Re: Xen 4.7 crash

From: Andrew Cooper <andrew.cooper3@citrix.com>
To: Julien Grall <julien.grall@arm.com>,
	Aaron Cornelius <Aaron.Cornelius@dornerworks.com>,
	Xen-devel <xen-devel@lists.xenproject.org>,
	Stefano Stabellini <sstabellini@kernel.org>
Subject: Re: Xen 4.7 crash
Date: Wed, 1 Jun 2016 23:31:21 +0100	[thread overview]
Message-ID: <40a51386-8c50-d5b4-259e-ca577b86789f@citrix.com> (raw)
In-Reply-To: <f7916387-200e-8f8b-1820-4e23d4fc9367@arm.com>

On 01/06/2016 23:24, Julien Grall wrote:
> Hi,
>
> On 01/06/2016 22:35, Andrew Cooper wrote:
>> On 01/06/2016 20:54, Aaron Cornelius wrote:
>>> <snip>
>>> (XEN) Xen call trace:
>>> (XEN)    [<0021fdd4>] free_domheap_pages+0x1c/0x324 (PC)
>>> (XEN)    [<0025b0cc>] p2m_teardown+0xa0/0x108 (LR)
>>> (XEN)    [<0025b0cc>] p2m_teardown+0xa0/0x108
>>> (XEN)    [<0024f668>] arch_domain_destroy+0x20/0x50
>>> (XEN)    [<0024f8f0>] arch_domain_create+0x258/0x284
>>> (XEN)    [<0020854c>] domain_create+0x2dc/0x510
>>> (XEN)    [<00206d6c>] do_domctl+0x5b4/0x1928
>>> (XEN)    [<00260130>] do_trap_hypervisor+0x1170/0x15b0
>>> (XEN)    [<00263b10>] entry.o#return_from_trap+0/0x4
>>> (XEN)
>>> (XEN)
>>> (XEN) ****************************************
>>> (XEN) Panic on CPU 0:
>>> (XEN) CPU0: Unexpected Trap: Data Abort
>>> (XEN)
>>> (XEN) ****************************************
>>> (XEN)
>>> (XEN) Reboot in five seconds...
>>
>> As for this specific crash itself,  In the case of an early error path,
>> p2m->root can be NULL in p2m_teardown(), in which case
>> free_domheap_pages() will fall over in a heap.  This patch should
>> resolve it.
>
> Good catch!
>
>>
>> @@ -1408,7 +1411,8 @@ void p2m_teardown(struct domain *d)
>>      while ( (pg = page_list_remove_head(&p2m->pages)) )
>>          free_domheap_page(pg);
>>
>> -    free_domheap_pages(p2m->root, P2M_ROOT_ORDER);
>> +    if ( p2m->root )
>> +        free_domheap_pages(p2m->root, P2M_ROOT_ORDER);
>>
>>      p2m->root = NULL;
>>
>> I would be tempted to suggest making free_domheap_pages() tolerate NULL
>> pointers, except that would only be a safe thing to do if we assert that
>> the order parameter is 0, which won't help this specific case.
>
> free_xenheap_pages already tolerates NULL (even if an order != 0). Is
> there any reason to not do the same for free_domheap_pages?

The xenheap allocation functions deal in terms of plain virtual
addresses, while the domheap functions deal in terms of struct page_info *.

Overall, this means that the domheap functions have a more restricted
input/output set than their xenheap variants.

As there is already precedent with xenheap, making domheap tolerate NULL
is probably fine, and indeed the preferred course of action.

~Andrew

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