Re: [PATCH 1/1] x86/mm: Fix limit mmap() of /dev/mem to valid physical addresses

Thomas Gleixner
2019-03-23T19:02:43Z

Ralph,

On Mon, 18 Mar 2019, rcampbell@nvidia.com wrote:
> From: Ralph Campbell <rcampbell@nvidia.com>
>
> If CONFIG_DEBUG_VIRTUAL is enabled, a read or write to /dev/mem can
> trigger a VIRTUAL_BUG_ON() depending on the value of high_memory.
> For example:
>
> read_mem()
> valid_phys_addr_range(p=401f1550, count=8)
> __pa(high_memory)
> __phys_addr(x=ffffc88000000000)
> // __START_KERNEL_map = ffffffff80000000
> // y = ffffc88000000000 - ffffffff80000000
> VIRTUAL_BUG_ON(phys_addr_valid(400000000000))
> // boot_cpu_data.x86_phys_bits=46

I have no idea why all the irrelevant information in this example would be
helpful, but after extracting the meat I think I know what you want to say.

> Since by design high_memory is outside the range of valid physical
> addresses, use the non-error checking version __pa_nodebug(high_memory).

high_memory is not outside the range of valid physical addresses by
design. It's only outside when memory is populated right at the end of the
physical address space.

So what you really want to say in the changelog is:

valid_phys_addr_range() is used to sanity check the physical address range
of an operation, e.g. access to /dev/mem. It uses __pa(high_memory)
internally.

If memory is populated at the end of the physical address space, then
__pa(high_memory) is outside of the physical address space because:

high_memory = (void *)__va(max_pfn * PAGE_SIZE - 1) + 1;

For the comparison in valid_phys_addr_range() this is not an issue, but if
CONFIG_DEBUG_VIRTUAL is enabled, __pa() maps to __phys_addr(), which
verifies that the resulting physical address is within the valid physical
address space of the CPU. So in the case that memory is populated at the
end of the physical address space, this is not true and triggers a
VIRTUAL_BUG_ON().

Use ... instead, because ...

> Fixes: be62a32044061cb4a3b70a10598e093f1319102e ("x86/mm: Limit mmap() of

Please limit the sha1 to the first 12 characters.

> /dev/mem to valid physical addresses")
>

No newline between Fixes and the rest please.

> Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>

> --- a/arch/x86/mm/mmap.c
> +++ b/arch/x86/mm/mmap.c
> @@ -230,7 +230,7 @@ bool mmap_address_hint_valid(unsigned long addr, unsigned long len)
> /* Can we access it for direct reading/writing? Must be RAM: */
> int valid_phys_addr_range(phys_addr_t addr, size_t count)
> {
> - return addr + count <= __pa(high_memory);
> + return addr + count <= __pa_nodebug(high_memory);

This lacks a comment. Aside of that I think there is no point in using
__pa(high_memory) here. This is all about the physical address range. So
this can be simply expressed via:

return addr + count <= max_pfn * PAGE_SIZE;

which is much more obvious.

Thanks,

tglx