linux-kernel - Re: [PATCH v2 2/2] KVM: arm/arm64: harden unmap_stage2_ptes in case end is not PAGE

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Date:   Fri, 8 Jun 2018 10:34:14 +0200
From:   Christoffer Dall <christoffer.dall@....com>
To:     Jia He <hejianet@...il.com>
Cc:     Marc Zyngier <marc.zyngier@....com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        Suzuki.Poulose@....com, linux-kernel@...r.kernel.org,
        jia.he@...-semitech.com
Subject: Re: [PATCH v2 2/2] KVM: arm/arm64: harden unmap_stage2_ptes in case
 end is not PAGE_SIZE aligned

On Fri, May 18, 2018 at 09:04:40PM +0800, Jia He wrote:
> 
> 
> On 5/18/2018 5:48 PM, Marc Zyngier Wrote:
> > On 18/05/18 10:27, Jia He wrote:
> >> If it passes addr=0x202920000,size=0xfe00 to unmap_stage2_range->
> >> ...->unmap_stage2_ptes, unmap_stage2_ptes will get addr=0x202920000,
> >> end=0x20292fe00. After first while loop addr=0x202930000, end=0x20292fe00,
> >> then addr!=end. Thus it will touch another pages by put_pages() in the
> >> 2nd loop.
> >>
> >> This patch fixes it by hardening the break condition of while loop.
> >>
> >> Signed-off-by: jia.he@...-semitech.com
> >> ---
> >> v2: newly added
> >>
> >>  virt/kvm/arm/mmu.c | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
> >> index 8dac311..45cd040 100644
> >> --- a/virt/kvm/arm/mmu.c
> >> +++ b/virt/kvm/arm/mmu.c
> >> @@ -217,7 +217,7 @@ static void unmap_stage2_ptes(struct kvm *kvm, pmd_t *pmd,
> >>  
> >>  			put_page(virt_to_page(pte));
> >>  		}
> >> -	} while (pte++, addr += PAGE_SIZE, addr != end);
> >> +	} while (pte++, addr += PAGE_SIZE, addr < end);
> >>  
> >>  	if (stage2_pte_table_empty(start_pte))
> >>  		clear_stage2_pmd_entry(kvm, pmd, start_addr);
> >>
> > 
> > I don't think this change is the right thing to do. You get that failure
> > because you're being passed a size that is not a multiple of PAGE_SIZE.
> > That's the mistake.
> > 
> > You should ensure that this never happens, rather than changing the page
> > table walkers (which are consistent with the way this kind of code is
> > written in other places of the kernel). As you mentioned in your first
> > patch, the real issue is that KSM is broken, and this is what should be
> > fixed.
> > 
> Got it, thanks
> Should I resend the patch 1/2 without any changes after droping patch 2/2?
> 

That's fine if you trim the commit message to make it look nicer.

Also, in the future, please include a cover letter if you send more than
one patch to the list.

Thanks,
-Christoffer