[RFC PATCH] drivers/char: kmem: disable read/write if VMALLOC_START < PAGE_OFFSET

[will.deacon@arm.com (Will Deacon)]

On Fri, Jun 09, 2017 at 04:32:28PM +0000, Ard Biesheuvel wrote:
> As it turns out, arm64 deviates from other architectures in the way it
> maps the VMALLOC region: on most (all?) other architectures, it resides
> strictly above the kernel's direct mapping of DRAM, but on arm64, this
> is the other way around. For instance, for a 48-bit VA configuration,
> we have
> 
>   modules : 0xffff000000000000 - 0xffff000008000000   (   128 MB)
>   vmalloc : 0xffff000008000000 - 0xffff7dffbfff0000   (129022 GB)
>   ...
>   vmemmap : 0xffff7e0000000000 - 0xffff800000000000   (  2048 GB maximum)
>             0xffff7e0000000000 - 0xffff7e0003ff0000   (    63 MB actual)
>   memory  : 0xffff800000000000 - 0xffff8000ffc00000   (  4092 MB)
> 
> This has mostly gone unnoticed until now, but it does appear that it
> breaks an assumption in the kcore read/write code, which does something
> like
> 
>   if (p < (unsigned long) high_memory) {
>     ... use straight copy_[to|from]_user() using p as virtual address ...
>   }
>   ...
>   if (count > 0) {
>     ... use vread/vwrite for accesses past high_memory ...
>   }
> 
> The first condition will inadvertently hold for the VMALLOC region if
> VMALLOC_START < PAGE_OFFSET, but the read/write will subsequently fail
> the virt_addr_valid() check, resulting in a -ENXIO return value.
> 
> Given how kmem seems to be living in borrowed time anyway, and given
> the fact that nobody noticed that the read/write interface is broken
> on arm64 in the first place, let's not bother trying to fix it, but
> simply fail such calls with a warning if VMALLOC_START < PAGE_OFFSET.
> (Note that kmem's mmap() interface is not affected by this)
> 
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> ---
> 
> This is just an RFC. There may be better ways to deal with this, including
> disabling /dev/kmem altogether on arm64.

FWIW: I'm fine with either approach.

Will