aio: simplify - and fix - fget/fput for io_submit()
commit 84c4e1f89fefe70554da0ab33be72c9be7994379 upstream.
Al Viro root-caused a race where the IOCB_CMD_POLL handling of
fget/fput() could cause us to access the file pointer after it had
already been freed:
"In more details - normally IOCB_CMD_POLL handling looks so:
1) io_submit(2) allocates aio_kiocb instance and passes it to
aio_poll()
2) aio_poll() resolves the descriptor to struct file by req->file =
fget(iocb->aio_fildes)
3) aio_poll() sets ->woken to false and raises ->ki_refcnt of that
aio_kiocb to 2 (bumps by 1, that is).
4) aio_poll() calls vfs_poll(). After sanity checks (basically,
"poll_wait() had been called and only once") it locks the queue.
That's what the extra reference to iocb had been for - we know we
can safely access it.
5) With queue locked, we check if ->woken has already been set to
true (by aio_poll_wake()) and, if it had been, we unlock the
queue, drop a reference to aio_kiocb and bugger off - at that
point it's a responsibility to aio_poll_wake() and the stuff
called/scheduled by it. That code will drop the reference to file
in req->file, along with the other reference to our aio_kiocb.
6) otherwise, we see whether we need to wait. If we do, we unlock the
queue, drop one reference to aio_kiocb and go away - eventual
wakeup (or cancel) will deal with the reference to file and with
the other reference to aio_kiocb
7) otherwise we remove ourselves from waitqueue (still under the
queue lock), so that wakeup won't get us. No async activity will
be happening, so we can safely drop req->file and iocb ourselves.
If wakeup happens while we are in vfs_poll(), we are fine - aio_kiocb
won't get freed under us, so we can do all the checks and locking
safely. And we don't touch ->file if we detect that case.
However, vfs_poll() most certainly *does* touch the file it had been
given. So wakeup coming while we are still in ->poll() might end up
doing fput() on that file. That case is not too rare, and usually we
are saved by the still present reference from descriptor table - that
fput() is not the final one.
But if another thread closes that descriptor right after our fget()
and wakeup does happen before ->poll() returns, we are in trouble -
final fput() done while we are in the middle of a method:
Al also wrote a patch to take an extra reference to the file descriptor
to fix this, but I instead suggested we just streamline the whole file
pointer handling by submit_io() so that the generic aio submission code
simply keeps the file pointer around until the aio has completed.
Fixes: bfe4037e722e ("aio: implement IOCB_CMD_POLL")
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Reported-by: syzbot+503d4cc169fcec1cb18c@syzkaller.appspotmail.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/fs/aio.c b/fs/aio.c
index d74fc9e..46229e6 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -161,9 +161,13 @@
unsigned id;
};
+/*
+ * First field must be the file pointer in all the
+ * iocb unions! See also 'struct kiocb' in <linux/fs.h>
+ */
struct fsync_iocb {
- struct work_struct work;
struct file *file;
+ struct work_struct work;
bool datasync;
};
@@ -177,8 +181,15 @@
struct work_struct work;
};
+/*
+ * NOTE! Each of the iocb union members has the file pointer
+ * as the first entry in their struct definition. So you can
+ * access the file pointer through any of the sub-structs,
+ * or directly as just 'ki_filp' in this struct.
+ */
struct aio_kiocb {
union {
+ struct file *ki_filp;
struct kiocb rw;
struct fsync_iocb fsync;
struct poll_iocb poll;
@@ -1054,6 +1065,8 @@
{
if (refcount_read(&iocb->ki_refcnt) == 0 ||
refcount_dec_and_test(&iocb->ki_refcnt)) {
+ if (iocb->ki_filp)
+ fput(iocb->ki_filp);
percpu_ref_put(&iocb->ki_ctx->reqs);
kmem_cache_free(kiocb_cachep, iocb);
}
@@ -1418,7 +1431,6 @@
file_end_write(kiocb->ki_filp);
}
- fput(kiocb->ki_filp);
aio_complete(iocb, res, res2);
}
@@ -1426,9 +1438,6 @@
{
int ret;
- req->ki_filp = fget(iocb->aio_fildes);
- if (unlikely(!req->ki_filp))
- return -EBADF;
req->ki_complete = aio_complete_rw;
req->private = NULL;
req->ki_pos = iocb->aio_offset;
@@ -1445,7 +1454,7 @@
ret = ioprio_check_cap(iocb->aio_reqprio);
if (ret) {
pr_debug("aio ioprio check cap error: %d\n", ret);
- goto out_fput;
+ return ret;
}
req->ki_ioprio = iocb->aio_reqprio;
@@ -1454,14 +1463,10 @@
ret = kiocb_set_rw_flags(req, iocb->aio_rw_flags);
if (unlikely(ret))
- goto out_fput;
+ return ret;
req->ki_flags &= ~IOCB_HIPRI; /* no one is going to poll for this I/O */
return 0;
-
-out_fput:
- fput(req->ki_filp);
- return ret;
}
static int aio_setup_rw(int rw, const struct iocb *iocb, struct iovec **iovec,
@@ -1515,24 +1520,19 @@
if (ret)
return ret;
file = req->ki_filp;
-
- ret = -EBADF;
if (unlikely(!(file->f_mode & FMODE_READ)))
- goto out_fput;
+ return -EBADF;
ret = -EINVAL;
if (unlikely(!file->f_op->read_iter))
- goto out_fput;
+ return -EINVAL;
ret = aio_setup_rw(READ, iocb, &iovec, vectored, compat, &iter);
if (ret)
- goto out_fput;
+ return ret;
ret = rw_verify_area(READ, file, &req->ki_pos, iov_iter_count(&iter));
if (!ret)
aio_rw_done(req, call_read_iter(file, req, &iter));
kfree(iovec);
-out_fput:
- if (unlikely(ret))
- fput(file);
return ret;
}
@@ -1549,16 +1549,14 @@
return ret;
file = req->ki_filp;
- ret = -EBADF;
if (unlikely(!(file->f_mode & FMODE_WRITE)))
- goto out_fput;
- ret = -EINVAL;
+ return -EBADF;
if (unlikely(!file->f_op->write_iter))
- goto out_fput;
+ return -EINVAL;
ret = aio_setup_rw(WRITE, iocb, &iovec, vectored, compat, &iter);
if (ret)
- goto out_fput;
+ return ret;
ret = rw_verify_area(WRITE, file, &req->ki_pos, iov_iter_count(&iter));
if (!ret) {
/*
@@ -1576,9 +1574,6 @@
aio_rw_done(req, call_write_iter(file, req, &iter));
}
kfree(iovec);
-out_fput:
- if (unlikely(ret))
- fput(file);
return ret;
}
@@ -1588,7 +1583,6 @@
int ret;
ret = vfs_fsync(req->file, req->datasync);
- fput(req->file);
aio_complete(container_of(req, struct aio_kiocb, fsync), ret, 0);
}
@@ -1599,13 +1593,8 @@
iocb->aio_rw_flags))
return -EINVAL;
- req->file = fget(iocb->aio_fildes);
- if (unlikely(!req->file))
- return -EBADF;
- if (unlikely(!req->file->f_op->fsync)) {
- fput(req->file);
+ if (unlikely(!req->file->f_op->fsync))
return -EINVAL;
- }
req->datasync = datasync;
INIT_WORK(&req->work, aio_fsync_work);
@@ -1615,10 +1604,7 @@
static inline void aio_poll_complete(struct aio_kiocb *iocb, __poll_t mask)
{
- struct file *file = iocb->poll.file;
-
aio_complete(iocb, mangle_poll(mask), 0);
- fput(file);
}
static void aio_poll_complete_work(struct work_struct *work)
@@ -1743,9 +1729,6 @@
INIT_WORK(&req->work, aio_poll_complete_work);
req->events = demangle_poll(iocb->aio_buf) | EPOLLERR | EPOLLHUP;
- req->file = fget(iocb->aio_fildes);
- if (unlikely(!req->file))
- return -EBADF;
req->head = NULL;
req->woken = false;
@@ -1788,10 +1771,8 @@
spin_unlock_irq(&ctx->ctx_lock);
out:
- if (unlikely(apt.error)) {
- fput(req->file);
+ if (unlikely(apt.error))
return apt.error;
- }
if (mask)
aio_poll_complete(aiocb, mask);
@@ -1829,6 +1810,11 @@
if (unlikely(!req))
goto out_put_reqs_available;
+ req->ki_filp = fget(iocb->aio_fildes);
+ ret = -EBADF;
+ if (unlikely(!req->ki_filp))
+ goto out_put_req;
+
if (iocb->aio_flags & IOCB_FLAG_RESFD) {
/*
* If the IOCB_FLAG_RESFD flag of aio_flags is set, get an
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 7b60848..111c94c 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -304,13 +304,19 @@
struct kiocb {
struct file *ki_filp;
+
+ /* The 'ki_filp' pointer is shared in a union for aio */
+ randomized_struct_fields_start
+
loff_t ki_pos;
void (*ki_complete)(struct kiocb *iocb, long ret, long ret2);
void *private;
int ki_flags;
u16 ki_hint;
u16 ki_ioprio; /* See linux/ioprio.h */
-} __randomize_layout;
+
+ randomized_struct_fields_end
+};
static inline bool is_sync_kiocb(struct kiocb *kiocb)
{