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SubjectRe: [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@gmail.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@gmail.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)
> ---

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