linux-kernel - Re: 2.6.29 pat issue

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Message-ID: <m1tz78pdwy.fsf@fess.ebiederm.org>
Date:	Thu, 05 Feb 2009 17:11:41 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Thomas Hellstrom <thellstrom@...are.com>
Cc:	"Pallipadi\, Venkatesh" <venkatesh.pallipadi@...el.com>,
	Linux kernel mailing list <linux-kernel@...r.kernel.org>,
	"Siddha\, Suresh B" <suresh.b.siddha@...el.com>
Subject: Re: 2.6.29 pat issue

Thomas Hellstrom <thellstrom@...are.com> writes:

> Indeed, it's crucial to keep the mappings consistent, but failure to do so is a
> kernel driver bug, it should never be the result of invalid user data.

It easily can be.  Think of an X server mmaping frame buffers. Or other
device bars.

> There are other more common kernel bugs that can be even worse and hang / crash
> the system. For example using uninitialized spinlocks, writing to kfreed memory
> etc. There is code in the kernel to detect these as well, but this code is
> behind debug defines.

There are cpu errata on almost every cpu in existence that come into play
if you have the cacheabilty attributes wrong on a page.  CPUs have been known
to do very weird things when you hit those errata.  At the best of it you
are no longer running on a deterministic machine, at the worst of it you
crash the cpu.

> IMHO checking each vm_insert_pfn() for caching attribute correctness is not
> something that should be enabled by default, due to the CPU overhead. Production
> drivers should never violate this.

If it is a problem the implementation should become more efficient.  Userspace
as well as drivers can generate these mappings so even with a perfect driver
you cannot guarantee that someone else does not have that area of memory
mapped differently.

Eric
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