From: Mikhail Lobanov Date: Mon, 15 Jun 2026 11:36:13 +0000 (+0300) Subject: f2fs: read COW data with the original inode during atomic write X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=a41075acde0124d2f8a5f563068a5d63e8ffd57b;p=thirdparty%2Flinux.git 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: stable@vger.kernel.org Signed-off-by: Mikhail Lobanov Reviewed-by: Chao Yu Signed-off-by: Jaegeuk Kim --- diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c index cea34b4595b11..60dea95b0295e 100644 --- a/fs/f2fs/data.c +++ b/fs/f2fs/data.c @@ -3860,7 +3860,7 @@ unlock_out: 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; @@ -3875,14 +3875,14 @@ static int prepare_atomic_write_begin(struct f2fs_sb_info *sbi, /* Look for the block in COW inode first */ err = __find_data_block(cow_inode, index, blk_addr); - if (err) { + if (err) return err; - } else if (__is_valid_data_blkaddr(*blk_addr)) { - *use_cow = true; + + if (__is_valid_data_blkaddr(*blk_addr)) return 0; - } else if (*blk_addr == NEW_ADDR) { + + if (*blk_addr == NEW_ADDR) cow_has_reserved_block = true; - } if (is_inode_flag_set(inode, FI_ATOMIC_REPLACE)) goto reserve_block; @@ -3917,7 +3917,6 @@ 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; @@ -3980,7 +3979,7 @@ repeat: 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); @@ -4020,8 +4019,15 @@ repeat: 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);