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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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>



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