Re: [PATCH 1/3] hfs/hfsplus: fix u32 overflow in check_and_correct_requested_length

[Viacheslav Dubeyko <vdubeyko@redhat.com>]

On Fri, 2026-05-01 at 11:02 +0000, Tristan Madani wrote:
> From: Tristan Madani <tristan@talencesecurity.com>
> 
> check_and_correct_requested_length() compares (off + len) against
> node_size using u32 arithmetic.  When the caller passes a large len
> value (e.g. from an underflowed subtraction in hfs_brec_remove()),
> off + len can wrap past 2^32 and produce a small result, causing the
> bounds check to pass when it should fail.
> 
> For example, with off=14 and len=0xFFFFFFF2 (underflowed from
> data_off - keyoffset - size in hfs_brec_remove), off + len wraps to 6,
> which is less than a typical node_size of 512, so the check passes and
> the subsequent memmove reads ~4GB past the node buffer.
> 
> Fix this by comparing len against (node_size - off) instead.  Since
> is_bnode_offset_valid() already guarantees off < node_size before this
> point, the subtraction cannot underflow.
> 
> Reported-by: syzbot+6df204b70bf3261691c5@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=6df204b70bf3261691c5
> Tested-by: syzbot+6df204b70bf3261691c5@syzkaller.appspotmail.com
> Reported-by: syzbot+e76bf3d19b85350571ac@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=e76bf3d19b85350571ac
> Tested-by: syzbot+e76bf3d19b85350571ac@syzkaller.appspotmail.com
> Fixes: a431930c9bac ("hfs: fix slab-out-of-bounds in hfs_bnode_read()")
> Cc: stable@vger.kernel.org
> Signed-off-by: Tristan Madani <tristan@talencesecurity.com>
> ---
>  fs/hfs/bnode.c          | 2 +-
>  fs/hfsplus/hfsplus_fs.h | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/hfs/bnode.c b/fs/hfs/bnode.c
> index 13d58c51fc46b..c00645a4a5733 100644
> --- a/fs/hfs/bnode.c
> +++ b/fs/hfs/bnode.c
> @@ -41,7 +41,7 @@ u32 check_and_correct_requested_length(struct hfs_bnode *node, u32 off, u32 len)
>  
>  	node_size = node->tree->node_size;
>  
> -	if ((off + len) > node_size) {
> +	if (len > node_size - off) {

I don't agree with likewise change. Probably, we need to have:

(u64)off + len

Thanks,
Slava.

>  		u32 new_len = node_size - off;
>  
>  		pr_err("requested length has been corrected: "
> diff --git a/fs/hfsplus/hfsplus_fs.h b/fs/hfsplus/hfsplus_fs.h
> index 3545b8dbf11c5..10b2dda3f8044 100644
> --- a/fs/hfsplus/hfsplus_fs.h
> +++ b/fs/hfsplus/hfsplus_fs.h
> @@ -600,7 +600,7 @@ u32 check_and_correct_requested_length(struct hfs_bnode *node, u32 off, u32 len)
>  
>  	node_size = node->tree->node_size;
>  
> -	if ((off + len) > node_size) {
> +	if (len > node_size - off) {
>  		u32 new_len = node_size - off;
>  
>  		pr_err("requested length has been corrected: "