linux-kernel - Re: [PATCH] efi_64: Fix a missing-check bug in arch/x86/platform/efi/efi

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Date:   Fri, 17 May 2019 10:41:28 +0200
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Gen Zhang <blackgod016574@...il.com>,
        linux-efi <linux-efi@...r.kernel.org>
Cc:     Darren Hart <dvhart@...radead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] efi_64: Fix a missing-check bug in arch/x86/platform/efi/efi_64.c
 of Linux 5.1

Hello Gen,

Thanks for the patch.

On Fri, 17 May 2019 at 10:26, Gen Zhang <blackgod016574@...il.com> wrote:
>
> save_pgd is allocated by kmalloc_array. And it is dereferenced in the
> following codes. However, memory allocation functions such as
> kmalloc_array may fail. Dereferencing this save_pgd null pointer may
> cause the kernel go wrong. Thus we should check this allocation and add
> error handling code.
>
> Signed-off-by: Gen Zhang <blackgod016574@...il.com>
>
> ---
> diff --git a/arch/x86/platform/efi/efi_64.c b/arch/x86/platform/efi/efi_64.c
> index cf0347f..fb9ae57 100644
> --- a/arch/x86/platform/efi/efi_64.c
> +++ b/arch/x86/platform/efi/efi_64.c
> @@ -91,6 +91,8 @@ pgd_t * __init efi_call_phys_prolog(void)
>
>         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)
> +               goto err;
>
>         /*
>          * Build 1:1 identity mapping for efi=old_map usage. Note that
> @@ -142,6 +144,9 @@ pgd_t * __init efi_call_phys_prolog(void)
>         __flush_tlb_all();
>
>         return save_pgd;
> +err:
> +       __flush_tlb_all();

What is the point of the goto and the TLB flush?

> +       return ERR_PTR(-ENOMEM);

Returning an error here is not going to make much difference, given
that the caller of efi_call_phys_prolog() does not bother to check it,
and passes the result straight into efi_call_phys_epilog(), which
happily attempts to dereference it.

So if you want to fix this properly, please fix it at the call site as
well. I'd prefer to avoid ERR_PTR() and just return NULL for a failed
allocation though.

>  }
>
>  void __init efi_call_phys_epilog(pgd_t *save_pgd)
> ---