Re: CVE-2023-52451: powerpc/pseries/memhp: Fix access beyond end of drmem array

[Michal Hocko]

On Mon 26-02-24 17:12:19, Greg KH wrote:
> On Mon, Feb 26, 2024 at 04:25:09PM +0100, Michal Hocko wrote:
> > On Mon 26-02-24 16:06:51, Greg KH wrote:
> > > On Mon, Feb 26, 2024 at 03:52:11PM +0100, Michal Hocko wrote:
> > > > On Thu 22-02-24 17:21:58, Greg KH wrote:
> > > > > Description
> > > > > ===========
> > > > > 
> > > > > In the Linux kernel, the following vulnerability has been resolved:
> > > > > 
> > > > > powerpc/pseries/memhp: Fix access beyond end of drmem array
> > > > > 
> > > > > dlpar_memory_remove_by_index() may access beyond the bounds of the
> > > > > drmem lmb array when the LMB lookup fails to match an entry with the
> > > > > given DRC index. When the search fails, the cursor is left pointing to
> > > > > &drmem_info->lmbs[drmem_info->n_lmbs], which is one element past the
> > > > > last valid entry in the array. The debug message at the end of the
> > > > > function then dereferences this pointer:
> > > > > 
> > > > >         pr_debug("Failed to hot-remove memory at %llx\n",
> > > > >                  lmb->base_addr);
> > > > 
> > > > While this is a reasonable fix and the stable material it is really
> > > > unclear to me why it has gained a CVE. Memory hotplug is a privileged
> > > > operation. Could you clarify please?
> > > 
> > > As you know, history has shown us that accessing out of your allocated
> > > memory can cause problems, and we can not assume use-cases, as we don't
> > > know how everyone uses our codebase, so marking places where we fix
> > > out-of-bound memory accesses is resolving a weakness in the codebase,
> > > hence a CVE assignment.
> > 
> > Does that mean that any potentially incorrect input provided by an admin is
> > considered CVE now? I guess we would need to ban interfaces like
> > /dev/mem and many others.
> 
> If you have your system set up to prevent admins from accessing /dev/mem
> (isn't there a config option for that), and you can access it, then yes,
> that would be a CVE-worthy issue.
> 
> But if you configure your system to allow an admin access to /dev/mem
> then you wanted that :)

OK, I thought we are having a serious discussion here. Now I am not
really sure about that.

> > > If your systems are not vulnerable to this specific issue, wonderful, no
> > > need to take it, but why wouldn't you want to take a fix that resolves a
> > > known weakness?
> > 
> > I have explicitly said, the fix is reasonable. I just do not see a point
> > to mark it as CVE. I fail to see any thread model where this would
> > matter as it would require untrusted privileged actor to trigger it
> > AFAICS. I am happy to be proven wrong here.
> 
> We can not determine threat models when filing CVEs as we do not know
> what your threat model is.  All we can do is determine if this resolves
> a weakness in the system.  A use-after-free is a weakness and this
> resolves that issue.

Sure, you cannot make any assumptions and nobody questions that. We do
have a certain common sense though. And if somebody comes up with a
usecase that we haven't anticipated then we can surely assign a CVE.
I though that level of control is exactly the reason to own the process
by the community.

All that being said I dispute the issue fixed here has any more security
relevance than allowing untrusted user to control memory hotplug which
could easily result in DoS of the system.
-- 
Michal Hocko
SUSE Labs