Re: [Xen-devel] [PATCH v3 for-4.10 1/2] x86/mm: fix potential race conditions in map_pages_to_xen().

[Julien Grall <julien.grall@xxxxxxxxxx>]

Hi,

On 14/11/17 08:20, Jan Beulich wrote:
> > > > On 14.11.17 at 07:53, <yu.c.zhang@xxxxxxxxxxxxxxx> wrote:
> > From: Min He <min.he@xxxxxxxxx>
> >
> > 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.
> >
> > This patch fixes the above problem by protecting the `pl1e` with the lock.
> >
> > Also, there're other potential race conditions. For instance, the L2/L3
> > entry may be modified concurrently on different CPUs, by routines such as
> > map_pages_to_xen(), modify_xen_mappings() etc. To fix this, this patch will
> > check the _PAGE_PRESENT and _PAGE_PSE flags, after the spinlock is obtained,
> > for the corresponding L2/L3 entry.
> >
> > 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>
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
Please try to have a cover letter in the future when you have multiple 
patches. This will make easier to give comments/release-ack for the all 
the patches. Anyway for the 2 patches:
Release-acked-by: Julien Grall <julien.grall@xxxxxxxxxx>

Cheers,

--
Julien Grall

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