linux-kernel - Re: [PATCH] x86/mm/64: Do not dereference non-present PGD entries

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Date:   Fri, 7 Aug 2020 13:07:39 +0300
From:   Mike Rapoport <rppt@...ux.ibm.com>
To:     Joerg Roedel <joro@...tes.org>
Cc:     x86@...nel.org, Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, hpa@...or.com,
        Joerg Roedel <jroedel@...e.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jason@...c4.com, Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/mm/64: Do not dereference non-present PGD entries

On Fri, Aug 07, 2020 at 10:40:13AM +0200, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@...e.de>
> 
> The code for preallocate_vmalloc_pages() was written under the
> assumption that the p4d_offset() and pud_offset() functions will perform
> present checks before dereferencing the parent entries.
> 
> This assumption is wrong an leads to a bug in the code which causes the
> physical address found in the PGD be used as a page-table page, even if
> the PGD is not present.
> 
> So the code flow currently is:
> 
> 	pgd = pgd_offset_k(addr);
> 	p4d = p4d_offset(pgd, addr);
> 	if (p4d_none(*p4d))
> 		p4d = p4d_alloc(&init_mm, pgd, addr);
> 
> This lacks a check for pgd_none() at least, the correct flow would be:
> 
> 	pgd = pgd_offset_k(addr);
> 	if (pgd_none(*pgd))
> 		p4d = p4d_alloc(&init_mm, pgd, addr);
> 	else
> 		p4d = p4d_offset(pgd, addr);
> 
> But this is the same flow that the p4d_alloc() and the pud_alloc()
> functions use internally, so there is no need to duplicate them.
> 
> Remove the p?d_none() checks from the function and just call into
> p4d_alloc() and pud_alloc() to correctly pre-allocate the PGD entries.
> 
> Reported-by: Jason A. Donenfeld <Jason@...c4.com>
> Fixes: 6eb82f994026 ("x86/mm: Pre-allocate P4D/PUD pages for vmalloc area")
> Signed-off-by: Joerg Roedel <jroedel@...e.de>

LGTM,

Reviewed-by: Mike Rapoport <rppt@...ux.ibm.com>

> ---
>  arch/x86/mm/init_64.c | 31 +++++++++++++------------------
>  1 file changed, 13 insertions(+), 18 deletions(-)
> 
> diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
> index 3f4e29a78f2b..449e071240e1 100644
> --- a/arch/x86/mm/init_64.c
> +++ b/arch/x86/mm/init_64.c
> @@ -1253,28 +1253,23 @@ static void __init preallocate_vmalloc_pages(void)
>  		p4d_t *p4d;
>  		pud_t *pud;
>  
> -		p4d = p4d_offset(pgd, addr);
> -		if (p4d_none(*p4d)) {
> -			/* Can only happen with 5-level paging */
> -			p4d = p4d_alloc(&init_mm, pgd, addr);
> -			if (!p4d) {
> -				lvl = "p4d";
> -				goto failed;
> -			}
> -		}
> +		lvl = "p4d";
> +		p4d = p4d_alloc(&init_mm, pgd, addr);
> +		if (!p4d)
> +			goto failed;
>  
> +		/*
> +		 * With 5-level paging the P4D level is not folded. So the PGDs
> +		 * are now populated and there is no need to walk down to the
> +		 * PUD level.
> +		 */
>  		if (pgtable_l5_enabled())
>  			continue;
>  
> -		pud = pud_offset(p4d, addr);
> -		if (pud_none(*pud)) {
> -			/* Ends up here only with 4-level paging */
> -			pud = pud_alloc(&init_mm, p4d, addr);
> -			if (!pud) {
> -				lvl = "pud";
> -				goto failed;
> -			}
> -		}
> +		lvl = "pud";
> +		pud = pud_alloc(&init_mm, p4d, addr);
> +		if (!pud)
> +			goto failed;
>  	}
>  
>  	return;
> -- 
> 2.26.2
> 

-- 
Sincerely yours,
Mike.