Re: [Xen-devel] [PATCH] xen/arm: p2m: Don't use default access permission when shattering a superpage

[Tamas K Lengyel <tamas@xxxxxxxxxxxxx>]

On Fri, Jul 29, 2016 at 12:53 PM, Julien Grall <julien.grall@xxxxxxx> wrote:
> The following message flood the console when memaccess is enabled on
> various platforms:
>
> traps.c:2510:d1v0 HSR=0x9383004f pc=0xffff000008b7d4c4 gva=0xffff000008eeb8e0 
> gpa=0x0000004903f8e0
>
> This is because a data abort from a guest was received due to a
> permission fault but memaccess thought there are no permission fault.
>
> On ARM, memaccess permissions are stored in a radix tree because there
> are not enough available bits in the p2m entry to store the access
> restriction. When memaccess is restricting the access (i.e any other
> access than p2m_access_rwx), the access will be added in the radix tree
> using the GFN as a key. This will be done for all 4KB pages.
>
> This means that memaccess has to shatter all the superpages in a given
> region to set the permission on a 4KB granularity. Currently, when a
> superpage is shattered, the new entries are using the value
> p2m->default_access which will restrict permission (because memaccess
> has been enabled). However the radix tree does not yet contain
> an entry for this GFN.
>
> If a guest VCPU is running at the same time and trying to access the
> modified region, it will result to a stage-2 permission fault. As
> the radix tree does not yet contain an entry for the GFN, memaccess will
> deduce that the fault was not valid and a data abort will be injecting
> to the guest (and crash it).
>
> Furthermore, the permission may be restricted outside of the requested
> region if it is only a subset of a 1GB/2MB superpage.
>
> The two issues can be fixed by re-using the permission of the superpage
> entry and override the necessary fields. This is not a problem because
> memaccess cannot work on superpage.
>
> Lastly, document the code which call mfn_to_p2m_entry when creating a
> the p2m entry for a table to explain that create the p2m entry to page table
> to explain that permission are ignored by the hardware (See D4.3.1 in ARM DDI
> 0487A.j). so the value of the parameter 'access' of mfn_to_p2m_entry does
> not matter.
>
> Signed-off-by: Julien Grall <julien.grall@xxxxxxx>

Thanks for looking into this, it is very strange that this issue has
not surfaced before as mem_access was extensively tested on the
Arndale and the XU during the 4.6 merge. Maybe it just happened that
the superpage shattering path was never hit during the tests. It still
does work fine on my Cubietruck as of the latest staging..

Tamas

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