linux-kernel - Re: [PATCH 4.1 11/86] iommu/amd: Fix BUG when faulting a PROT

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Date:	Fri, 6 Nov 2015 12:55:08 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	stable <stable@...r.kernel.org>, Jay Cornwall <jay@...rnwall.me>,
	Joerg Roedel <jroedel@...e.de>, linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH 4.1 11/86] iommu/amd: Fix BUG when faulting a PROT_NONE VMA

On Fri, Nov 6, 2015 at 12:49 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> And some "handle_mm_fault would BUG_ON()" comment is just bogus. It's
> not handle_mm_fault()'s case that you called it without checking
> proper permissions.

Side note: as to why handle_mm_fault() doesn't just do things itself,
there's a historical situation where we used to let people do things
in ptrace() that they couldn't do directly, and punch through
protections (and turn shared read-only pages into a dirty private
page).

So the permissions checking was up to the caller, because some callers
could do things that other callers could not.

I *think* we have gotten rid of all those cases, and I guess we could
consider just making handle_mm_fault() itself stricter. But that's the
historical background on why callers need to check this.

Adding linux-mm to the cc, to see if anybody there has some comments
wrt just moving all the EFAULT handling into handle_mm_fault() and
relaxing the caller requirements.

               Linus
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