linux-kernel - Re: [PATCH v4 04/29] x86/cpa: In populate_pgd, don't set the pgd entry until it's populated

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Message-ID: <20160628184810.GA2855@pd.tnic>
Date:	Tue, 28 Jun 2016 20:48:10 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Andy Lutomirski <luto@...nel.org>
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, Nadav Amit <nadav.amit@...il.com>,
	Kees Cook <keescook@...omium.org>,
	Brian Gerst <brgerst@...il.com>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Jann Horn <jann@...jh.net>,
	Heiko Carstens <heiko.carstens@...ibm.com>
Subject: Re: [PATCH v4 04/29] x86/cpa: In populate_pgd, don't set the pgd
 entry until it's populated

On Sun, Jun 26, 2016 at 02:55:26PM -0700, Andy Lutomirski wrote:
> This avoids pointless races in which another CPU or task might see a
> partially populated global pgd entry.  These races should normally
> be harmless, but, if another CPU propagates the entry via
> vmalloc_fault and then populate_pgd fails (due to memory allocation
> failure, for example), this prevents a use-after-free of the pgd
> entry.
> 
> Signed-off-by: Andy Lutomirski <luto@...nel.org>
> ---
>  arch/x86/mm/pageattr.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c
> index 7a1f7bbf4105..6a8026918bf6 100644
> --- a/arch/x86/mm/pageattr.c
> +++ b/arch/x86/mm/pageattr.c
> @@ -1113,7 +1113,9 @@ static int populate_pgd(struct cpa_data *cpa, unsigned long addr)
>  
>  	ret = populate_pud(cpa, addr, pgd_entry, pgprot);
>  	if (ret < 0) {
> -		unmap_pgd_range(cpa->pgd, addr,
> +		if (pud)
> +			free_page((unsigned long)pud);
> +		unmap_pud_range(pgd_entry, addr,
>  				addr + (cpa->numpages << PAGE_SHIFT));
>  		return ret;
>  	}
> -- 

So something's amiss here. Subject says:

"x86/cpa: In populate_pgd, don't set the pgd entry until it's populated"

but you haven't moved

	set_pgd(pgd_entry, __pgd(__pa(pud) | _KERNPG_TABLE));

after populate_pud() succeeds... Which is a good catch but your patch
should do it too. :-)

-- 
Regards/Gruss,
    Boris.

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