[PATCH v2] fanotify: fix use-after-free of file range info

[Chengfeng Ye]

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

Store the range position directly in the permission event and use
FANOTIFY_NO_RANGE when range information is unavailable. 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: stable@xxxxxxxxxxxxxxx
Suggested-by: Jan Kara <jack@xxxxxxx>
Signed-off-by: Chengfeng Ye <nicoyip.dev@xxxxxxxxx>
---

Changes in v2:
- Remove ppos from fanotify_perm_event and use FANOTIFY_NO_RANGE as the
  sentinel for unavailable range information, as suggested by Jan Kara.
- Read the event-owned position directly when reporting range information.

Link: https://lore.kernel.org/linux-fsdevel/20260730085801.2068723-1-nicoyip.dev@xxxxxxxxx/ [v1]

 fs/notify/fanotify/fanotify.c      | 3 +--
 fs/notify/fanotify/fanotify.h      | 6 ++++--
 fs/notify/fanotify/fanotify_user.c | 4 ++--
 3 files changed, 7 insertions(+), 6 deletions(-)

diff --git a/fs/notify/fanotify/fanotify.c b/fs/notify/fanotify/fanotify.c
index a3555bebad63..b05b6d3abb87 100644
--- a/fs/notify/fanotify/fanotify.c
+++ b/fs/notify/fanotify/fanotify.c
@@ -600,8 +600,7 @@ static struct fanotify_event *fanotify_alloc_perm_event(const void *data,
 	pevent->hdr.len = 0;
 	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 : FANOTIFY_NO_RANGE;
 	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..3710543dbf82 100644
--- a/fs/notify/fanotify/fanotify.h
+++ b/fs/notify/fanotify/fanotify.h
@@ -428,6 +428,8 @@ FANOTIFY_ME(struct fanotify_event *event)
 	return container_of(event, struct fanotify_mnt_event, fae);
 }
 
+#define FANOTIFY_NO_RANGE	((loff_t)-1)
+
 /*
  * Structure for permission fanotify events. It gets allocated and freed in
  * fanotify_handle_event() since we wait there for user response. When the
@@ -438,7 +440,7 @@ 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;			/* FANOTIFY_NO_RANGE if unavailable */
 	size_t count;
 	u32 response;			/* userspace answer to the event */
 	unsigned short state;		/* state of the event */
@@ -468,7 +470,7 @@ static inline bool fanotify_event_has_access_range(struct fanotify_event *event)
 	if (!(event->mask & FANOTIFY_PRE_CONTENT_EVENTS))
 		return false;
 
-	return FANOTIFY_PERM(event)->ppos;
+	return FANOTIFY_PERM(event)->pos != FANOTIFY_NO_RANGE;
 }
 
 static inline struct fanotify_event *FANOTIFY_E(struct fsnotify_event *fse)
diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
index b604e3da58ad..bba92d691f0d 100644
--- a/fs/notify/fanotify/fanotify_user.c
+++ b/fs/notify/fanotify/fanotify_user.c
@@ -675,12 +675,12 @@ static size_t copy_range_info_to_user(struct fanotify_event *event,
 	if (WARN_ON_ONCE(info_len > count))
 		return -EFAULT;
 
-	if (WARN_ON_ONCE(!pevent->ppos))
+	if (WARN_ON_ONCE(pevent->pos == FANOTIFY_NO_RANGE))
 		return -EINVAL;
 
 	info.hdr.info_type = FAN_EVENT_INFO_TYPE_RANGE;
 	info.hdr.len = info_len;
-	info.offset = *(pevent->ppos);
+	info.offset = pevent->pos;
 	info.count = pevent->count;
 
 	if (copy_to_user(buf, &info, info_len))
-- 
2.43.0