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Re: [PATCH] revert "x86/mm: re-implement get_page_light() using an atomic increment"


  • To: Jan Beulich <jbeulich@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Mon, 29 Apr 2024 14:09:52 +0100
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  • Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxx>
  • Delivery-date: Mon, 29 Apr 2024 13:10:03 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

On 29/04/2024 2:01 pm, Jan Beulich wrote:
> revert "x86/mm: re-implement get_page_light() using an atomic increment"
>
> This reverts commit c40bc0576dcc5acd4d7e22ef628eb4642f568533.
>
> That change aimed at eliminating an open-coded lock-like construct,
> which really isn't all that similar to, in particular, get_page(). The
> function always succeeds. Any remaining concern would want taking care
> of by placing block_lock_speculation() at the end of the function.
> Since the function is called only during page (de)validation, any
> possible performance concerns over such extra serialization could
> likely be addressed by pre-validating (e.g. via pinning) page tables.
>
> The fundamental issue with the change being reverted is that it detects
> bad state only after already having caused possible corruption. While
> the system is going to be halted in such an event, there is a time
> window during which the resulting incorrect state could be leveraged by
> a clever (in particular: fast enough) attacker.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>



 


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