Re: [syzbot] [hfs?] WARNING in hfs_write_inode

[Linus Torvalds]

On Wed, Jan 4, 2023 at 6:43 AM Arnd Bergmann <arnd@xxxxxxxx> wrote:
>
> My patch was a mechanical conversion from '/* panic? */'
> to 'WARN_ON()' to work around a compiler warning,
> and the previous code had been in there since the
> 2004 HFS rewrite by Roman Zippel.

Yeah. Looking at the code, the warning does make sense, in that the
code then does a

   hfs_bnode_write(...)

with the updated data, using the size of "struct hfs_cat_file", so
checking that the entry has that length does seem somewhat sensible.

At the same time, the HFS_IS_RSRC(inode) case in the same function
never had that "panic ?" thing, and obviously the two cases that *do*
have the check never actually did anything (since 2004, as you point
out - you have to go back to before the git days to even see any
development in this area).

> I know nothing about what this function actually does,
> so my best answer is that we could revert my patch
> and use pr_debug() instead of WARN_ON() for all of these.

Looks like this is syzbot just mounting a garbage image (or is it
actually some real hfs thing?)

I'm not sure a pr_debug() would even be appropriate. I think "return
-EIO" (but with a hfs_find_exit()) is likely the right answer. We've
done that in the past (see commit 8d824e69d9f3: "hfs: fix OOB Read in
__hfs_brec_find").

I suspect this code is basically all dead. From what I can tell, hfs
only gets updates for

 (a) syzbot reports

 (b) vfs interface changes

and the last real changes seem to have been by Ernesto A. Fernández
back in 2018.

Hmm. Looking at that code, we have another bug in there, introduced by
an earlier fix for a similar issue: commit 8d824e69d9f3 ("hfs: fix OOB
Read in __hfs_brec_find") added

+       if (HFS_I(main_inode)->cat_key.CName.len > HFS_NAMELEN)
+               return -EIO;

but it's after hfs_find_init(), so it should actually have done a
hfs_find_exit() to not leak memory.

So we should probably fix that too.

Something like this ENTIRELY UNTESTED patch?

Do we have anybody who looks at hfs?

                      Linus
 fs/hfs/inode.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/fs/hfs/inode.c b/fs/hfs/inode.c
index 9c329a365e75..3a155c1d810e 100644
--- a/fs/hfs/inode.c
+++ b/fs/hfs/inode.c
@@ -458,15 +458,16 @@ int hfs_write_inode(struct inode *inode, struct writeback_control *wbc)
 		/* panic? */
 		return -EIO;
 
+	res = -EIO;
 	if (HFS_I(main_inode)->cat_key.CName.len > HFS_NAMELEN)
-		return -EIO;
+		goto out;
 	fd.search_key->cat = HFS_I(main_inode)->cat_key;
 	if (hfs_brec_find(&fd))
-		/* panic? */
 		goto out;
 
 	if (S_ISDIR(main_inode->i_mode)) {
-		WARN_ON(fd.entrylength < sizeof(struct hfs_cat_dir));
+		if (fd.entrylength < sizeof(struct hfs_cat_dir))
+			goto out;
 		hfs_bnode_read(fd.bnode, &rec, fd.entryoffset,
 			   sizeof(struct hfs_cat_dir));
 		if (rec.type != HFS_CDR_DIR ||
@@ -479,6 +480,8 @@ int hfs_write_inode(struct inode *inode, struct writeback_control *wbc)
 		hfs_bnode_write(fd.bnode, &rec, fd.entryoffset,
 			    sizeof(struct hfs_cat_dir));
 	} else if (HFS_IS_RSRC(inode)) {
+		if (fd.entrylength < sizeof(struct hfs_cat_file))
+			goto out;
 		hfs_bnode_read(fd.bnode, &rec, fd.entryoffset,
 			       sizeof(struct hfs_cat_file));
 		hfs_inode_write_fork(inode, rec.file.RExtRec,
@@ -486,7 +489,8 @@ int hfs_write_inode(struct inode *inode, struct writeback_control *wbc)
 		hfs_bnode_write(fd.bnode, &rec, fd.entryoffset,
 				sizeof(struct hfs_cat_file));
 	} else {
-		WARN_ON(fd.entrylength < sizeof(struct hfs_cat_file));
+		if (fd.entrylength < sizeof(struct hfs_cat_file))
+			goto out;
 		hfs_bnode_read(fd.bnode, &rec, fd.entryoffset,
 			   sizeof(struct hfs_cat_file));
 		if (rec.type != HFS_CDR_FIL ||
@@ -503,9 +507,10 @@ int hfs_write_inode(struct inode *inode, struct writeback_control *wbc)
 		hfs_bnode_write(fd.bnode, &rec, fd.entryoffset,
 			    sizeof(struct hfs_cat_file));
 	}
+	res = 0;
 out:
 	hfs_find_exit(&fd);
-	return 0;
+	return res;
 }
 
 static struct dentry *hfs_file_lookup(struct inode *dir, struct dentry *dentry,