On Tue, Aug 20, 2024 at 7:33 AM Linus Torvalds
[off-list ref] wrote:

On Mon, 19 Aug 2024 at 12:23, Barry Song [off-list ref] wrote:

quoted

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/ (local)

Don't make insane and incorrect changes to the MM code and spread
Fear, Uncertainty and Doubt.

quoted

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/ (local)

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