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SubjectRe: [PATCH] arm64: mm: correct the start of physical address in linear map
On 2021-02-12 20:23:16, Pavel Tatashin wrote:
> Memory hotplug may fail on systems with CONFIG_RANDOMIZE_BASE because the
> linear map range is not checked correctly.
>
> The start physical address that linear map covers can be actually at the
> end of the range because of randmomization. Check that and if so reduce it
> to 0.
>
> This can be verified on QEMU with setting kaslr-seed to ~0ul:
>
> memstart_offset_seed = 0xffff
> START: __pa(_PAGE_OFFSET(vabits_actual)) = ffff9000c0000000
> END: __pa(PAGE_END - 1) = 1000bfffffff
>
> Signed-off-by: Pavel Tatashin <pasha.tatashin@soleen.com>
> Fixes: 58284a901b42 ("arm64/mm: Validate hotplug range before creating linear mapping")

Tested-by: Tyler Hicks <tyhicks@linux.microsoft.com>

This fixes a memory hot plugging bug that I was seeing on 5.10, with the
introduction of 58284a901b42.

One comment below...

> ---
> arch/arm64/mm/mmu.c | 15 +++++++++++++--
> 1 file changed, 13 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index ae0c3d023824..6057ecaea897 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -1444,14 +1444,25 @@ static void __remove_pgd_mapping(pgd_t *pgdir, unsigned long start, u64 size)
>
> static bool inside_linear_region(u64 start, u64 size)
> {
> + u64 start_linear_pa = __pa(_PAGE_OFFSET(vabits_actual));
> + u64 end_linear_pa = __pa(PAGE_END - 1);
> +
> + /*
> + * Check for a wrap, it is possible because of randomized linear mapping
> + * the start physical address is actually bigger than the end physical
> + * address. In this case set start to zero because [0, end_linear_pa]
> + * range must still be able to cover all addressable physical addresses.
> + */
> + if (start_linear_pa > end_linear_pa)
> + start_linear_pa = 0;

We're ignoring the portion from the linear mapping's start PA to the
point of wraparound. Could the start and end of the hot plugged memory
fall within this range and, as a result, the hot plug operation be
incorrectly blocked?

Tyler

> +
> /*
> * Linear mapping region is the range [PAGE_OFFSET..(PAGE_END - 1)]
> * accommodating both its ends but excluding PAGE_END. Max physical
> * range which can be mapped inside this linear mapping range, must
> * also be derived from its end points.
> */
> - return start >= __pa(_PAGE_OFFSET(vabits_actual)) &&
> - (start + size - 1) <= __pa(PAGE_END - 1);
> + return start >= start_linear_pa && (start + size - 1) <= end_linear_pa;
> }
>
> int arch_add_memory(int nid, u64 start, u64 size,
> --
> 2.25.1
>

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