Subject: [PATCHv2 00/18] arm64: mm: rework page table creation

From: [email-masked] (Laura Abbott)
Date: Mon, 4 Jan 2016 17:08:58 -0800

On 01/04/2016 09:56 AM, Mark Rutland wrote:
> Hi all,
>
> This series reworks the arm64 early page table code, in order to:
>
> (a) Avoid issues with potentially-conflicting TTBR1 TLB entries (as raised in
>      Jeremy's thread [1]). This can happen when splitting/merging sections or
>      contiguous ranges, and per a pessimistic reading of the ARM ARM may happen
>      for changes to other fields in translation table entries.
>
> (b) Allow for more complex page table creation early on, with tables created
>      with fine-grained permissions as early as possible. In the cases where we
>      currently use fine-grained permissions (e.g. DEBUG_RODATA and marking .init
>      as non-executable), this is required for the same reasons as (a), as we
>      must ensure that changes to page tables do not split/merge sections or
>      contiguous regions for memory in active use.
>
> (c) Avoid edge cases where we need to allocate memory before a sufficient
>      proportion of the early linear map is in place to accommodate allocations.
>
> This series:
>
> * Introduces the necessary infrastructure to safely swap TTBR1_EL1 (i.e.
>    without risking conflicting TLB entries being allocated). The arm64 KASAN
>    code is migrated to this.
>
> * Adds helpers to walk page tables by physical address, independent of the
>    linear mapping, and modifies __create_mapping and friends to relying on a new
>    set of FIX_{PGD,PUD,PMD,PTE} to map tables as required for modification.
>
> * Removes the early memblock limit, now that create_mapping does not rely on the
>    early linear map. This solves (c), and allows for (b).
>
> * Generates an entirely new set of kernel page tables with fine-grained (i.e.
>    page-level) permission boundaries, which can then be safely installed. These
>    are created with sufficient granularity such that later changes (currently
>    only fixup_init) will not split/merge sections or contiguous regions, and can
>    follow a break-before-make approach without affecting the rest of the page
>    tables.
>
> There are still opportunities for improvement:
>
> * BUG() when splitting sections or creating overlapping entries in
>    create_mapping, as these both indicate serious bugs in kernel page table
>    creation.
>
>    This will require rework to the EFI runtime services pagetable creation, as
>    for >4K page kernels EFI memory descriptors may share pages (and currently
>    such overlap is assumed to be benign).

Given the split_{pmd,pud} were added for DEBUG_RODATA, is there any reason
those can't be dropped now since it sounds like the EFI problem is for overlapping
entries and not splitting?

>
> * Use ROX mappings for the kernel text and rodata when creating the new tables.
>    This avoiding potential conflicts from changes to translation tables, and
>    giving us better protections earlier.
>
>    Currently the alternatives patching code relies on being able to use the
>    kernel mapping to update the text. We cannot rely on any text which itself
>    may be patched, and updates may straddle page boundaries, so this is
>    non-trivial.
>
> * Clean up usage of swapper_pg_dir so we can switch to the new tables without
>    having to reuse the existing pgd. This will allow us to free the original
>    pgd (i.e. we can free all the initial tables in one go).
>
> Any and all feedback is welcome.

This series points out that my attempt to allow set_memory_* to
work on regular kernel memory[1] is broken right now because it breaks down
the larger block sizes. Do you have any suggestions for a cleaner approach
short of requiring all memory mapped with 4K pages? The only solution I see
right now is having a separate copy of page tables to switch to. Any idea
other idea I come up with would have problems if we tried to invalidate an
entry before breaking it down.

Thanks,
Laura

[1]https://lkml.kernel.org/g/<[email-masked]>

>
> This series is based on today's arm64 [2] for-next/core branch (commit
> c9cd0ed925c0b927), and this version is tagged as
> arm64-pagetable-rework-20160104 while the latest version should be in the
> unstable branch arm64/pagetable-rework in my git repo [3].
>
> Since v1 [4] (tagged arm64-pagetable-rework-20151209):
> * Drop patches taken into the arm64 tree.
> * Rebase to arm64 for-next/core.
> * Copy early KASAN tables.
> * Fix KASAN pgd manipulation.
> * Specialise allocators for page tables, in function and naming.
> * Update comments.
>
> Thanks,
> Mark.
>
> [1] http://lists.infradead.org/pipermail/linux-arm-kernel/2015-November/386178.html
> [2] git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git
> [3] git://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git
> [4] http://lists.infradead.org/pipermail/linux-arm-kernel/2015-December/392292.html
>
> Mark Rutland (18):
>    asm-generic: make __set_fixmap_offset a static inline
>    arm64: mm: specialise pagetable allocators
>    arm64: mm: place empty_zero_page in bss
>    arm64: unify idmap removal
>    arm64: unmap idmap earlier
>    arm64: add function to install the idmap
>    arm64: mm: add code to safely replace TTBR1_EL1
>    arm64: kasan: avoid TLB conflicts
>    arm64: mm: move pte_* macros
>    arm64: mm: add functions to walk page tables by PA
>    arm64: mm: avoid redundant __pa(__va(x))
>    arm64: mm: add __{pud,pgd}_populate
>    arm64: mm: add functions to walk tables in fixmap
>    arm64: mm: use fixmap when creating page tables
>    arm64: mm: allocate pagetables anywhere
>    arm64: mm: allow passing a pgdir to alloc_init_*
>    arm64: ensure _stext and _etext are page-aligned
>    arm64: mm: create new fine-grained mappings at boot
>
>   arch/arm64/include/asm/fixmap.h      |  10 ++
>   arch/arm64/include/asm/kasan.h       |   3 +
>   arch/arm64/include/asm/mmu_context.h |  63 ++++++-
>   arch/arm64/include/asm/pgalloc.h     |  26 ++-
>   arch/arm64/include/asm/pgtable.h     |  87 +++++++---
>   arch/arm64/kernel/head.S             |   1 +
>   arch/arm64/kernel/setup.c            |   7 +
>   arch/arm64/kernel/smp.c              |   4 +-
>   arch/arm64/kernel/suspend.c          |  20 +--
>   arch/arm64/kernel/vmlinux.lds.S      |   5 +-
>   arch/arm64/mm/kasan_init.c           |  32 ++--
>   arch/arm64/mm/mmu.c                  | 311 ++++++++++++++++++-----------------
>   arch/arm64/mm/proc.S                 |  27 +++
>   include/asm-generic/fixmap.h         |  14 +-
>   14 files changed, 381 insertions(+), 229 deletions(-)
>