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fanotify: fix use-after-free of file range info

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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: 870499bc1d ("fanotify: report file range info with pre-content events")
Cc: stable@vger.kernel.org
Suggested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Chengfeng Ye <nicoyip.dev@gmail.com>
Link: https://patch.msgid.link/20260730134316.2085087-1-nicoyip.dev@gmail.com
Signed-off-by: Jan Kara <jack@suse.cz>
This commit is contained in:
Chengfeng Ye 2026-07-31 13:31:45 +02:00 • committed by Jan Kara
commit d7f1cf5be3
3 changed files with 6 additions and 8 deletions
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