linux-kernel - Re: introduce probe_slab

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Date:	Wed, 22 Oct 2014 16:08:45 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	oleg@...hat.com
Cc:	peterz@...radead.org, cl@...ux.com, tkhai@...dex.ru,
	ktkhai@...allels.com, linux-kernel@...r.kernel.org,
	mingo@...hat.com, vdavydov@...allels.com
Subject: Re: introduce probe_slab_address?

From: Oleg Nesterov <oleg@...hat.com>
Date: Wed, 22 Oct 2014 21:42:28 +0200

> Now the question: is this LOAD is safe in case when this (freed) page
> already has another mapping? This is black magic to me, I do not know.
> And Peter has some concerns.

It is immatieral, because you can read garbage and it's "don't care"
in this context.

And later if it is used again at this virtual address, it will be
initialized with stores at that virtual address first.

So no problem.

> And, say, copy_from_user_page() on sparc does
> 
> 	flush_cache_page();
> 	memcpy();
> 	flush_ptrace_access();

In this case, as I tried to explain, it matters because the physical
address is being accessed from two virtual address at the same time
"for the same usage".

That's what distinguishes this from the SLAB and RCU cases you cite.

Illegal aliases only matter within a specific usage of a piece of
memory.

So if a page is mapped into userspace, and we touch the kernel side
PAGE_OFFSET alias mapping of it, we can have problems.  And that is
what is happening in the ptrace case above.

But if we are using the page later in SLAB, the previous userspace
mapping will not cause problems wrt. aliasing.  It cannot, because
once SLAB gets the page it will initialize it at the new virtual
address.
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