{"id":"CVE-2026-63811","summary":"f2fs: read COW data with the original inode during atomic write","details":"In the Linux kernel, the following vulnerability has been resolved:\n\nf2fs: read COW data with the original inode during atomic write\n\nWhen updating an atomic-write file, f2fs_write_begin() may read the\npreviously written data back from the COW inode:\nprepare_atomic_write_begin() locates the block in the COW inode and sets\nuse_cow, and the read bio is then built with the COW inode:\n\n\tf2fs_submit_page_read(use_cow ? F2FS_I(inode)-\u003ecow_inode : inode,\n\t\t\t      ...);\n\nand f2fs_grab_read_bio() decides whether to schedule fs-layer decryption\n(STEP_DECRYPT) for the bio based on that inode via\nfscrypt_inode_uses_fs_layer_crypto().\n\nHowever, the folio being filled belongs to the original inode\n(folio-\u003emapping-\u003ehost == inode), and the data stored in the COW block was\nencrypted (or left as plaintext) using the original inode's context, not\nthe COW inode's -- see f2fs_encrypt_one_page(), which keys off\nfio-\u003epage-\u003emapping-\u003ehost.  fscrypt_decrypt_pagecache_blocks() likewise\noperates on folio-\u003emapping-\u003ehost.\n\nThe COW inode is created as a tmpfile in the parent directory and inherits\nits encryption policy from there.  With test_dummy_encryption the newly\ncreated COW inode gets the dummy policy and becomes encrypted, while a\npre-existing regular file -- created before the policy applied, e.g.\nalready present in the on-disk image -- stays unencrypted.  The read\npath then sets STEP_DECRYPT based on the encrypted COW inode and calls\nfscrypt_decrypt_pagecache_blocks() on a folio whose host (the unencrypted\noriginal inode) has a NULL -\u003ei_crypt_info, dereferencing it:\n\n  Oops: general protection fault, probably for non-canonical address ...\n  KASAN: null-ptr-deref in range [0x0000000000000008-0x000000000000000f]\n  RIP: 0010:fscrypt_decrypt_pagecache_blocks+0xa0/0x310\n  Workqueue: f2fs_post_read_wq f2fs_post_read_work\n  Call Trace:\n   fscrypt_decrypt_bio+0x1eb/0x340\n   f2fs_post_read_work+0xba/0x140\n   process_one_work+0x91c/0x1a40\n   worker_thread+0x677/0xe90\n   kthread+0x2bc/0x3a0\n\nThe COW inode is only needed to locate the on-disk block, and that block\naddress is already resolved into @blkaddr by prepare_atomic_write_begin()\nvia __find_data_block(cow_inode, ...); f2fs_submit_page_read() then reads\nfrom that physical @blkaddr directly, so the inode argument only selects\nthe post-read crypto context, not which block is fetched.  Reading with\n@inode therefore returns the same (latest, not-yet-committed) COW data,\nwhile making both the fs-layer decryption decision and the inline crypto\npath use the correct (original inode's) key.\n\nWith the COW inode no longer used at the read site, the use_cow flag has no\nremaining consumer; drop it from f2fs_write_begin() and\nprepare_atomic_write_begin().","modified":"2026-07-21T03:47:27.534870886Z","published":"2026-07-19T12:02:13.057Z","database_specific":{"cna_assigner":"Linux","osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/63xxx/CVE-2026-63811.json"},"references":[{"type":"WEB","url":"https://git.kernel.org/stable/c/a41075acde0124d2f8a5f563068a5d63e8ffd57b"},{"type":"WEB","url":"https://git.kernel.org/stable/c/a92332f32a8d31a7eee47b1dc1d751cb3319908f"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/63xxx/CVE-2026-63811.json"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-63811"},{"type":"PACKAGE","url":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","events":[{"introduced":"591fc34e1f98b0d7eef4aa3440bfdff3c5a1cadd"},{"fixed":"a92332f32a8d31a7eee47b1dc1d751cb3319908f"},{"fixed":"a41075acde0124d2f8a5f563068a5d63e8ffd57b"}]}],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-63811.json"}},{"package":{"name":"Kernel","ecosystem":"Linux"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"6.4.0"},{"fixed":"7.1.3"}]}],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-63811.json"}}],"schema_version":"1.7.5"}