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

["David Hildenbrand (Arm)" <david@kernel.org>]

On 10/29/26 03:42, Joas Antonio dos Santos wrote:
> 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));
>  }
>  

Should we rename the tracepoint then?

-- 
Cheers,

David