Re: [PATCH 6.4 7/8] fork: lock VMAs of the parent process when forking

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

On Sun, Jul 09, 2023 at 02:39:00PM +0200, Thorsten Leemhuis wrote:
> On 09.07.23 13:14, Greg Kroah-Hartman wrote:
> > From: Suren Baghdasaryan <surenb@google.com>
> > 
> > commit 2b4f3b4987b56365b981f44a7e843efa5b6619b9 upstream.
> > 
> > Patch series "Avoid memory corruption caused by per-VMA locks", v4.
> > 
> > A memory corruption was reported in [1] with bisection pointing to the
> > patch [2] enabling per-VMA locks for x86.  Based on the reproducer
> > provided in [1] we suspect this is caused by the lack of VMA locking while
> > forking a child process.
> > [...]
> 
> Question from someone that is neither a C nor a git expert -- and thus
> might say something totally stupid below (and thus maybe should not have
> sent this mail at all).
> 
> But I have to wonder: is adding this patch to stable necessary given
> patch 8/8?
> 
> FWIW, this change looks like this:
> 
> > ---
> >  kernel/fork.c |    6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > --- a/kernel/fork.c
> > +++ b/kernel/fork.c
> > @@ -662,6 +662,12 @@ static __latent_entropy int dup_mmap(str
> >  		retval = -EINTR;
> >  		goto fail_uprobe_end;
> >  	}
> > +#ifdef CONFIG_PER_VMA_LOCK
> > +	/* Disallow any page faults before calling flush_cache_dup_mm */
> > +	for_each_vma(old_vmi, mpnt)
> > +		vma_start_write(mpnt);
> > +	vma_iter_set(&old_vmi, 0);
> > +#endif
> >  	flush_cache_dup_mm(oldmm);
> >  	uprobe_dup_mmap(oldmm, mm);
> >  	/*
> 
> But when I look at kernel/fork.c in mainline I can't see this bit. I
> also only see Linus' change (e.g. patch 8/8 in this series) when I look at
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/log/kernel/fork.c

Look at 946c6b59c56d ("Merge tag 'mm-hotfixes-stable-2023-07-08-10-43'
of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm")

Where Linus manually dropped those #ifdefs.

Hm, I'll leave them for now in 6.4.y as that is "safer", but if Suren
feels comfortable, I'll gladly take a patch from him to drop them in the
6.4.y tree as well.

thanks,

greg k-h