Re: [Xen-devel] [PATCH] page-alloc: detect double free earlier

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 09/05/2019 14:25, Jan Beulich wrote:
>>>> On 09.05.19 at 14:50, <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 09/05/2019 13:35, Jan Beulich wrote:
>>> Right now this goes unnoticed until some subsequent page allocator
>>> operation stumbles across the thus corrupted list. We can do better:
>>> Only PGC_state_inuse and PGC_state_offlining pages can legitimately be
>>> passed to free_heap_pages().
>>>
>>> Take the opportunity and also restrict the PGC_broken check to the
>>> PGC_state_offlining case, as only pages of that type or
>>> PGC_state_offlined may have this flag set on them. Similarly, since
>>> PGC_state_offlined is not a valid input state, the setting of "tainted"
>>> can be restricted to just this case.
>>>
>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, with a suggestion.
> Thanks.
>
>>> --- a/xen/common/page_alloc.c
>>> +++ b/xen/common/page_alloc.c
>>> @@ -1409,13 +1409,22 @@ static void free_heap_pages(
>>>           *     in its pseudophysical address space).
>>>           * In all the above cases there can be no guest mappings of this 
>>> page.
>>>           */
>>> -        ASSERT(!page_state_is(&pg[i], offlined));
>>> -        pg[i].count_info =
>>> -            ((pg[i].count_info & PGC_broken) |
>>> -             (page_state_is(&pg[i], offlining)
>>> -              ? PGC_state_offlined : PGC_state_free));
>>> -        if ( page_state_is(&pg[i], offlined) )
>>> +        switch ( pg[i].count_info & PGC_state )
>>> +        {
>>> +        case PGC_state_inuse:
>>> +            BUG_ON(pg[i].count_info & PGC_broken);
>>> +            pg[i].count_info = PGC_state_free;
>>> +            break;
>>> +
>>> +        case PGC_state_offlining:
>>> +            pg[i].count_info = (pg[i].count_info & PGC_broken) |
>>> +                               PGC_state_offlined;
>>>              tainted = 1;
>>> +            break;
>>> +
>>> +        default:
>> Given that this is a fully fatal condition, it would be helpful to at
>> least print the state we found here.  For cases other than
>> PGC_state_free, it would probably be a very useful piece of information
>> for diagnosing what went wrong.
> Funny you should say this - I have the debugging patch below on top
> in my tree. I could easily submit this as a standalone follow-on patch.

TBH, I think it would be fine folded in, although with...

>
> Jan
>
> --- unstable.orig/xen/common/page_alloc.c
> +++ unstable/xen/common/page_alloc.c
> @@ -1014,7 +1014,14 @@ static struct page_info *alloc_heap_page
>      for ( i = 0; i < (1 << order); i++ )
>      {
>          /* Reference count must continuously be zero for free pages. */
> -        BUG_ON((pg[i].count_info & ~PGC_need_scrub) != PGC_state_free);
> +        if ( (pg[i].count_info & ~PGC_need_scrub) != PGC_state_free )
> +        {
> +            printk(XENLOG_ERR "pg[%x] m=%lx c=%lx o=%x v=%lx t=%x\n",

"pg[%u] mfn %"PRImfn" c=%#lx o=%u v=%#lx t=%#x\n"

so we don't end up printing numbers which are ambiguous between hex/dec.

With at least the ambiguity removed, my ack stands.

~Andrew

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