linux-kernel - Re: [patch 1/2] x86/mm/pti: Handle unaligned address gracefully in pti_clone

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Date:   Wed, 28 Aug 2019 20:58:32 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Song Liu <songliubraving@...com>,
        Joerg Roedel <jroedel@...e.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Rik van Riel <riel@...riel.com>
Subject: Re: [patch 1/2] x86/mm/pti: Handle unaligned address gracefully in
 pti_clone_pagetable()


* Thomas Gleixner <tglx@...utronix.de> wrote:

> From: Song Liu <songliubraving@...com>
> 
> pti_clone_pmds() assumes that the supplied address is either:
> 
>  - properly PUD/PMD aligned
> or
>  - the address is actually mapped which means that independent
>    of the mapping level (PUD/PMD/PTE) the next higher mapping
>    exist.

s/independent
 /independently

s/exist
 /exists

> If that's not the case the unaligned address can be incremented by PUD or
> PMD size wrongly. All callers supply mapped and/or aligned addresses, but
> for robustness sake, it's better to handle that case proper and to emit a
> warning.

s/wrongly
 /incorrectly

s/robustness sake
 /robustness's sake

s/proper
 /properly

With that:

>  		pud = pud_offset(p4d, addr);
>  		if (pud_none(*pud)) {
> -			addr += PUD_SIZE;
> +			WARN_ON_ONCE(addr & PUD_MASK);
> +			addr = round_up(addr + 1, PUD_SIZE);
>  			continue;
>  		}
>  
>  		pmd = pmd_offset(pud, addr);
>  		if (pmd_none(*pmd)) {
> -			addr += PMD_SIZE;
> +			WARN_ON_ONCE(addr & PMD_MASK);
> +			addr = round_up(addr + 1, PMD_SIZE);

So given that PUD_MASK and PMD_MASK are masking out the *offset*:

 arch/x86/include/asm/pgtable_64_types.h:#define PMD_MASK	(~(PMD_SIZE - 1))

Didn't we want something like:

			WARN_ON_ONCE(addr & ~PUD_MASK);

			WARN_ON_ONCE(addr & ~PMD_MASK);

to warn about an unaligned 'addr', or am I misreading the intent here?

Thanks,

	Ingo