Re: [PATCH 1/1] x86/ioremap: Use is_vmalloc_addr in iounmap

Dan Williams
2024-08-09T03:55:06Z

Alistair Popple wrote:
>
> Dan Williams <dan.j.williams@intel.com> writes:
>
> > [ add Alistair and John ]
> >
> > Thomas Gleixner wrote:
> >> On Thu, Aug 08 2024 at 09:39, Dan Williams wrote:
> >> > Dan Williams wrote:
> >> >> Apologies was trying to quickly reverse engineer how private memory
> >> >> might be different than typical memremap_pages(), but it is indeed the
> >> >> same in this aspect.
> >> >>
> >> >> So the real difference is that the private memory case tries to
> >> >> allocate physical memory by searching for holes in the iomem_resource
> >> >> starting from U64_MAX. That might explain why only the private memory
> >> >> case is violating assumptions with respect to high_memory spilling into
> >> >> vmalloc space.
> >> >
> >> > Not U64_MAX, but it starts searching for free physical address space
> >> > starting at MAX_PHYSMEM_BITS, see gfr_start().
> >>
> >> Wait. MAX_PHYSMEM_BITS is either 46 (4-level) or 52 (5-level), which is
> >> fully covered by the identity map space.
> >>
> >> So even if the search starts from top of that space, how do we end up
> >> with high_memory > VMALLOC_START?
> >>
> >> That does not make any sense at all
> >
> > Max, or Alistair can you provide more details of how private memory spills over
> > into the VMALLOC space on these platforms?
>
> Well I was hoping pleading ignorance on x86 memory maps would get me out
> of having to look too deeply :-) But alas...
>
> It appears the problem originates in KASLR which can cause the VMALLOC
> region to overlap with the top of the linear map.
>
> > I too would have thought that MAX_PHYSMEM_BITS protects against this?
>
> Me too, until about an hour ago. As noted above
> request_free_mem_region() allocates from (1 << MAX_PHYSMEM_BITS) - 1
> down. Therefore VMALLOC_START needs to be greater than PAGE_OFFSET + (1
> << MAX_PHYSMEM_BITS) - 1. However the default configuration for KASLR
> as set by RANDOMIZE_MEMORY_PHYSICAL_PADDING is to only provide 10TB
> above what max_pfn is set to at boot time (and even then only if
> CONFIG_MEMORY_HOTPLUG is enabled).
>
> Obviously ZONE_DEVICE memory ends up being way above that and crosses
> into the VMALLOC region. So I think the actual fix is something like:
>
> ---
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index e36261b4ea14..c58d7b0f5bca 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -2277,6 +2277,7 @@ config RANDOMIZE_MEMORY_PHYSICAL_PADDING
> depends on RANDOMIZE_MEMORY
> default "0xa" if MEMORY_HOTPLUG
> default "0x0"
> + range 0x40 0x40 if GET_FREE_REGION
> range 0x1 0x40 if MEMORY_HOTPLUG
> range 0x0 0x40
> help

Oh, good find! Alternatively if you wanted some kaslr + private memory
then something like this? (untested):

diff --git a/kernel/resource.c b/kernel/resource.c
index 14777afb0a99..2eeb8d8a40d4 100644
--- a/kernel/resource.c
+++ b/kernel/resource.c
@@ -1824,10 +1824,11 @@ static resource_size_t gfr_start(struct resource *base, resource_size_t size,
resource_size_t align, unsigned long flags)
{
if (flags & GFR_DESCENDING) {
+ u64 kaslr_pad = CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING << 40;
resource_size_t end;

end = min_t(resource_size_t, base->end,
- (1ULL << MAX_PHYSMEM_BITS) - 1);
+ (1ULL << MAX_PHYSMEM_BITS) - kaslr_pad - 1);
return end - size + 1;
}