Re: Catching use-after-free easily in linux kernel.

[Amit]

On Tue, 11 Mar 2025 at 10:12, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
>
> On Tue, Mar 11, 2025 at 08:46:36AM +0530, Amit wrote:
> > On Tue, Mar 11, 2025, 6:05 AM Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> >
> > > On Mon, Mar 10, 2025 at 01:24:54PM +0530, Amit wrote:
> > > > Hi,
> > > >
> > > > We can catch use-after-free easily if we do the following:
> > > >
> > > > kfree(x);
> > > > (x) = NULL;
> > > >
> > > > Now, if someone uses 'x' again then the kernel will crash and we will
> > > know where
> > > > the use-after-free is happening and then we can fix it.
> > >
> > > That assumes that no pointer is ever stored in more than one place.
> > > Which is very clearly false.
> > >
> >
> >
> > I will do some experiments and then I will reply if I find something.
> >
> > I will introduce a global macro and then change all kfree() to this macro
> > name using cscope probably.
> >
> > Then I will compile the kernel and run the new kernel and see if some crash
> > is happening or not.
>
> What would that test, exactly?  And why would that be any more useful than
> adding global variables named wank and magic and replacing every kfree(p) with
> ((magic = wank++),kfree(p))?  That also would not introduce any crashes...

If I don't find any kernel crashes then it's fine. But I will do the
experiment to check whether I am right or wrong. Theoretical
discussion about whether the kernel will crash or not in my experiment
won't be much useful probably.

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