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

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Wed, Apr 17, 2024 at 04:54:46PM +0200, Jan Kara wrote:
> On Wed 17-04-24 15:30:03, Greg Kroah-Hartman wrote:
> > 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.
> 
> OK, understood. But let me state that with the current state of affairs in
> the filesystem land, it will not take a determined attacker long to get
> arbitrary code execution out of "maliciously corrupted fs mounted". The
> code of most filesystems has simply never been written with the assumption
> that it can be presented with malicious data and we have hundreds of
> thousands lines of code like that. We have fixed the most glaring problems
> but by far not all (partly because of performance and maintenance costs,
> partly because they are baked into on-disk formats).

I totally agree.  It's up to the distros to stop doing this if they wish
to stop this problem from happening.

thanks,

greg k-h