Re: [PATCH v3 0/4] mm: clarify nofail memory allocation

[Barry Song <21cnbao@gmail.com>]

On Tue, Aug 20, 2024 at 7:33 AM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Mon, 19 Aug 2024 at 12:23, Barry Song <21cnbao@gmail.com> wrote:
> >
> >
> > That could be an exploit taking advantage of those improper callers,
>
> So?
>
> FIX THE BUGGY CODE.

That's definitely in progress, with patch 1/4 addressing vdpa. There's also
an RFC to enforce DIRECT_RECLAMATION for __GFP_NOFAIL, which
will prevent passing unsupported flags to the memory management
system:

https://lore.kernel.org/all/20240724085544.299090-6-21cnbao@gmail.com/

>
> Don't make insane and incorrect changes to the MM code and spread
> Fear, Uncertainty and Doubt.
>
> > thus it wouldn’t necessarily result in an immediate oops in callers but
> > result in an exploit
>
> No. Any bug can be an exploit. Don't try to make this something
> special by calling it an exploit.
>
> NULL pointer dereferences are some of the *least* worrisome bugs,
> because we don't allow people to mmap the NULL area anyway.
>
> So just stop spreading FUD. We don't improve the kernel by making
> excuses for bugs, we improve it by fixing things.
>
> And any caller that asks for NOFAIL with bad parameters is buggy. The
> MM code should NOT try to fix it up, and dammit, BUG_ON() is not
> acceptable as a debugging help. Never was, never will be.

Okay, I see your point. However, the discussion originally began with just
a simple WARN_ON() to flag improper usage:
https://lore.kernel.org/linux-mm/20240717230025.77361-1-21cnbao@gmail.com/

Now, it seems we've come full circle and are opting to use
WARN_ON_ONCE() instead?

>
> Worry-warts already do "reboot-on-warn".
>
>             Linus

Thanks
Barry