[f2fs-dev] [PATCH v2] f2fs: read COW data with the original inode during atomic write

When updating an atomic-write file, f2fs_write_begin() may read the
previously written data back from the COW inode:
prepare_atomic_write_begin() locates the block in the COW inode and sets
use_cow, and the read bio is then built with the COW inode:

	f2fs_submit_page_read(use_cow ? F2FS_I(inode)->cow_inode : inode,
			      ...);

and f2fs_grab_read_bio() decides whether to schedule fs-layer decryption
(STEP_DECRYPT) for the bio based on that inode via
fscrypt_inode_uses_fs_layer_crypto().

However, the folio being filled belongs to the original inode
(folio->mapping->host == inode), and the data stored in the COW block was
encrypted (or left as plaintext) using the original inode's context, not
the COW inode's -- see f2fs_encrypt_one_page(), which keys off
fio->page->mapping->host.  fscrypt_decrypt_pagecache_blocks() likewise
operates on folio->mapping->host.

The COW inode is created as a tmpfile in the parent directory and inherits
its encryption policy from there.  With test_dummy_encryption the newly
created COW inode gets the dummy policy and becomes encrypted, while a
pre-existing regular file -- created before the policy applied, e.g.
already present in the on-disk image -- stays unencrypted.  The read
path then sets STEP_DECRYPT based on the encrypted COW inode and calls
fscrypt_decrypt_pagecache_blocks() on a folio whose host (the unencrypted
original inode) has a NULL ->i_crypt_info, dereferencing it:

  Oops: general protection fault, probably for non-canonical address ...
  KASAN: null-ptr-deref in range [0x0000000000000008-0x000000000000000f]
  RIP: 0010:fscrypt_decrypt_pagecache_blocks+0xa0/0x310
  Workqueue: f2fs_post_read_wq f2fs_post_read_work
  Call Trace:
   fscrypt_decrypt_bio+0x1eb/0x340
   f2fs_post_read_work+0xba/0x140
   process_one_work+0x91c/0x1a40
   worker_thread+0x677/0xe90
   kthread+0x2bc/0x3a0

The COW inode is only needed to locate the on-disk block, and that block
address is already resolved into @blkaddr by prepare_atomic_write_begin()
via __find_data_block(cow_inode, ...); f2fs_submit_page_read() then reads
from that physical @blkaddr directly, so the inode argument only selects
the post-read crypto context, not which block is fetched.  Reading with
@inode therefore returns the same (latest, not-yet-committed) COW data,
while making both the fs-layer decryption decision and the inline crypto
path use the correct (original inode's) key.

With the COW inode no longer used at the read site, the use_cow flag has no
remaining consumer; drop it from f2fs_write_begin() and
prepare_atomic_write_begin().

Fixes: 591fc34e1f98 ("f2fs: use cow inode data when updating atomic write")
Cc: [email protected]
Signed-off-by: Mikhail Lobanov <[email protected]>
Reviewed-by: Chao Yu <[email protected]>
---
v2: drop the now-unused use_cow flag from f2fs_write_begin() and
    prepare_atomic_write_begin() (Chao Yu); no functional change beyond
    v1.  Carried Chao's Reviewed-by as the cleanup was his request.
    Rebased on current mainline (f2fs_submit_page_read() now takes a
    fsverity_info argument and returns void).

 fs/f2fs/data.c | 17 ++++++++++++-----
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
index 8d4f1e75dee3..9016272b68c7 100644
--- a/fs/f2fs/data.c
+++ b/fs/f2fs/data.c
@@ -3822,7 +3822,7 @@ static int __reserve_data_block(struct inode *inode, pgoff_t index,
 
 static int prepare_atomic_write_begin(struct f2fs_sb_info *sbi,
 			struct folio *folio, loff_t pos, unsigned int len,
-			block_t *blk_addr, bool *node_changed, bool *use_cow)
+			block_t *blk_addr, bool *node_changed)
 {
 	struct inode *inode = folio->mapping->host;
 	struct inode *cow_inode = F2FS_I(inode)->cow_inode;
@@ -3839,7 +3839,6 @@ static int prepare_atomic_write_begin(struct f2fs_sb_info *sbi,
 	if (err) {
 		return err;
 	} else if (*blk_addr != NULL_ADDR) {
-		*use_cow = true;
 		return 0;
 	}
 
@@ -3873,7 +3872,6 @@ static int f2fs_write_begin(const struct kiocb *iocb,
 	struct folio *folio;
 	pgoff_t index = pos >> PAGE_SHIFT;
 	bool need_balance = false;
-	bool use_cow = false;
 	block_t blkaddr = NULL_ADDR;
 	int err = 0;
 
@@ -3936,7 +3934,7 @@ static int f2fs_write_begin(const struct kiocb *iocb,
 
 	if (f2fs_is_atomic_file(inode))
 		err = prepare_atomic_write_begin(sbi, folio, pos, len,
-					&blkaddr, &need_balance, &use_cow);
+					&blkaddr, &need_balance);
 	else
 		err = prepare_write_begin(sbi, folio, pos, len,
 					&blkaddr, &need_balance);
@@ -3976,8 +3974,15 @@ static int f2fs_write_begin(const struct kiocb *iocb,
 			err = -EFSCORRUPTED;
 			goto put_folio;
 		}
-		f2fs_submit_page_read(use_cow ? F2FS_I(inode)->cow_inode :
-						inode,
+		/*
+		 * Although the block may be stored in the COW inode, the folio
+		 * belongs to @inode and its data was encrypted (or not) using
+		 * @inode's context (see f2fs_encrypt_one_page()).  Read with
+		 * @inode so the post-read decryption decision matches the
+		 * folio's owner; otherwise an unencrypted @inode whose COW inode
+		 * is encrypted hits a NULL ->i_crypt_info on decryption.
+		 */
+		f2fs_submit_page_read(inode,
 				      NULL, /* can't write to fsverity files */
 				      folio, blkaddr, 0, true);
 

-- 
2.34.1



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