Re: [PATCH v2] drivers/kmem: disable on arm64
From: Ard Biesheuvel
Date: Tue Jun 20 2017 - 02:48:25 EST
On 19 June 2017 at 17:03, Will Deacon <will.deacon@xxxxxxx> wrote:
> On Mon, Jun 19, 2017 at 04:37:24PM +0200, Ard Biesheuvel wrote:
>> On arm64, the /dev/kmem driver barely works, given that it assumes that
>> VMALLOC_START > PAGE_OFFSET, which is not the case on arm64. Due to the
> Probably worth being explicit about the failure mode here (the commit
> message you had in your RFC was much better ;).
Yeah, that's a fair point, I was being lazy :-)
I will repost with the v1 commit log integrated
>> dubious value of kmem on an architecture such as arm64, which is fairly
>> new and is not as forgiving as other architectures when it comes to
>> userland accesses to kernel memory using mismatched attributes*, let's
>> disable it altogether if CONFIG_ARM64=y.
>> * Mappings in the vmalloc region will be accessed using vread/vwrite when
>> reading/writing to/from /dev/kmem, which will perform these accesses by
>> kmap()'ing each page using writeback cacheable attributes. This will be
>> correct in most cases, but this is not guaranteed.
>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
>> drivers/char/Kconfig | 2 ++
>> 1 file changed, 2 insertions(+)
>> diff --git a/drivers/char/Kconfig b/drivers/char/Kconfig
>> index 31adbebf812e..8102ee7b3247 100644
>> --- a/drivers/char/Kconfig
>> +++ b/drivers/char/Kconfig
>> @@ -17,6 +17,8 @@ config DEVMEM
>> config DEVKMEM
>> bool "/dev/kmem virtual device support"
>> + # On arm64, VMALLOC_START < PAGE_OFFSET, which confuses kmem read/write
>> + depends on !ARM64
>> Say Y here if you want to support the /dev/kmem device. The
>> /dev/kmem device is rarely used, but can be used for certain