Re: CVE-2024-26774: ext4: avoid dividing by 0 in mb_update_avg_fragment_size() when block bitmap corrupt

[Greg Kroah-Hartman]

On Wed, Apr 17, 2024 at 01:43:24PM +0200, Jan Kara wrote:
> Hello!
> 
> On Wed 03-04-24 19:31:41, Greg Kroah-Hartman wrote:
> > Description
> > ===========
> > 
> > In the Linux kernel, the following vulnerability has been resolved:
> > 
> > ext4: avoid dividing by 0 in mb_update_avg_fragment_size() when block bitmap corrupt
> > 
> > Determine if bb_fragments is 0 instead of determining bb_free to eliminate
> > the risk of dividing by zero when the block bitmap is corrupted.
> > 
> > The Linux kernel CVE team has assigned CVE-2024-26774 to this issue.
> 
> I'd like to understand what is the imagined security threat fixed by this
> patch (as multiple patches of similar nature got assigned a CVE). The patch
> fixes a bug that if a corrupted filesystem is read-write mounted, we can do
> division-by-zero. Now if you can make the system mount a corrupted
> filesystem, you can do many interesting things to the system other than
> create a division by zero... So what is the presumed threat model here?

Exactly what you said, "if you mount a corrupted file system, you will
get a divide by zero fault."

Many systems auto-mount any filesystem plugged into it.  If yours do
not, then yours does not need to worry about this type of CVE.

thanks,

greg k-h