linux-kernel - Re: [PATCH 1/1] mm/madvise: initialize prev pointer in madvise_walk

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <9e80cea5-fa78-46a2-b2bd-2f3846c24a34@lucifer.local>
Date: Tue, 17 Jun 2025 09:34:21 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Lance Yang <ioworker0@...il.com>, akpm@...ux-foundation.org,
        21cnbao@...il.com, Liam.Howlett@...cle.com, vbabka@...e.cz,
        jannh@...gle.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Lance Yang <lance.yang@...ux.dev>
Subject: Re: [PATCH 1/1] mm/madvise: initialize prev pointer in
 madvise_walk_vmas

On Tue, Jun 17, 2025 at 10:28:58AM +0200, David Hildenbrand wrote:
> On 17.06.25 10:21, Lorenzo Stoakes wrote:
> > On Tue, Jun 17, 2025 at 09:54:29AM +0200, David Hildenbrand wrote:
> > > On 17.06.25 04:05, Lance Yang wrote:
> > > > From: Lance Yang <lance.yang@...ux.dev>
> > > >
> > > > The prev pointer was uninitialized, which could lead to undefined behavior
> > > > where its address is taken and passed to the visit() callback without being
> > > > assigned a value.
> > >
> > > So, we are passing the pointer value to visit(), which is not undefined
> > > behavior.
> > >
> > > The issue would be if anybody takes a look at the value stored at that
> > > pointer. Because, already passing an uninitialized value to a (non-inlined)
> > > function is undefined behavior according to C.
> > >
> > > In madvise_update_vma()->vma_modify_flags_name() we do exactly that,
> > > correct?
> >
> > Err the parameter there is struct vm_area_struct **prev...
> >
> > We deref to the prev ptr which is unassigned yes but the pointer to the pointer isn't...
> >
>
> struct vm_area_struct *prev;
>
> is uninitialized.
>
> We pass &prev -> prevp, which now points at something uninitialized.
>
> Doing "*prevp =" is fine, because we will initialize.
>
> Doing "= *prep" is not fine, because the value was not initialized.

Yep sorry pre-coffee.

>
> > >
> > > 	vma = vma_modify_flags_name(&vmi, *prev, ...
> > >
> > > We should use Fixes: then.
> >
> > So no we shouldn't...
> >
> > >
> > >
> > > Acked-by: David Hildenbrand <david@...hat.com>
> >
> > Sure? :)
>
> Unless I am missing something important, yes :)

This solution isn't correct as prev == NULL when prev != NULL is wholly
incorrect.

We need a better solution.

>
> --
> Cheers,
>
> David / dhildenb
>