__arm_ioremap creates page table with domain set to kernel

Catalin Marinas catalin.marinas at arm.com
Sat Jan 22 05:10:03 EST 2011

On 22 January 2011 03:46, Colin Cross <ccross at google.com> wrote:
> On Fri, Jan 21, 2011 at 7:37 PM, Nicolas Pitre <nico at fluxnic.net> wrote:
>> On Fri, 21 Jan 2011, Colin Cross wrote:
>>> If CONFIG_SMP is set, __arm_ioremap always creates a page table
>>> mapping by calling ioremap_page_range in lib/ioremap.c, and passes it
>>> the memory prot value but not the domain.  iormap_page_range
>>> eventually calls pte_alloc_kernel, which sets the domain to
>>> DOMAIN_KERNEL, instead of DOMAIN_IO.
>>> The kernel domain is normally set to client, the same as the IO
>>> domain, but it can get temporarily switched to manager mode.  When it
>>> is in manager mode, it ignores the memory protection bits, and the
>>> instruction prefetcher is allowed by the ARMv7 spec to ignore the XN
>>> (eXecute Never) bit and fetch from IO memory.  I don't know that this
>>> would ever actually occur, but the ARM spec says in B3.6.2:
>>> The XN attribute is not checked for domains marked as Manager.
>>> Read-sensitive memory must not be
>>> included in domains marked as Manager, because the XN bit does not
>>> prevent prefetches in these cases.
>>> If this is a real problem, I don't see any quick fix.  The domain bits
>>> are set in the pmd, so ioremapped memory can not share a pmd with
>>> regular vmalloc memory, and ioremap_page_range has no way to carry a
>>> domain to pte_alloc_kernel.
>> This has been fixed already. Have a look at:
>> |commit 247055aa21ffef1c49dd64710d5e94c2aee19b58
>> |Author: Catalin Marinas <catalin.marinas at arm.com>
>> |Date:   Mon Sep 13 16:03:21 2010 +0100
>> |
>> |    ARM: 6384/1: Remove the domain switching on ARMv6k/v7 CPUs
>> |
>> |[...]
> Ah, thanks.  For the linux-tegra-2.6.36 kernel I worked around the
> problem by statically mapping all of the IO regions and preventing
> ioremaps outside of the static mappings or physical memory.

The domains removal patch has been around for nearly two years, you
can get an earlier version (a 2.6.35 variant went into the Linaro
kernel as well.


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