Re: [PATCH] x86/mm: fix a potential race condition in map_pages_to_xen().

[Yu Zhang <yu.c.zhang@linux.intel.com>]

On 11/9/2017 5:22 PM, Jan Beulich wrote:
>>>> On 09.11.17 at 16:29, <yu.c.zhang@linux.intel.com> wrote:
>> In map_pages_to_xen(), a L2 page table entry may be reset to point to
>> a superpage, and its corresponding L1 page table need be freed in such
>> scenario, when these L1 page table entries are mapping to consecutive
>> page frames and having the same mapping flags.
>>
>> However, variable `pl1e` is not protected by the lock before L1 page table
>> is enumerated. A race condition may happen if this code path is invoked
>> simultaneously on different CPUs.
>>
>> For example, `pl1e` value on CPU0 may hold an obsolete value, pointing
>> to a page which has just been freed on CPU1. Besides, before this page
>> is reused, it will still be holding the old PTEs, referencing consecutive
>> page frames. Consequently the `free_xen_pagetable(l2e_to_l1e(ol2e))` will
>> be triggered on CPU0, resulting the unexpected free of a normal page.
>>
>> Protecting the `pl1e` with the lock will fix this race condition.
>>
>> Signed-off-by: Min He <min.he@intel.com>
>> Signed-off-by: Yi Zhang <yi.z.zhang@intel.com>
>> Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
> Oh, one more thing: Is it really the case that all three of you
> contributed to the patch? We don't use the Linux model of
> everyone through whose hands a patch passes adding an
> S-o-b of their own - that would rather be Reviewed-by then (if
> applicable).
>
> Also generally I would consider the first S-o-b to be that of the
> original author, yet the absence of an explicit From: tag makes
> authorship ambiguous here. Please clarify this in v2.

Oh, we three found this issue when debugging a bug together. And Min is
the author of this patch. So I'd like to add

"From: Min He <min.he@intel.com> "

at the beginning of the commit message in v2. :-)

Yu
> Jan
>
>


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