CVE-2026-63811

In the Linux kernel, the following vulnerability has been resolved:

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().