linux-kernel - Re: [PATCH] powerpc/32: Fix missing NULL pmd check in virt_to

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Message-ID: <20200313033517.GA37606@ubuntu-m2-xlarge-x86>
Date:   Thu, 12 Mar 2020 20:35:17 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Christophe Leroy <christophe.leroy@....fr>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>, ndesaulniers@...gle.com,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc/32: Fix missing NULL pmd check in virt_to_kpte()

On Sat, Mar 07, 2020 at 10:09:15AM +0000, Christophe Leroy wrote:
> Commit 2efc7c085f05 ("powerpc/32: drop get_pteptr()"),
> replaced get_pteptr() by virt_to_kpte(). But virt_to_kpte() lacks a
> NULL pmd check and returns an invalid non NULL pointer when there
> is no page table.
> 
> Reported-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Fixes: 2efc7c085f05 ("powerpc/32: drop get_pteptr()")
> Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
> ---
>  arch/powerpc/include/asm/pgtable.h | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/include/asm/pgtable.h b/arch/powerpc/include/asm/pgtable.h
> index b80bfd41828d..b1f1d5339735 100644
> --- a/arch/powerpc/include/asm/pgtable.h
> +++ b/arch/powerpc/include/asm/pgtable.h
> @@ -54,7 +54,9 @@ static inline pmd_t *pmd_ptr_k(unsigned long va)
>  
>  static inline pte_t *virt_to_kpte(unsigned long vaddr)
>  {
> -	return pte_offset_kernel(pmd_ptr_k(vaddr), vaddr);
> +	pmd_t *pmd = pmd_ptr_k(vaddr);
> +
> +	return pmd_none(*pmd) ? NULL : pte_offset_kernel(pmd, vaddr);
>  }
>  #endif
>  
> -- 
> 2.25.0
> 

With QEMU 4.2.0, I can confirm this fixes the panic:

Tested-by: Nathan Chancellor <natechancellor@...il.com>