Re: [PATCH v8 0/4] Introduce mseal

[Jeff Xu]

On Thu, Feb 1, 2024 at 9:00 PM Theo de Raadt <deraadt@xxxxxxxxxxx> wrote:
>
> Jeff Xu <jeffxu@xxxxxxxxxxxx> wrote:
>
> > Even without free.
> > I personally do not like the heap getting sealed like that.
> >
> > Component A.
> > p=malloc(4096);
> > writing something to p.
> >
> > Component B:
> > mprotect(p,4096, RO)
> > mseal(p,4096)
> >
> > This will split the heap VMA, and prevent the heap from shrinking, if
> > this is in a frequent code path, then it might hurt the process's
> > memory usage.
> >
> > The existing code is more likely to use malloc than mmap(), so it is
> > easier for dev to seal a piece of data belonging to another component.
> > I hope this pattern is not wide-spreading.
> >
> > The ideal way will be just changing the library A to use mmap.
>
> I think you are lacking some test programs to see how it actually
> behaves; the effect is worse than you think, and the impact is immediately
> visible to the programmer, and the lesson is clear:
>
>         you can only seal objects which you gaurantee never get recycled.
>
>         Pushing a sealed object back into reuse is a disasterous bug.
>
>         Noone should call this interface, unless they understand that.
>
> I'll say again, you don't have a test program for various allocators to
> understand how it behaves.  The failure modes described in your docuemnts
> are not correct.
>
I understand what you mean: I will add that part to the document:
Try to recycle a sealed memory is disastrous, e.g.
p=malloc(4096);
mprotect(p,4096,RO)
mseal(p,4096)
free(p);

My point is:
I think sealing an object from the heap is a bad pattern in general,
even dev doesn't free it. That was one of the reasons for the sealable
flag, I hope saying this doesn't be perceived as looking for excuses.

>