From d7f1cf5be33ef0175a4e8ed8687aeb98fb00a851 Mon Sep 17 00:00:00 2001 From: Chengfeng Ye Date: Fri, 31 Jul 2026 13:31:45 +0200 Subject: 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 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@vger.kernel.org Suggested-by: Jan Kara Signed-off-by: Chengfeng Ye Link: https://patch.msgid.link/20260730134316.2085087-1-nicoyip.dev@gmail.com Signed-off-by: Jan Kara --- fs/notify/fanotify/fanotify.c | 3 +-- fs/notify/fanotify/fanotify.h | 6 ++++-- fs/notify/fanotify/fanotify_user.c | 5 +---- 3 files changed, 6 insertions(+), 8 deletions(-) diff --git a/fs/notify/fanotify/fanotify.c b/fs/notify/fanotify/fanotify.c index b59f0aa43c4b..a208a7ec1692 100644 --- a/fs/notify/fanotify/fanotify.c +++ b/fs/notify/fanotify/fanotify.c @@ -601,8 +601,7 @@ static struct fanotify_event *fanotify_alloc_perm_event(const void *data, pevent->state = FAN_EVENT_INIT; pevent->watchdog_cnt = 0; 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 9ee373ff5840..faffe8f0cb12 100644 --- a/fs/notify/fanotify/fanotify_user.c +++ b/fs/notify/fanotify/fanotify_user.c @@ -680,12 +680,9 @@ 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)) - 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)) -- cgit v1.2.3