Re: [PATCH v5] hfs: initialize B-tree inodes and validate catalog CNIDs

From: Viacheslav Dubeyko

Date: Mon Oct 19 2026 - 19:48:23 EST

Hi Yangguikun,

Could you please resend the v6 patch with proper subject? Otherwise, it
looks like a mess. I see it as Re: [PATCH v5] hfs: initialize B-tree
inodes and validate catalog CNIDs (but not as v6 patch).

The patch looks good, by the way. I think I would like to run the
xfstests for the v6 patch after receiving it as proper patch. ;) And I
will need to have the second patch too for the check.

Thanks,
Slava.

On Sat, 2026-10-17 at 13:14 +0800, yangguikun wrote:
> Hi Slava,
>
> Thanks for the detailed review.
>
> On set_nlink(): I removed the explicit call, but please note that it
> was a no-op: inode_init_always_gfp() sets __i_nlink = 1
> (fs/inode.c:238)
> for every new inode, and the original iget_locked() path in
> hfs_btree_open() never cleared it. So the B-tree inodes have had
> i_nlink == 1 since before this series, inode_generic_drop() returns
> false for them, and the final iput() in hfs_btree_close() already
> took
> the !drop branch with writeback. I could not construct the
> still-dirty-at-put_super scenario outside of an error in the umount
> sync; if you have a concrete case in mind, I am happy to test it. The
> redundant set_nlink() is still worth removing for clarity.
>
> tz_secondswest is only consumed by hfs_revalidate_dentry(), which
> runs
> from dentries; the B-tree inodes never get one, so I removed that
> initialization as well.
>
> I will use the WARN_ON_ONCE() form for the already-cached inode case.
>
> On hfs_iget(): agreed on both counts. Returning ERR_PTR() lets the
> corruption cases (-EIO) and allocation failure (-ENOMEM) stay
> distinguishable, and both callers currently guess: hfs_lookup()
> translates NULL to -EACCES and hfs_fill_super() reports -EINVAL.
> minix_iget() already works this way. In v5 I only did what the v4
> review suggested (-EIO), but the complete fix is the ERR_PTR()
> conversion, so I reverted the dir.c change and will send it as a
> separate patch with the caller updates.
>
> Patch v6 follows.
>
> Thanks,
> yangguikun
>
> From 014ad11799ffc5fb857c23337e26121ee571f76a Mon Sep 17 00:00:00
> 2001
> From: yangguikun <yangguikun19@xxxxxx>
> Date: Sat, 17 Oct 2026 01:49:17 +0800
> Subject: [PATCH v6] hfs: initialize B-tree inodes and validate
> catalog CNIDs
>
> hfs_btree_open() leaves the internal B-tree inodes with i_mode == 0.
> A crafted catalog record naming CNID 3 or 4 can return one of these
> cached inodes through hfs_iget(), triggering VFS_BUG_ON_INODE() in
> may_open() with CONFIG_DEBUG_VFS=y.
>
> Initialize the B-tree inodes with S_IFREG and their address-space
> operations before publishing them. Reject attempts to reuse an
> already-cached B-tree inode.
>
> Validate catalog CNIDs and record types before the inode-cache
> lookup,
> since cached inodes bypass hfs_read_inode() validation. This also
> prevents a crafted file record from aliasing the root directory
> inode.
>
> Found by syzkaller-style fuzzing of v7.2-rc7.
>
> Suggested-by: Viacheslav Dubeyko <slava@xxxxxxxxxxx>
> Link:
> https://lore.kernel.org/linux-fsdevel/tencent_24D928451FEFA8A35482EFFA9132A7305307@xxxxxx/
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: yangguikun <yangguikun19@xxxxxx>
> ---
> Changes in v6:
> - Drop redundant set_nlink(inode, 1); the link count remains 1.
> - Drop tz_secondswest initialization for B-tree inodes.
> - Use WARN_ON_ONCE() for an already-cached B-tree inode.
> - Defer hfs_iget() error propagation to a separate patch.
>
> diff --git a/fs/hfs/btree.c b/fs/hfs/btree.c
> index 41b4e8f..1891ad5 100644
> --- a/fs/hfs/btree.c
> +++ b/fs/hfs/btree.c
> @@ -155,44 +155,9 @@ struct hfs_btree *hfs_btree_open(struct
> super_block *sb, u32 id, btree_keycmp ke
> tree->cnid = id;
> tree->keycmp = keycmp;
>
> - tree->inode = iget_locked(sb, id);
> - if (!tree->inode)
> + tree->inode = hfs_system_iget(sb, id);
> + if (IS_ERR(tree->inode))
> goto free_tree;
> - BUG_ON(!(inode_state_read_once(tree->inode) & I_NEW));
> - {
> - struct hfs_mdb *mdb = HFS_SB(sb)->mdb;
> - HFS_I(tree->inode)->flags = 0;
> - mutex_init(&HFS_I(tree->inode)->extents_lock);
> - switch (id) {
> - case HFS_EXT_CNID:
> - hfs_inode_read_fork(tree->inode, mdb->drXTExtRec,
> mdb->drXTFlSize,
> -     mdb->drXTFlSize,
> be32_to_cpu(mdb->drXTClpSiz));
> - if (HFS_I(tree->inode)->alloc_blocks >
> - HFS_I(tree->inode)-
> >first_blocks) {
> - pr_err("invalid btree extent records\n");
> - unlock_new_inode(tree->inode);
> - goto free_inode;
> - }
> -
> - tree->inode->i_mapping->a_ops = &hfs_btree_aops;
> - break;
> - case HFS_CAT_CNID:
> - hfs_inode_read_fork(tree->inode, mdb->drCTExtRec,
> mdb->drCTFlSize,
> -     mdb->drCTFlSize,
> be32_to_cpu(mdb->drCTClpSiz));
> -
> - if (!HFS_I(tree->inode)->first_blocks) {
> - pr_err("invalid btree extent records (0
> size)\n");
> - unlock_new_inode(tree->inode);
> - goto free_inode;
> - }
> -
> - tree->inode->i_mapping->a_ops = &hfs_btree_aops;
> - break;
> - default:
> - BUG();
> - }
> - }
> - unlock_new_inode(tree->inode);
>
> mapping = tree->inode->i_mapping;
> folio = filemap_grab_folio(mapping, 0);
> diff --git a/fs/hfs/hfs_fs.h b/fs/hfs/hfs_fs.h
> index e250f87..8d52170 100644
> --- a/fs/hfs/hfs_fs.h
> +++ b/fs/hfs/hfs_fs.h
> @@ -163,6 +163,11 @@ extern int hfs_cat_move(u32 cnid, struct inode
> *src_dir,
> extern void hfs_cat_build_key(struct super_block *sb, btree_key
> *key,
>       u32 parent, const struct qstr *name);
>
> +static inline bool hfs_is_system_reserved_cnid(u32 cnid)
> +{
> + return cnid > HFS_ROOT_CNID && cnid < HFS_FIRSTUSER_CNID;
> +}
> +
> /*
> * Validate the CNID of a catalog record.
> */
> @@ -219,6 +224,7 @@ extern void hfs_inode_read_fork(struct inode
> *inode, struct hfs_extent *ext,
> u32 clump_size);
> extern struct inode *hfs_iget(struct super_block *sb, struct
> hfs_cat_key *key,
> hfs_cat_rec *rec);
> +extern struct inode *hfs_system_iget(struct super_block *sb, u32
> cnid);
> extern void hfs_evict_inode(struct inode *inode);
> extern void hfs_delete_inode(struct inode *inode);
>
> diff --git a/fs/hfs/inode.c b/fs/hfs/inode.c
> index 2aef3c3..a66de7b 100644
> --- a/fs/hfs/inode.c
> +++ b/fs/hfs/inode.c
> @@ -421,6 +421,69 @@ static int hfs_read_inode(struct inode *inode,
> void *data)
> return 0;
> }
>
> +static int hfs_system_read_inode(struct inode *inode)
> +{
> + struct hfs_sb_info *hsb = HFS_SB(inode->i_sb);
> + struct hfs_mdb *mdb = hsb->mdb;
> +
> + HFS_I(inode)->flags = 0;
> + HFS_I(inode)->rsrc_inode = NULL;
> + mutex_init(&HFS_I(inode)->extents_lock);
> +
> + inode->i_uid = hsb->s_uid;
> + inode->i_gid = hsb->s_gid;
> +
> + inode->i_mode = S_IFREG;
> + inode->i_mapping->a_ops = &hfs_btree_aops;
> +
> + switch (inode->i_ino) {
> + case HFS_EXT_CNID:
> + hfs_inode_read_fork(inode, mdb->drXTExtRec, mdb-
> >drXTFlSize,
> +     mdb->drXTFlSize,
> be32_to_cpu(mdb->drXTClpSiz));
> + if (HFS_I(inode)->alloc_blocks > HFS_I(inode)-
> >first_blocks) {
> + pr_err("invalid btree extent records\n");
> + return -EIO;
> + }
> + break;
> + case HFS_CAT_CNID:
> + hfs_inode_read_fork(inode, mdb->drCTExtRec, mdb-
> >drCTFlSize,
> +     mdb->drCTFlSize,
> be32_to_cpu(mdb->drCTClpSiz));
> + if (!HFS_I(inode)->first_blocks) {
> + pr_err("invalid btree extent records (0
> size)\n");
> + return -EIO;
> + }
> + break;
> + default:
> + return -EIO;
> + }
> +
> + return 0;
> +}
> +
> +struct inode *hfs_system_iget(struct super_block *sb, u32 cnid)
> +{
> + struct inode *inode;
> + int err;
> +
> + inode = iget_locked(sb, cnid);
> + if (!inode)
> + return ERR_PTR(-ENOMEM);
> +
> + if (WARN_ON_ONCE(!(inode_state_read_once(inode) & I_NEW))) {
> + iput(inode);
> + return ERR_PTR(-EEXIST);
> + }
> +
> + err = hfs_system_read_inode(inode);
> + if (err) {
> + iget_failed(inode);
> + return ERR_PTR(err);
> + }
> +
> + unlock_new_inode(inode);
> + return inode;
> +}
> +
> /*
> * __hfs_iget()
> *
> @@ -446,6 +509,11 @@ struct inode *hfs_iget(struct super_block *sb,
> struct hfs_cat_key *key, hfs_cat_
> default:
> return NULL;
> }
> +
> + if (!hfs_is_valid_cnid(cnid, rec->type) ||
> +     hfs_is_system_reserved_cnid(cnid))
> + return NULL;
> +
> inode = iget5_locked(sb, cnid, hfs_test_inode,
> hfs_read_inode, &data);
> if (inode && (inode_state_read_once(inode) & I_NEW))
> unlock_new_inode(inode);