linux-kernel - Re: Fwd: Memory corruption in multithreaded user space program while calling fork

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Date:   Sat, 8 Jul 2023 10:29:42 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Thorsten Leemhuis <regressions@...mhuis.info>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Bagas Sanjaya <bagasdotme@...il.com>,
        Jacob Young <jacobly.alt@...il.com>,
        Laurent Dufour <ldufour@...ux.ibm.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Memory Management <linux-mm@...ck.org>,
        Linux PowerPC <linuxppc-dev@...ts.ozlabs.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Linux regressions mailing list <regressions@...ts.linux.dev>
Subject: Re: Fwd: Memory corruption in multithreaded user space program while
 calling fork

On Sat, 8 Jul 2023 at 04:35, Thorsten Leemhuis
<regressions@...mhuis.info> wrote:
>
> The plan since early this week is to mark CONFIG_PER_VMA_LOCK as broken;
> latest patch that does this is this one afaics:

Bah.

Both marking it as broken and the pending fix seems excessive.

Why isn't the trivial fix just to say "yes, fork() gets the mmap_lock
for writing for a reason, and that reason is that it acts kind of like
mprotect()".

And then just do what those functions do.

IOW, why isn't the fix just to do

  --- a/kernel/fork.c
  +++ b/kernel/fork.c
  @@ -686,6 +686,7 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
        for_each_vma(old_vmi, mpnt) {
                struct file *file;

  +             vma_start_write(mpnt);
                if (mpnt->vm_flags & VM_DONTCOPY) {
                        vm_stat_account(mm, mpnt->vm_flags, -vma_pages(mpnt));
                        continue;

and be done with this? Yes, we could move it down a bit more, ignoring
the VM_DONTCOPY vma's, but they are so uncommon as to not matter, so
who cares?

              Linus