[PATCH v2] arm64: head: avoid over-mapping in map_memory

[Will Deacon]

On Mon, Aug 23, 2021 at 11:12:53AM +0100, Mark Rutland wrote:
> The `compute_indices` and `populate_entries` macros operate on inclusive
> bounds, and thus the `map_memory` macro which uses them also operates
> on inclusive bounds.
> 
> We pass `_end` and `_idmap_text_end` to `map_memory`, but these are
> exclusive bounds, and if one of these is sufficiently aligned (as a
> result of kernel configuration, physical placement, and KASLR), then:
> 
> * In `compute_indices`, the computed `iend` will be in the page/block *after*
>   the final byte of the intended mapping.
> 
> * In `populate_entries`, an unnecessary entry will be created at the end
>   of each level of table. At the leaf level, this entry will map up to
>   SWAPPER_BLOCK_SIZE bytes of physical addresses that we did not intend
>   to map.
> 
> As we may map up to SWAPPER_BLOCK_SIZE bytes more than intended, we may
> violate the boot protocol and map physical address past the 2MiB-aligned
> end address we are permitted to map. As we map these with Normal memory
> attributes, this may result in further problems depending on what these
> physical addresses correspond to.
> 
> The final entry at each level may require an additional table at that
> level. As EARLY_ENTRIES() calculates an inclusive bound, we allocate
> enough memory for this.
> 
> Avoid the extraneous mapping by having map_memory convert the exclusive
> end address to an inclusive end address by subtracting one, and do
> likewise in EARLY_ENTRIES() when calculating the number of required
> tables. For clarity, comments are updated to more clearly document which
> boundaries the macros operate on.  For consistency with the other
> macros, the comments in map_memory are also updated to describe `vstart`
> and `vend` as virtual addresses.
> 
> Fixes: 0370b31e48454d8c ("arm64: Extend early page table code to allow for larger kernel")
> Signed-off-by: Mark Rutland <mark.rutland at arm.com>
> Cc: Anshuman Khandual <anshuman.khandual at arm.com>
> Cc: Ard Biesheuvel <ard.biesheuvel at linaro.org>
> Cc: Catalin Marinas <catalin.marinas at arm.com>
> Cc: Steve Capper <steve.capper at arm.com>
> Cc: Will Deacon <will at kernel.org>
> ---
>  arch/arm64/include/asm/kernel-pgtable.h |  4 ++--
>  arch/arm64/kernel/head.S                | 11 ++++++-----
>  2 files changed, 8 insertions(+), 7 deletions(-)

Acked-by: Will Deacon <will at kernel.org>

Will