linux-kernel - Re: [PATCH] xen/p2m: Fix one by off error in checking the P2M tree directory.

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Date:	Tue, 4 Sep 2012 16:44:31 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	linux-kernel@...r.kernel.org
Cc:	xen-devel@...ts.xensource.com
Subject: Re: [PATCH] xen/p2m: Fix one by off error in checking the P2M tree
 directory.

On Tue, Sep 04, 2012 at 04:17:14PM -0400, Konrad Rzeszutek Wilk wrote:
> We would the full P2M top directory from 0->MAX_DOMAIN_PAGES (inclusive).

.. We would traverse the full P2M top directory (from 0->MAX_DOMAIN_PAGES
inclusive) when trying to figure out whether we can re-use some of the
P2M middle leafs.

> 
> Which meant that if the kernel was compiled with MAX_DOMAIN_PAGES=512
> we would try to use the 512th entry. Fortunately for us the p2m_top_index
> has a check for this:
> 
>  BUG_ON(pfn >= MAX_P2M_PFN);
> 
> which we hit and saw this:
> 
> (XEN) domain_crash_sync called from entry.S
> (XEN) Domain 0 (vcpu#0) crashed on cpu#0:
> (XEN) ----[ Xen-4.1.2-OVM  x86_64  debug=n  Tainted:    C ]----
> (XEN) CPU:    0
> (XEN) RIP:    e033:[<ffffffff819cadeb>]
> (XEN) RFLAGS: 0000000000000212   EM: 1   CONTEXT: pv guest
> (XEN) rax: ffffffff81db5000   rbx: ffffffff81db4000   rcx: 0000000000000000
> (XEN) rdx: 0000000000480211   rsi: 0000000000000000   rdi: ffffffff81db4000
> (XEN) rbp: ffffffff81793db8   rsp: ffffffff81793d38   r8:  0000000008000000
> (XEN) r9:  4000000000000000   r10: 0000000000000000   r11: ffffffff81db7000
> (XEN) r12: 0000000000000ff8   r13: ffffffff81df1ff8   r14: ffffffff81db6000
> (XEN) r15: 0000000000000ff8   cr0: 000000008005003b   cr4: 00000000000026f0
> (XEN) cr3: 0000000661795000   cr2: 0000000000000000
> 
> Fixes-Oracle-Bug: 14570662
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> ---
>  arch/x86/xen/p2m.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
> index 0bfaf5b..af11f00 100644
> --- a/arch/x86/xen/p2m.c
> +++ b/arch/x86/xen/p2m.c
> @@ -695,7 +695,7 @@ bool __init early_can_reuse_p2m_middle(unsigned long set_pfn, unsigned long set_
>  	if (p2m_index(set_pfn))
>  		return false;
>  
> -	for (pfn = 0; pfn <= MAX_DOMAIN_PAGES; pfn += P2M_PER_PAGE) {
> +	for (pfn = 0; pfn < MAX_DOMAIN_PAGES; pfn += P2M_PER_PAGE) {
>  		topidx = p2m_top_index(pfn);
>  
>  		if (!p2m_top[topidx])
> -- 
> 1.7.7.6
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