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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