Chao Yu <c...@kernel.org> 于2025年6月4日周三 17:48写道：
>
> On 6/4/25 13:54, Zhiguo Niu wrote:
> > The decompress_io_ctx may be released asynchronously after
> > I/O completion. If this file is deleted immediately after read,
> > and the kworker of processing post_read_wq has not been executed yet
> > due to high workloads, It is possible that the inode(f2fs_inode_info)
> > is evicted and freed before it is used f2fs_free_dic.
> >
> > The UAF case as below:
> > Thread A Thread B
> > - f2fs_decompress_end_io
> > - f2fs_put_dic
> > - queue_work
> > add free_dic work to post_read_wq
> > - do_unlink
> > - iput
> > - evict
> > - call_rcu
> > This file is deleted after read.
> >
> > Thread C kworker to process post_read_wq
> > - rcu_do_batch
> > - f2fs_free_inode
> > - kmem_cache_free
> > inode is freed by rcu
> > - process_scheduled_works
> > - f2fs_late_free_dic
> > - f2fs_free_dic
> > - f2fs_release_decomp_mem
> > read
> > (dic->inode)->i_compress_algorithm
> >
> > This patch increase inode->i_count before f2fs_free_dic and decrease it
> > after free the dic.
> >
> > Cc: Daeho Jeong <daehoje...@google.com>
> > Fixes: bff139b49d9f ("f2fs: handle decompress only post processing in
> > softirq")
> > Signed-off-by: Zhiguo Niu <zhiguo....@unisoc.com>
> > Signed-off-by: Baocong Liu <baocong....@unisoc.com>
> > ---
> > fs/f2fs/compress.c | 19 ++++++++++++++-----
> > 1 file changed, 14 insertions(+), 5 deletions(-)
> >
> > diff --git a/fs/f2fs/compress.c b/fs/f2fs/compress.c
> > index b3c1df9..6b3b3a7 100644
> > --- a/fs/f2fs/compress.c
> > +++ b/fs/f2fs/compress.c
> > @@ -1687,7 +1687,7 @@ static void f2fs_release_decomp_mem(struct
> > decompress_io_ctx *dic,
> > }
> >
> > static void f2fs_free_dic(struct decompress_io_ctx *dic,
> > - bool bypass_destroy_callback);
> > + bool bypass_destroy_callback, bool late_free);
> >
> > struct decompress_io_ctx *f2fs_alloc_dic(struct compress_ctx *cc)
> > {
> > @@ -1743,12 +1743,12 @@ struct decompress_io_ctx *f2fs_alloc_dic(struct
> > compress_ctx *cc)
> > return dic;
> >
> > out_free:
> > - f2fs_free_dic(dic, true);
> > + f2fs_free_dic(dic, true, false);
> > return ERR_PTR(ret);
> > }
> >
> > static void f2fs_free_dic(struct decompress_io_ctx *dic,
> > - bool bypass_destroy_callback)
> > + bool bypass_destroy_callback, bool late_free)
> > {
> > int i;
> >
> > @@ -1775,6 +1775,11 @@ static void f2fs_free_dic(struct decompress_io_ctx
> > *dic,
> > }
> >
> > page_array_free(dic->inode, dic->rpages, dic->nr_rpages);
> > + if (late_free) {
> > + spin_lock(&dic->inode->i_lock);
> > + atomic_dec(&dic->inode->i_count);
> > + spin_unlock(&dic->inode->i_lock);
>
> If it is the last one release i_count, it needs to call iput_final to evict
> inode
> like what iput did, so we'd better to call iput() here?
Hi Chao,
Yes, we have also tested this method(iput/__iget), and it worked.
Just think It is simpler and easier to read to directly operate
i_count, and then free it
by relying on the memory module when i_count=0.
But It seems iput/__iget is better.
>
> > + }
> > kmem_cache_free(dic_entry_slab, dic);
> > }
> >
> > @@ -1783,16 +1788,20 @@ static void f2fs_late_free_dic(struct work_struct
> > *work)
> > struct decompress_io_ctx *dic =
> > container_of(work, struct decompress_io_ctx, free_work);
> >
> > - f2fs_free_dic(dic, false);
> > + f2fs_free_dic(dic, false, true);
> > }
> >
> > static void f2fs_put_dic(struct decompress_io_ctx *dic, bool in_task)
> > {
> > if (refcount_dec_and_test(&dic->refcnt)) {
> > if (in_task) {
> > - f2fs_free_dic(dic, false);
> > + f2fs_free_dic(dic, false, false);
> > } else {
> > INIT_WORK(&dic->free_work, f2fs_late_free_dic);
> > + /* to avoid inode is evicted simultaneously */
> > + spin_lock(&dic->inode->i_lock);
> > + atomic_inc(&dic->inode->i_count);
> > + spin_unlock(&dic->inode->i_lock);
>
> iget()?
>
> BTW, can we store i_compress_algorithm in dic to avoid inode access?
Also thought of this method, but it would require more changes.
dic->inode used in f2fs_free_dic are all needed to modify except
i_compress_algorithm.
such as page_array_free(dic->inode),
allow_memalloc_for_decomp(F2FS_I_SB(dic->inode)).
Do you have any other suggestions?
thanks！
>
> Thanks,
>
> > queue_work(F2FS_I_SB(dic->inode)->post_read_wq,
> > &dic->free_work);
> > }
>
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