Re: [Xen-devel] [PATCH] x86: moke show_page_walk() more robust

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

It's not merely a guest crash. It shouldn't be possible for a guest to end
up with bogus PTEs. It's a host consistency issue and hence a host crash is
appropriate.

 -- Keir

On 20/07/2009 07:39, "Jan Beulich" <JBeulich@xxxxxxxxxx> wrote:

> I very much think it is worthwhile: In the case given, a supposed guest crash
> resulted in a host crash - the expected behavior would have been to do the
> page table walk, then forward the fault to the guest.
> 
> Jan
> 
>>>> Keir Fraser <keir.fraser@xxxxxxxxxxxxx> 17.07.09 21:48 >>>
> Is this worthwhile? It's kind of game over by this point, and we already
> dumped out the useful stuff. And the ensuing crash makes it very clear that
> the page walk discovered an invalid mfn (rather than this alternative of
> silently truncating the walk).
> 
>  -- Keir
> 
> On 17/07/2009 10:20, "Jan Beulich" <JBeulich@xxxxxxxxxx> wrote:
> 
>> This should fix the host crash reported in the thread "d273:v3:
>> reserved bit in page table (ec=0019)" (but it specifically doesn't deal
>> with the yet unknown original cause of the problem).
>> 
>> Also add in a missing line in x86-64's do_page_walk().
>> 
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxxxx>
>> 
>> --- 2009-07-10.orig/xen/arch/x86/x86_32/traps.c 2009-07-10 13:58:56.000000000
>> +0200
>> +++ 2009-07-10/xen/arch/x86/x86_32/traps.c 2009-07-17 11:12:42.000000000
>> +0200
>> @@ -165,7 +165,8 @@ void show_page_walk(unsigned long addr)
>>      printk(" L3[0x%03lx] = %"PRIpte" %08lx\n",
>>             l3_table_offset(addr), l3e_get_intpte(l3e), pfn);
>>      unmap_domain_page(l3t);
>> -    if ( !(l3e_get_flags(l3e) & _PAGE_PRESENT) )
>> +    if ( !(l3e_get_flags(l3e) & _PAGE_PRESENT) ||
>> +         !mfn_valid(mfn) )
>>          return;
>>  
>>      l2t = map_domain_page(mfn);
>> @@ -178,7 +179,8 @@ void show_page_walk(unsigned long addr)
>>             (l2e_get_flags(l2e) & _PAGE_PSE) ? "(PSE)" : "");
>>      unmap_domain_page(l2t);
>>      if ( !(l2e_get_flags(l2e) & _PAGE_PRESENT) ||
>> -         (l2e_get_flags(l2e) & _PAGE_PSE) )
>> +         (l2e_get_flags(l2e) & _PAGE_PSE) ||
>> +         !mfn_valid(mfn) )
>>          return;
>>  
>>      l1t = map_domain_page(mfn);
>> --- 2009-07-10.orig/xen/arch/x86/x86_64/mm.c 2009-07-10 13:58:56.000000000
>> +0200
>> +++ 2009-07-10/xen/arch/x86/x86_64/mm.c 2009-07-17 11:10:29.000000000 +0200
>> @@ -124,6 +124,7 @@ void *do_page_walk(struct vcpu *v, unsig
>>      l3e = l3t[l3_table_offset(addr)];
>>      mfn = l3e_get_pfn(l3e);
>>      if ( !(l3e_get_flags(l3e) & _PAGE_PRESENT) || !mfn_valid(mfn) )
>> +        return NULL;
>>      if ( (l3e_get_flags(l3e) & _PAGE_PSE) )
>>          return mfn_to_virt(mfn) + (addr & ((1UL << L3_PAGETABLE_SHIFT) -
>> 1));
>>  
>> --- 2009-07-10.orig/xen/arch/x86/x86_64/traps.c 2009-07-10 13:58:56.000000000
>> +0200
>> +++ 2009-07-10/xen/arch/x86/x86_64/traps.c 2009-07-17 11:11:59.000000000
>> +0200
>> @@ -180,7 +180,8 @@ void show_page_walk(unsigned long addr)
>>      pfn = mfn_valid(mfn) ? get_gpfn_from_mfn(mfn) : INVALID_M2P_ENTRY;
>>      printk(" L4[0x%03lx] = %"PRIpte" %016lx\n",
>>             l4_table_offset(addr), l4e_get_intpte(l4e), pfn);
>> -    if ( !(l4e_get_flags(l4e) & _PAGE_PRESENT) )
>> +    if ( !(l4e_get_flags(l4e) & _PAGE_PRESENT) ||
>> +         !mfn_valid(mfn) )
>>          return;
>>  
>>      l3t = mfn_to_virt(mfn);
>> @@ -192,7 +193,8 @@ void show_page_walk(unsigned long addr)
>>             l3_table_offset(addr), l3e_get_intpte(l3e), pfn,
>>             (l3e_get_flags(l3e) & _PAGE_PSE) ? " (PSE)" : "");
>>      if ( !(l3e_get_flags(l3e) & _PAGE_PRESENT) ||
>> -         (l3e_get_flags(l3e) & _PAGE_PSE) )
>> +         (l3e_get_flags(l3e) & _PAGE_PSE) ||
>> +         !mfn_valid(mfn) )
>>          return;
>>  
>>      l2t = mfn_to_virt(mfn);
>> @@ -204,7 +206,8 @@ void show_page_walk(unsigned long addr)
>>             l2_table_offset(addr), l2e_get_intpte(l2e), pfn,
>>             (l2e_get_flags(l2e) & _PAGE_PSE) ? "(PSE)" : "");
>>      if ( !(l2e_get_flags(l2e) & _PAGE_PRESENT) ||
>> -         (l2e_get_flags(l2e) & _PAGE_PSE) )
>> +         (l2e_get_flags(l2e) & _PAGE_PSE) ||
>> +         !mfn_valid(mfn) )
>>          return;
>>  
>>      l1t = mfn_to_virt(mfn);
>> 
>> 
>> 
>> 
>> _______________________________________________
>> Xen-devel mailing list
>> Xen-devel@xxxxxxxxxxxxxxxxxxx
>> http://lists.xensource.com/xen-devel
> 
> 



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