[PATCH] f2fs: compress: fix UAF of f2fs_inode_info in f2fs_free_dic

[Zhiguo Niu]

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 <daehojeong@xxxxxxxxxx>
Fixes: bff139b49d9f ("f2fs: handle decompress only post processing in softirq")
Signed-off-by: Zhiguo Niu <zhiguo.niu@xxxxxxxxxx>
Signed-off-by: Baocong Liu <baocong.liu@xxxxxxxxxx>
---
 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);
+	}
 	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);
 			queue_work(F2FS_I_SB(dic->inode)->post_read_wq,
 					&dic->free_work);
 		}
-- 
1.9.1