Re: [PATCH] [4/8] CPA: Fix set_memory_x for ioremap

Ingo Molnar
2008-02-11T12:27:41Z

* Andi Kleen <ak@suse.de> wrote:

> EFI currently calls set_memory_x() on potentially ioremapped
> addresses.
>
> This is problematic for several reasons:
>
> - The cpa code internally calls __pa on it which does not work for
> remapped addresses and will give some random result.

Wrong. We do call __pa() on vmalloc ranges (which is a known
uncleanliness that we intend to fix), but contrary to your claim the
result is not "random result". On 64-bit it's guaranteed to have a value
above ~66 TB on 64-bit and hence fails all the filters later on so it
has zero practical relevance at the moment. On 32-bit we transform it
down to somewhere around 1GB - where we check it against the BIOS range
filters - which again cannot trigger. But I do agree that it's unclean
and needs fixing up.

Detailed analysis on 64-bit: we call __pa() here:

static int change_page_attr_addr(struct cpa_data *cpa)
...
unsigned long phys_addr = __pa(address);

which for vmalloc area virtual addresses will indeed yield some really
high (and invalid) physical address. That address will never trigger
this check:

if (within(address, HIGH_MAP_START, HIGH_MAP_END))
address = (unsigned long) __va(phys_addr);

or this check:

if (within(phys_addr, 0, KERNEL_TEXT_SIZE)) {

so we'll never actuall _use_ that phys_addr.

> - cpa will try to change all potential aliases (like the kernel
> mapping on x86-64), but that is not needed for NX because the caller
> does only needs its specific virtual address executable. There is no
> requirement in the x86 architecture for nx bits to be coherent between
> mapping aliases. Also with the previous problem of __pa returning a
> wrong address it would likely try to change some random other page if
> you're unlucky and the random result would match the kernel text
> range.

wrong. That "random other page" is guaranteed to be above 66 TB
physical.

Anyway, i agree that it's ugly and unintuitive and it's on our clean-up
list. But your patch is not a good cleanup because it just hides the
underlying weakness.

Ingo