Re: [PATCH AUTOSEL 4.19 17/49] efi/x86/Add missing error handling to old_memmap 1:1 mapping code

[Ard Biesheuvel]

On Sat, 8 Jun 2019 at 13:43, Sasha Levin <sashal@xxxxxxxxxx> wrote:
>
> From: Gen Zhang <blackgod016574@xxxxxxxxx>
>
> [ Upstream commit 4e78921ba4dd0aca1cc89168f45039add4183f8e ]
>
> The old_memmap flow in efi_call_phys_prolog() performs numerous memory
> allocations, and either does not check for failure at all, or it does
> but fails to propagate it back to the caller, which may end up calling
> into the firmware with an incomplete 1:1 mapping.
>
> So let's fix this by returning NULL from efi_call_phys_prolog() on
> memory allocation failures only, and by handling this condition in the
> caller. Also, clean up any half baked sets of page tables that we may
> have created before returning with a NULL return value.
>
> Note that any failure at this level will trigger a panic() two levels
> up, so none of this makes a huge difference, but it is a nice cleanup
> nonetheless.
>
> [ardb: update commit log, add efi_call_phys_epilog() call on error path]
>
> Signed-off-by: Gen Zhang <blackgod016574@xxxxxxxxx>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Rob Bradford <robert.bradford@xxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: linux-efi@xxxxxxxxxxxxxxx
> Link: http://lkml.kernel.org/r/20190525112559.7917-2-ard.biesheuvel@xxxxxxxxxx
> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

This was already discussed in the thread that proposed this patch for
stable: please don't queue this right now, the patches are more likely
to harm than hurt, and they certainly don't fix a security
vulnerability, as has been claimed.


> ---
>  arch/x86/platform/efi/efi.c    | 2 ++
>  arch/x86/platform/efi/efi_64.c | 9 ++++++---
>  2 files changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/platform/efi/efi.c b/arch/x86/platform/efi/efi.c
> index 9061babfbc83..353019d4e6c9 100644
> --- a/arch/x86/platform/efi/efi.c
> +++ b/arch/x86/platform/efi/efi.c
> @@ -86,6 +86,8 @@ static efi_status_t __init phys_efi_set_virtual_address_map(
>         pgd_t *save_pgd;
>
>         save_pgd = efi_call_phys_prolog();
> +       if (!save_pgd)
> +               return EFI_ABORTED;
>
>         /* Disable interrupts around EFI calls: */
>         local_irq_save(flags);
> diff --git a/arch/x86/platform/efi/efi_64.c b/arch/x86/platform/efi/efi_64.c
> index ee5d08f25ce4..dfc809b31c7c 100644
> --- a/arch/x86/platform/efi/efi_64.c
> +++ b/arch/x86/platform/efi/efi_64.c
> @@ -84,13 +84,15 @@ pgd_t * __init efi_call_phys_prolog(void)
>
>         if (!efi_enabled(EFI_OLD_MEMMAP)) {
>                 efi_switch_mm(&efi_mm);
> -               return NULL;
> +               return efi_mm.pgd;
>         }
>
>         early_code_mapping_set_exec(1);
>
>         n_pgds = DIV_ROUND_UP((max_pfn << PAGE_SHIFT), PGDIR_SIZE);
>         save_pgd = kmalloc_array(n_pgds, sizeof(*save_pgd), GFP_KERNEL);
> +       if (!save_pgd)
> +               return NULL;
>
>         /*
>          * Build 1:1 identity mapping for efi=old_map usage. Note that
> @@ -138,10 +140,11 @@ pgd_t * __init efi_call_phys_prolog(void)
>                 pgd_offset_k(pgd * PGDIR_SIZE)->pgd &= ~_PAGE_NX;
>         }
>
> -out:
>         __flush_tlb_all();
> -
>         return save_pgd;
> +out:
> +       efi_call_phys_epilog(save_pgd);
> +       return NULL;
>  }
>
>  void __init efi_call_phys_epilog(pgd_t *save_pgd)
> --
> 2.20.1
>