[PATCH] ARM: mm: use fully constructed struct pages for EFI pgd allocations

[Ard Biesheuvel]

On 22 July 2016 at 20:42, Ard Biesheuvel <ard.biesheuvel at linaro.org> wrote:
> The late_alloc() PTE allocation function used by create_mapping_late()
> does not call pgtable_page_ctor() on PTE pages it allocates, leaving
> the per-page spinlock uninitialized.
>
> Since generic page table manipulation code may assume that translation
> table pages that are not owned by init_mm are covered by fully
> constructed struct pages, the following crash may occur with the new
> UEFI memory attributes table code.
>
>   efi: memattr: Processing EFI Memory Attributes table:
>   efi: memattr:  0x0000ffa16000-0x0000ffa82fff [Runtime Code       |RUN|  |  |XP|  |  |  |   |  |  |  |  ]
>   Unable to handle kernel NULL pointer dereference at virtual address 00000010
>   pgd = c0204000
>   [00000010] *pgd=00000000
>   Internal error: Oops: 5 [#1] SMP ARM
>   Modules linked in:
>   CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.7.0-rc4-00063-g3882aa7b340b #361
>   Hardware name: Generic DT based system
>   task: ed858000 ti: ed842000 task.ti: ed842000
>   PC is at __lock_acquire+0xa0/0x19a8
>   ...
>   [<c038c830>] (__lock_acquire) from [<c038e4f8>] (lock_acquire+0x6c/0x88)
>   [<c038e4f8>] (lock_acquire) from [<c0c06134>] (_raw_spin_lock+0x2c/0x3c)
>   [<c0c06134>] (_raw_spin_lock) from [<c0410384>] (apply_to_page_range+0xe8/0x238)
>   [<c0410384>] (apply_to_page_range) from [<c1205f34>] (efi_set_mapping_permissions+0x54/0x5c)
>   [<c1205f34>] (efi_set_mapping_permissions) from [<c1247474>] (efi_memattr_apply_permissions+0x2b8/0x378)
>   [<c1247474>] (efi_memattr_apply_permissions) from [<c1248258>] (arm_enable_runtime_services+0x1f0/0x22c)
>   [<c1248258>] (arm_enable_runtime_services) from [<c0301f0c>] (do_one_initcall+0x44/0x174)
>   [<c0301f0c>] (do_one_initcall) from [<c1200d10>] (kernel_init_freeable+0x90/0x1e8)
>   [<c1200d10>] (kernel_init_freeable) from [<c0bff690>] (kernel_init+0x8/0x114)
>   [<c0bff690>] (kernel_init) from [<c0307ed0>] (ret_from_fork+0x14/0x24)
>
> The crash is due to the fact that the UEFI page tables are not owned by
> init_mm, but are not covered by fully constructed struct pages.
>
> Given that the UEFI subsystem is currently the only user of
> create_mapping_late(), add an unconditional call to pgtable_page_ctor() to
> late_alloc().
>
> Fixes: 9fc68b717c24 ("ARM/efi: Apply strict permissions for UEFI Runtime Services regions")
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
> ---
>
> The commit in question was introduced in v4.7, so ideally this should go in
> as a late fix. However, EFI on ARM is not widely used [yet], and the memory
> attributes table even less so, so I am perfectly happy for this to go in
> later.
>
>  arch/arm/mm/mmu.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/arm/mm/mmu.c b/arch/arm/mm/mmu.c
> index 62f4d01941f7..d0ac45451805 100644
> --- a/arch/arm/mm/mmu.c
> +++ b/arch/arm/mm/mmu.c
> @@ -728,7 +728,7 @@ static void *__init late_alloc(unsigned long sz)
>  {
>         void *ptr = (void *)__get_free_pages(PGALLOC_GFP, get_order(sz));
>
> -       BUG_ON(!ptr);
> +       BUG_ON(!ptr || !pgtable_page_ctor(virt_to_page(ptr)));

Actually, putting code with side effects inside BUG_ON() is not a good
idea, and I deliberately avoided doing so in the arm64 counterpart of
this patch.

If there are no other comments on this patch, I will fix that up
before dropping this in the patch system