_ctx *ctx = file->private_data;
|
|
|
|
wake_up_poll(&ctx->wqh, EPOLLHUP);
|
|
eventfd_ctx_put(ctx);
|
|
return 0;
|
|
}
|
|
|
|
static __poll_t eventfd_poll(struct file *file, poll_table *wait)
|
|
{
|
|
struct eventfd_ctx *ctx = file->private_data;
|
|
__poll_t events = 0;
|
|
u64 count;
|
|
|
|
poll_wait(file, &ctx->wqh, wait);
|
|
|
|
/*
|
|
* All writes to ctx->count occur within ctx->wqh.lock. This read
|
|
* can be done outside ctx->wqh.lock because we know that poll_wait
|
|
* takes that lock (through add_wait_queue) if our caller will sleep.
|
|
*
|
|
* The read _can_ therefore seep into add_wait_queue's critical
|
|
* section, but cannot move above it! add_wait_queue's spin_lock acts
|
|
* as an acquire barrier and ensures that the read be ordered properly
|
|
* against the writes. The following CAN happen and is safe:
|
|
*
|
|
* poll write
|
|
* ----------------- ------------
|
|
* lock ctx->wqh.lock (in poll_wait)
|
|
* count = ctx->count
|
|
* __add_wait_queue
|
|
* unlock ctx->wqh.lock
|
|
* lock ctx->qwh.lock
|
|
* ctx->count += n
|
|
* if (waitqueue_active)
|
|
* wake_up_locked_poll
|
|
* unlock ctx->qwh.lock
|
|
* eventfd_poll returns 0
|
|
*
|
|
* but the following, which would miss a wakeup, cannot happen:
|
|
*
|
|
* poll write
|
|
* ----------------- ------------
|
|
* count = ctx->count (INVALID!)
|
|
* lock ctx->qwh.lock
|
|
* ctx->count += n
|
|
* **waitqueue_active is false**
|
|
* **no wake_up_locked_poll!**
|
|
* unlock ctx->qwh.lock
|
|
* lock ctx->wqh.lock (in poll_wait)
|
|
* __add_wait_queue
|
|
* unlock ctx->wqh.lock
|
|
* eventfd_poll returns 0
|
|
*/
|
|
count = READ_ONCE(ctx->count);
|
|
|
|
if (count > 0)
|
|
events |= EPOLLIN;
|
|
if (count == ULLONG_MAX)
|
|
events |= EPOLLERR;
|
|
if (ULLONG_MAX - 1 > count)
|
|
events |= EPOLLOUT;
|
|
|
|
return events;
|
|
}
|
|
|
|
void eventfd_ctx_do_read(struct eventfd_ctx *ctx, __u64 *cnt)
|
|
{
|
|
lockdep_assert_held(&ctx->wqh.lock);
|
|
|
|
*cnt = ((ctx->flags & EFD_SEMAPHORE) && ctx->count) ? 1 : ctx->count;
|
|
ctx->count -= *cnt;
|
|
}
|
|
EXPORT_SYMBOL_GPL(eventfd_ctx_do_read);
|
|
|
|
/**
|
|
* eventfd_ctx_remove_wait_queue - Read the current counter and removes wait queue.
|
|
* @ctx: [in] Pointer to eventfd context.
|
|
* @wait: [in] Wait queue to be removed.
|
|
* @cnt: [out] Pointer to the 64-bit counter value.
|
|
*
|
|
* Returns %0 if successful, or the following error codes:
|
|
*
|
|
* -EAGAIN : The operation would have blocked.
|
|
*
|
|
* This is used to atomically remove a wait queue entry from the eventfd wait
|
|
* queue head, and read/reset the counter value.
|
|
*/
|
|
int eventfd_ctx_remove_wait_queue(struct eventfd_ctx *ctx, wait_queue_entry_t *wait,
|
|
__u64 *cnt)
|
|
{
|
|
unsigned long flags;
|
|
|
|
spin_lock_irqsave(&ctx->wqh.lock, flags);
|
|
eventfd_ctx_do_read(ctx, cnt);
|
|
__remove_wait_queue(&ctx->wqh, wait);
|
|
if (*cnt != 0 && waitqueue_active(&ctx->wqh))
|
|
wake_up_locked_poll(&ctx->wqh, EPOLLOUT);
|
|
spin_unlock_irqrestore(&ctx->wqh.lock, flags);
|
|
|
|
return *cnt != 0 ? 0 : -EAGAIN;
|
|
}
|
|
EXPORT_SYMBOL_GPL(eventfd_ctx_remove_wait_queue);
|
|
|
|
static ssize_t eventfd_read(struct kiocb *iocb, struct iov_iter *to)
|
|
{
|
|
struct file *file = iocb->ki_filp;
|
|
struct eventfd_ctx *ctx = file->private_data;
|
|
__u64 ucnt = 0;
|
|
|
|
if (iov_iter_count(to) < sizeof(ucnt))
|
|
return -EINVAL;
|
|
spin_lock_irq(&ctx->wqh.lock);
|
|
if (!ctx->count) {
|
|
if ((file->f_flags & O_NONBLOCK) ||
|
|
(iocb->ki_flags & IOCB_NOWAIT)) {
|
|
spin_unlock_irq(&ctx->wqh.lock);
|
|
return -EAGAIN;
|
|
}
|
|
|
|
if (wait_event_interruptible_locked_irq(ctx->wqh, ctx->count)) {
|
|
spin_unlock_irq(&ctx->wqh.lock);
|
|
return -ERESTARTSYS;
|
|
}
|
|
}
|
|
eventfd_ctx_do_read(ctx, &ucnt);
|
|
current->in_eventfd = 1;
|
|
if (waitqueue_active(&ctx->wqh))
|
|
wake_up_locked_poll(&ctx->wqh, EPOLLOUT);
|
|
current->in_eventfd = 0;
|
|
spin_unlock_irq(&ctx->wqh.lock);
|
|
if (unlikely(copy_to_iter(&ucnt, sizeof(ucnt), to) != sizeof(ucnt)))
|
|
return -EFAULT;
|
|
|
|
return sizeof(ucnt);
|
|
}
|
|
|
|
static ssize_t eventfd_write(struct file *file, const char __user *buf, size_t count,
|
|
loff_t *ppos)
|
|
{
|
|
struct eventfd_ctx *ctx = file->private_data;
|
|
ssize_t res;
|
|
__u64 ucnt;
|
|
|
|
if (count < sizeof(ucnt))
|
|
return -EINVAL;
|
|
if (copy_from_user(&ucnt, buf, sizeof(ucnt)))
|
|
return -EFAULT;
|
|
if (ucnt == ULLONG_MAX)
|
|
return -EINVAL;
|
|
spin_lock_irq(&ctx->wqh.lock);
|
|
res = -EAGAIN;
|
|
if (ULLONG_MAX - ctx->count > ucnt)
|
|
res = sizeof(ucnt);
|
|
else if (!(file->f_flags & O_NONBLOCK)) {
|
|
res = wait_event_interruptible_locked_irq(ctx->wqh,
|
|
ULLONG_MAX - ctx->count > ucnt);
|
|
if (!res)
|
|
res = sizeof(ucnt);
|
|
}
|
|
if (likely(res > 0)) {
|
|
ctx->count += ucnt;
|
|
current->in_eventfd = 1;
|
|
if (waitqueue_active(&ctx->wqh))
|
|
wake_up_locked_poll(&ctx->wqh, EPOLLIN);
|
|
current->in_eventfd = 0;
|
|
}
|
|
spin_unlock_irq(&ctx->wqh.lock);
|
|
|
|
return res;
|
|
}
|
|
|
|
#ifdef CONFIG_PROC_FS
|
|
static void eventfd_show_fdinfo(struct seq_file *m, struct file *f)
|
|
{
|
|
struct eventfd_ctx *ctx = f->private_data;
|
|
|
|
spin_lock_irq(&ctx->wqh.lock);
|
|
seq_printf(m, "eventfd-count: %16llx\n",
|
|
(unsigned long long)ctx->count);
|
|
spin_unlock_irq(&ctx->wqh.lock);
|
|
seq_printf(m, "eventfd-id: %d\n", ctx->id);
|
|
seq_printf(m, "eventfd-semaphore: %d\n",
|
|
!!(ctx->flags & EFD_SEMAPHORE));
|
|
}
|
|
#endif
|
|
|
|
static const struct file_operations eventfd_fops = {
|
|
#ifdef CONFIG_PROC_FS
|
|
.show_fdinfo = eventfd_show_fdinfo,
|
|
#endif
|
|
.release = eventfd_release,
|
|
.poll = eventfd_poll,
|
|
.read_iter = eventfd_read,
|
|
.write = eventfd_write,
|
|
.llseek = noop_llseek,
|
|
};
|
|
|
|
/**
|
|
* eventfd_fget - Acquire a reference of an eventfd file descriptor.
|
|
* @fd: [in] Eventfd file descriptor.
|
|
*
|
|
* Returns a pointer to the eventfd file structure in case of success, or the
|
|
* following error pointer:
|
|
*
|
|
* -EBADF : Invalid @fd file descriptor.
|
|
* -EINVAL : The @fd file descriptor is not an eventfd file.
|
|
*/
|
|
struct file *eventfd_fget(int fd)
|
|
{
|
|
struct file *file;
|
|
|
|
file = fget(fd);
|
|
if (!file)
|
|
return ERR_PTR(-EBADF);
|
|
if (file->f_op != &eventfd_fops) {
|
|
fput(file);
|
|
return ERR_PTR(-EINVAL);
|
|
}
|
|
|
|
return file;
|
|
}
|
|
EXPORT_SYMBOL_GPL(eventfd_fget);
|
|
|
|
/**
|
|
* eventfd_ctx_fdget - Acquires a reference to the internal eventfd context.
|
|
* @fd: [in] Eventfd file descriptor.
|
|
*
|
|
* Returns a pointer to the internal eventfd context, otherwise the error
|
|
* pointers returned by the following functions:
|
|
*
|
|
* eventfd_fget
|
|
*/
|
|
struct eventfd_ctx *eventfd_ctx_fdget(int fd)
|
|
{
|
|
struct eventfd_ctx *ctx;
|
|
struct fd f = fdget(fd);
|
|
if (!f.file)
|
|
return ERR_PTR(-EBADF);
|
|
ctx = eventfd_ctx_fileget(f.file);
|
|
fdput(f);
|
|
return ctx;
|
|
}
|
|
EXPORT_SYMBOL_GPL(eventfd_ctx_fdget);
|
|
|
|
/**
|
|
* eventfd_ctx_fileget - Acquires a reference to the internal eventfd context.
|
|
* @file: [in] Eventfd file pointer.
|
|
*
|
|
* Returns a pointer to the internal eventfd context, otherwise the error
|
|
* pointer:
|
|
*
|
|
* -EINVAL : The @fd file descriptor is not an eventfd file.
|
|
*/
|
|
struct eventfd_ctx *eventfd_ctx_fileget(struct file *file)
|
|
{
|
|
struct eventfd_ctx *ctx;
|
|
|
|
if (file->f_op != &eventfd_fops)
|
|
return ERR_PTR(-EINVAL);
|
|
|
|
ctx = file->private_data;
|
|
kref_get(&ctx->kref);
|
|
return ctx;
|
|
}
|
|
EXPORT_SYMBOL_GPL(eventfd_ctx_fileget);
|
|
|
|
static int do_eventfd(unsigned int count, int flags)
|
|
{
|
|
struct eventfd_ctx *ctx;
|
|
struct file *file;
|
|
int fd;
|
|
|
|
/* Check the EFD_* constants for consistency. */
|
|
BUILD_BUG_ON(EFD_CLOEXEC != O_CLOEXEC);
|
|
BUILD_BUG_ON(EFD_NONBLOCK != O_NONBLOCK);
|
|
|
|
if (flags & ~EFD_FLAGS_SET)
|
|
return -EINVAL;
|
|
|
|
ctx = kmalloc(sizeof(*ctx), GFP_KERNEL);
|
|
if (!ctx)
|
|
return -ENOMEM;
|
|
|
|
kref_init(&ctx->kref);
|
|
init_waitqueue_head(&ctx->wqh);
|
|
ctx->count = count;
|
|
ctx->flags = flags;
|
|
ctx->id = ida_simple_get(&eventfd_ida, 0, 0, GFP_KERNEL);
|
|
|
|
flags &= EFD_SHARED_FCNTL_FLAGS;
|
|
flags |= O_RDWR;
|
|
fd = get_unused_fd_flags(flags);
|
|
if (fd < 0)
|
|
goto err;
|
|
|
|
file = anon_inode_getfile("[eventfd]", &eventfd_fops, ctx, flags);
|
|
if (IS_ERR(file)) {
|
|
put_unused_fd(fd);
|
|
fd = PTR_ERR(file);
|
|
goto err;
|
|
}
|
|
|
|
file->f_mode |= FMODE_NOWAIT;
|
|
fd_install(fd, file);
|
|
return fd;
|
|
err:
|
|
eventfd_free_ctx(ctx);
|
|
return fd;
|
|
}
|
|
|
|
SYSCALL_DEFINE2(eventfd2, unsigned int, count, int, flags)
|
|
{
|
|
return do_eventfd(count, flags);
|
|
}
|
|
|
|
SYSCALL_DEFINE1(eventfd, unsigned int, count)
|
|
{
|
|
return do_eventfd(count, 0);
|
|
}
|
|
|