[PATCH] hugetlbfs: emit hugetlbfs_free_inode from ->destroy_inode()

[Joas Antonio dos Santos <joasantonio108@gmail.com>]

hugetlbfs_free_inode() calls trace_hugetlbfs_free_inode(), and the
hugetlbfs__inode event class records inode->i_sb->s_dev.  ->free_inode()
runs from an RCU callback (destroy_inode() queues i_callback() with
call_rcu()), and Documentation/filesystems/porting.rst says that this
callback "is *NOT* ordered wrt superblock destruction; as the matter of
fact, the superblock and all associated structures might be already
gone".  put_super() frees the superblock with call_rcu() followed by a
workqueue item (destroy_super_rcu() -> destroy_super_work()).  syzbot
hit this with the event enabled:

  BUG: KASAN: slab-use-after-free in trace_event_raw_event_hugetlbfs__inode+0x45b/0x4c0 include/trace/events/hugetlbfs.h:36
  Read of size 4 at addr ffff888038850010 by task syz-executor952/6134
   hugetlbfs_free_inode+0x15e/0x1f0 fs/hugetlbfs/inode.c:1182
   i_callback+0x46/0x80 fs/inode.c:317
   rcu_do_batch kernel/rcu/tree.c:2650 [inline]
   rcu_core+0x5f0/0x1220 kernel/rcu/tree.c:2919
  Allocated by task 6134:
   alloc_super fs/super.c:326 [inline]
   sget_fc+0x1ba/0x1f60 fs/super.c:874
   hugetlbfs_get_tree+0x1e7/0x2b0 fs/hugetlbfs/inode.c:1431
  Freed by task 855:
   kfree+0x22b/0x6d0 mm/slub.c:6923
   process_one_work+0xac7/0x1b10 kernel/workqueue.c:3495

Emit the event from hugetlbfs_destroy_inode() instead.  destroy_inode()
calls ->destroy_inode() synchronously, before it queues the RCU
callback, while the superblock is still alive; hugetlbfs_destroy_inode()
already dereferences inode->i_sb there.  ->free_inode() is left with
only the kmem_cache_free().

Fixes: 014ad7c42a69 ("hugetlbfs: use tracepoints in hugetlbfs functions.")
Reported-by: syzbot+91e1fc35f51774c479b2@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/6ae27525.5451848c.f7707.0001.GAE@google.com/
Cc: stable@vger.kernel.org
Signed-off-by: Joas Antonio dos Santos <joasantonio108@gmail.com>
Assisted-by: Yaga Agent HackerSec Claude:claude-opus-5-5
---
Tested on net/main 94cea342e891 (arm64, QEMU, generic KASAN, tracing
enabled) with the syzbot C reproducer, which enables
events/hugetlbfs/hugetlbfs_free_inode and races hugetlbfs mounts and
lazy unmounts with open + unlink.  Without this patch KASAN reports the
slab-use-after-free in trace_event_raw_event_hugetlbfs__inode() from
hugetlbfs_free_inode() <- i_callback() during the first 10 second run,
on 3 of 3 boots.  With it, 12 runs over 2 boots are clean, and the trace
buffer still holds the hugetlbfs_free_inode events (about 46k per boot)
with valid dev numbers.  fs/hugetlbfs/inode.o builds cleanly with W=1.

The event now fires when the inode is destroyed instead of when its
memory goes back to the slab cache, one RCU grace period earlier; the
recorded fields are the same.

 fs/hugetlbfs/inode.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index 6656807..27a3937 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -1179,12 +1179,16 @@ static struct inode *hugetlbfs_alloc_inode(struct super_block *sb)
 
 static void hugetlbfs_free_inode(struct inode *inode)
 {
-	trace_hugetlbfs_free_inode(inode);
 	kmem_cache_free(hugetlbfs_inode_cachep, HUGETLBFS_I(inode));
 }
 
 static void hugetlbfs_destroy_inode(struct inode *inode)
 {
+	/*
+	 * Not in ->free_inode(): that runs from an RCU callback, when the
+	 * superblock the event reads may already be freed.
+	 */
+	trace_hugetlbfs_free_inode(inode);
 	hugetlbfs_inc_free_inodes(HUGETLBFS_SB(inode->i_sb));
 }
 
-- 
2.39.5