syzbot is reporting that BUG() in hfs_write_inode() fires upon unmount
operation when the inode number of the record retrieved as a result of
hfs_cat_find_brec(HFS_ROOT_CNID) is not HFS_ROOT_CNID, for
commit b905bafdea21 ("hfs: Sanity check the root record") checked
the record size and the record type but did not check the inode number.

Viacheslav Dubeyko considers that the fix should be in hfs_read_inode()
but Viacheslav has no time for proposing the fix [1]. Also, we can't
guarantee that the inode number of the record retrieved as a result of
hfs_cat_find_brec(HFS_ROOT_CNID) is HFS_ROOT_CNID if we validate only in
hfs_read_inode(). Therefore, while what Viacheslav would propose might
partially overwrap with my proposal, let's fix an 1000+ days old bug by
adding a sanity check in hfs_fill_super().

Reported-by: syzbot+97e301b4b82ae803d21b@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=97e301b4b82ae803d21b
Link: https://lkml.kernel.org/r/a3d1464ee40df7f072ea1c19e1ccf533e34554ca.camel@ibm.com [1]
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
---
 fs/hfs/super.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/hfs/super.c b/fs/hfs/super.c
index 388a318297ec..ae6dbc4bb813 100644
--- a/fs/hfs/super.c
+++ b/fs/hfs/super.c
@@ -354,7 +354,7 @@ static int hfs_fill_super(struct super_block *sb, struct fs_context *fc)
 			goto bail_hfs_find;
 		}
 		hfs_bnode_read(fd.bnode, &rec, fd.entryoffset, fd.entrylength);
-		if (rec.type != HFS_CDR_DIR)
+		if (rec.type != HFS_CDR_DIR || rec.dir.DirID != cpu_to_be32(HFS_ROOT_CNID))
 			res = -EIO;
 	}
 	if (res)
-- 
2.51.0

On Fri, 2025-09-12 at 23:59 +0900, Tetsuo Handa wrote:
> syzbot is reporting that BUG() in hfs_write_inode() fires upon unmount
> operation when the inode number of the record retrieved as a result of
> hfs_cat_find_brec(HFS_ROOT_CNID) is not HFS_ROOT_CNID, for
> commit b905bafdea21 ("hfs: Sanity check the root record") checked
> the record size and the record type but did not check the inode number.
> 
> Viacheslav Dubeyko considers that the fix should be in hfs_read_inode()
> but Viacheslav has no time for proposing the fix [1]. Also, we can't
> guarantee that the inode number of the record retrieved as a result of
> hfs_cat_find_brec(HFS_ROOT_CNID) is HFS_ROOT_CNID if we validate only in
> hfs_read_inode(). Therefore, while what Viacheslav would propose might
> partially overwrap with my proposal, let's fix an 1000+ days old bug by
> adding a sanity check in hfs_fill_super().
> 

I cannot accept any fix with such comment. The commit message should explain the
issue and fix nature.

We are not in a hurry. We should fix the reason of the issue even if it is
"1000+ days old bug".

Thanks,
Slava.

> Reported-by: syzbot+97e301b4b82ae803d21b@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=97e301b4b82ae803d21b  
> Link: https://lkml.kernel.org/r/a3d1464ee40df7f072ea1c19e1ccf533e34554ca.camel@ibm.com   [1]
> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> ---
>  fs/hfs/super.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/hfs/super.c b/fs/hfs/super.c
> index 388a318297ec..ae6dbc4bb813 100644
> --- a/fs/hfs/super.c
> +++ b/fs/hfs/super.c
> @@ -354,7 +354,7 @@ static int hfs_fill_super(struct super_block *sb, struct fs_context *fc)
>  			goto bail_hfs_find;
>  		}
>  		hfs_bnode_read(fd.bnode, &rec, fd.entryoffset, fd.entrylength);
> -		if (rec.type != HFS_CDR_DIR)
> +		if (rec.type != HFS_CDR_DIR || rec.dir.DirID != cpu_to_be32(HFS_ROOT_CNID))
>  			res = -EIO;
>  	}
>  	if (res)

On 2025/09/16 7:14, Viacheslav Dubeyko wrote:
> On Fri, 2025-09-12 at 23:59 +0900, Tetsuo Handa wrote:
>> syzbot is reporting that BUG() in hfs_write_inode() fires upon unmount
>> operation when the inode number of the record retrieved as a result of
>> hfs_cat_find_brec(HFS_ROOT_CNID) is not HFS_ROOT_CNID, for
>> commit b905bafdea21 ("hfs: Sanity check the root record") checked
>> the record size and the record type but did not check the inode number.
>>
>> Viacheslav Dubeyko considers that the fix should be in hfs_read_inode()
>> but Viacheslav has no time for proposing the fix [1]. Also, we can't
>> guarantee that the inode number of the record retrieved as a result of
>> hfs_cat_find_brec(HFS_ROOT_CNID) is HFS_ROOT_CNID if we validate only in
>> hfs_read_inode(). Therefore, while what Viacheslav would propose might
>> partially overwrap with my proposal, let's fix an 1000+ days old bug by
>> adding a sanity check in hfs_fill_super().
>>
> 
> I cannot accept any fix with such comment. The commit message should explain the
> issue and fix nature.

Then, see v4 at https://lkml.kernel.org/r/427fcb57-8424-4e52-9f21-7041b2c4ae5b@I-love.SAKURA.ne.jp