linux-kernel - Re: [PATCH] arm: KVM: fix possible misalignment of PGDs and bounce page

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Date:	Fri, 28 Mar 2014 12:37:33 -0700
From:	Christoffer Dall <christoffer.dall@...aro.org>
To:	Mark Salter <msalter@...hat.com>
Cc:	Russell King <linux@....linux.org.uk>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, kvmarm@...ts.cs.columbia.edu
Subject: Re: [PATCH] arm: KVM: fix possible misalignment of PGDs and bounce
 page

On Fri, Mar 28, 2014 at 10:25:19AM -0400, Mark Salter wrote:
> The kvm/mmu code shared by arm and arm64 uses kalloc() to allocate
> a bounce page (if hypervisor init code crosses page boundary) and
> hypervisor PGDs. The problem is that kalloc() does not guarantee
> the proper alignment. In the case of the bounce page, the page sized
> buffer allocated may also cross a page boundary negating the purpose
> and leading to a hang during kvm initialization. Likewise the PGDs
> allocated may not meet the minimum alignment requirements of the
> underlying MMU. This patch uses __get_free_page() to guarantee the
> worst case alignment needs of the bounce page and PGDs on both arm
> and arm64.
> 
> Signed-off-by: Mark Salter <msalter@...hat.com>
> ---
>  arch/arm/kvm/mmu.c | 15 +++++++++------
>  1 file changed, 9 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
> index 7789857..575d790 100644
> --- a/arch/arm/kvm/mmu.c
> +++ b/arch/arm/kvm/mmu.c
> @@ -42,6 +42,8 @@ static unsigned long hyp_idmap_start;
>  static unsigned long hyp_idmap_end;
>  static phys_addr_t hyp_idmap_vector;
>  
> +#define pgd_order get_order(PTRS_PER_PGD * sizeof(pgd_t))
> +
>  #define kvm_pmd_huge(_x)	(pmd_huge(_x) || pmd_trans_huge(_x))
>  
>  static void kvm_tlb_flush_vmid_ipa(struct kvm *kvm, phys_addr_t ipa)
> @@ -199,14 +201,14 @@ void free_boot_hyp_pgd(void)
>  	if (boot_hyp_pgd) {
>  		unmap_range(NULL, boot_hyp_pgd, hyp_idmap_start, PAGE_SIZE);
>  		unmap_range(NULL, boot_hyp_pgd, TRAMPOLINE_VA, PAGE_SIZE);
> -		kfree(boot_hyp_pgd);
> +		free_pages((unsigned long)boot_hyp_pgd, pgd_order);
>  		boot_hyp_pgd = NULL;
>  	}
>  
>  	if (hyp_pgd)
>  		unmap_range(NULL, hyp_pgd, TRAMPOLINE_VA, PAGE_SIZE);
>  
> -	kfree(init_bounce_page);
> +	free_page((unsigned long)init_bounce_page);
>  	init_bounce_page = NULL;
>  
>  	mutex_unlock(&kvm_hyp_pgd_mutex);
> @@ -236,7 +238,7 @@ void free_hyp_pgds(void)
>  		for (addr = VMALLOC_START; is_vmalloc_addr((void*)addr); addr += PGDIR_SIZE)
>  			unmap_range(NULL, hyp_pgd, KERN_TO_HYP(addr), PGDIR_SIZE);
>  
> -		kfree(hyp_pgd);
> +		free_pages((unsigned long)hyp_pgd, pgd_order);
>  		hyp_pgd = NULL;
>  	}
>  
> @@ -930,7 +932,7 @@ int kvm_mmu_init(void)
>  		size_t len = __hyp_idmap_text_end - __hyp_idmap_text_start;
>  		phys_addr_t phys_base;
>  
> -		init_bounce_page = kmalloc(PAGE_SIZE, GFP_KERNEL);
> +		init_bounce_page = (void *)__get_free_page(GFP_KERNEL);
>  		if (!init_bounce_page) {
>  			kvm_err("Couldn't allocate HYP init bounce page\n");
>  			err = -ENOMEM;
> @@ -956,8 +958,9 @@ int kvm_mmu_init(void)
>  			 (unsigned long)phys_base);
>  	}
>  
> -	hyp_pgd = kzalloc(PTRS_PER_PGD * sizeof(pgd_t), GFP_KERNEL);
> -	boot_hyp_pgd = kzalloc(PTRS_PER_PGD * sizeof(pgd_t), GFP_KERNEL);
> +	hyp_pgd = (pgd_t *)__get_free_pages(GFP_KERNEL | __GFP_ZERO, pgd_order);
> +	boot_hyp_pgd = (pgd_t *)__get_free_pages(GFP_KERNEL | __GFP_ZERO, pgd_order);
> +
>  	if (!hyp_pgd || !boot_hyp_pgd) {
>  		kvm_err("Hyp mode PGD not allocated\n");
>  		err = -ENOMEM;
> -- 
> 1.8.5.3
> 
This looks right to me.  Funnily enough I seem to remember a discussion
from when we originally merged this code where someone (maybe me) argued
that kmalloc() would align to the size of the allocation, but I don't
see anything backing this up at this point.

So:

Acked-by: Christoffer Dall <christoffer.dall@...aro.org>

If Marc agrees I can queue this for -rc1.
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