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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 09.11.17 at 16:29, <yu.c.zhang@xxxxxxxxxxxxxxx> 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@xxxxxxxxx>
> Signed-off-by: Yi Zhang <yi.z.zhang@xxxxxxxxx>
> Signed-off-by: Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>

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.

Jan


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