[PATCH] fanotify: fix use-after-free of file range info

fsnotify_pre_content() builds its file_range on the triggering task's
stack. fanotify_alloc_perm_event() saves a pointer to range.pos in the
heap-allocated permission event so copy_range_info_to_user() can report
the offset later.

The event reader can set the event state to FAN_EVENT_REPORTED and then
sleep while preparing the file descriptor. If a signal interrupts the
triggering task at that point, fanotify_get_response() changes the state
to FAN_EVENT_CANCELED and returns. This unwinds the file_range stack
frame while the reader still owns the event. The reader then dereferences
pevent->ppos and copies the stale stack value to userspace.

KASAN reported:

  BUG: KASAN: use-after-free in fanotify_read+0x293e/0x2970
  Read of size 8 at addr ffff88811434fc50 by task fanotify_inotif/95
  Call Trace:
   fanotify_read+0x293e/0x2970
   vfs_read+0x177/0xa20
   ksys_read+0xf7/0x1c0
   do_syscall_64+0xf9/0x540
   entry_SYSCALL_64_after_hwframe+0x77/0x7f

Copy the range position into the permission event and make ppos refer to
that event-owned value. The event remains alive until the reader finishes,
so the reported offset no longer depends on the triggering task's stack.

Fixes: 870499bc1d4d ("fanotify: report file range info with pre-content events")
Cc: [email protected]
Signed-off-by: Chengfeng Ye <[email protected]>
---
 fs/notify/fanotify/fanotify.c | 3 ++-
 fs/notify/fanotify/fanotify.h | 3 ++-
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/notify/fanotify/fanotify.c b/fs/notify/fanotify/fanotify.c
index a3555bebad63..c97d1be70310 100644
--- a/fs/notify/fanotify/fanotify.c
+++ b/fs/notify/fanotify/fanotify.c
@@ -601,7 +601,8 @@ static struct fanotify_event *fanotify_alloc_perm_event(const void *data,
 	pevent->state = FAN_EVENT_INIT;
 	pevent->path = *path;
 	/* NULL ppos means no range info */
-	pevent->ppos = range ? &range->pos : NULL;
+	pevent->pos = range ? range->pos : 0;
+	pevent->ppos = range ? &pevent->pos : NULL;
 	pevent->count = range ? range->count : 0;
 	path_get(path);
 
diff --git a/fs/notify/fanotify/fanotify.h b/fs/notify/fanotify/fanotify.h
index a0619e7694d5..c964df6c0514 100644
--- a/fs/notify/fanotify/fanotify.h
+++ b/fs/notify/fanotify/fanotify.h
@@ -438,7 +438,8 @@ FANOTIFY_ME(struct fanotify_event *event)
 struct fanotify_perm_event {
 	struct fanotify_event fae;
 	struct path path;
-	const loff_t *ppos;		/* optional file range info */
+	loff_t pos;
+	const loff_t *ppos;		/* &pos if range info is available */
 	size_t count;
 	u32 response;			/* userspace answer to the event */
 	unsigned short state;		/* state of the event */
-- 
2.43.0