Re: x86/kaslr: Expose and use the end of the physical memory address space

Dan Williams
2024-08-14T14:33:50Z

Thomas Gleixner wrote:
> iounmap() on x86 occasionally fails to unmap because the provided valid
> ioremap address is not below high_memory. It turned out that this
> happens due to KASLR.
>
> KASLR uses the full address space between PAGE_OFFSET and vaddr_end to
> randomize the starting points of the direct map, vmalloc and vmemmap
> regions. It thereby limits the size of the direct map by using the
> installed memory size plus an extra configurable margin for hot-plug
> memory. This limitation is done to gain more randomization space
> because otherwise only the holes between the direct map, vmalloc,
> vmemmap and vaddr_end would be usable for randomizing.
>
> The limited direct map size is not exposed to the rest of the kernel, so
> the memory hot-plug and resource management related code paths still
> operate under the assumption that the available address space can be
> determined with MAX_PHYSMEM_BITS.
>
> request_free_mem_region() allocates from (1 << MAX_PHYSMEM_BITS) - 1
> downwards. That means the first allocation happens past the end of the
> direct map and if unlucky this address is in the vmalloc space, which
> causes high_memory to become greater than VMALLOC_START and consequently
> causes iounmap() to fail for valid ioremap addresses.
>
> MAX_PHYSMEM_BITS cannot be changed for that because the randomization
> does not align with address bit boundaries and there are other places
> which actually require to know the maximum number of address bits. All
> remaining usage sites of MAX_PHYSMEM_BITS have been analyzed and found
> to be correct.
>
> Cure this by exposing the end of the direct map via PHYSMEM_END and use
> that for the memory hot-plug and resource management related places
> instead of relying on MAX_PHYSMEM_BITS. In the KASLR case PHYSMEM_END
> maps to a variable which is initialized by the KASLR initialization and
> otherwise it is based on MAX_PHYSMEM_BITS as before.
>
> To prevent future hickups add a check into add_pages() to catch callers
> trying to add memory above PHYSMEM_END.
>
> Fixes: 0483e1fa6e09 ("x86/mm: Implement ASLR for kernel memory regions")
> Reported-by: Max Ramanouski <max8rr8@gmail.com>
> Reported-by: Alistair Popple <apopple@nvidia.com>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
[..]
> --- a/arch/x86/mm/kaslr.c
> +++ b/arch/x86/mm/kaslr.c
[..]
> @@ -134,6 +147,8 @@ void __init kernel_randomize_memory(void
> */
> vaddr += get_padding(&kaslr_regions[i]);
> vaddr = round_up(vaddr + 1, PUD_SIZE);
> + if (kaslr_regions[i].end)
> + *kaslr_regions[i].end = __pa(vaddr) - 1;

In the context of the patch it is clear that this is physmem_end, when
someone comes to read this later maybe a comment like:

/*
* KASLR trims the maximum possible size of the direct-map record that
* physmem_end boundary here
*/

With or without that the patch looks good to me:

Reviewed-by: Dan Williams <dan.j.williams@intel.com>