Re: [Xen-devel] [PATCH for-4.12 v2 05/17] xen/arm: p2m: Handle translation fault in get_page_from_gva

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 4 Dec 2018, Julien Grall wrote:
> A follow-up patch will re-purpose the valid bit of LPAE entries to
> generate fault even on entry containing valid information.
> 
> This means that when translating a guest VA to guest PA (e.g IPA) will
> fail if the Stage-2 entries used have the valid bit unset. Because of
> that, we need to fallback to walk the page-table in software to check
> whether the fault was expected.
> 
> This patch adds the software page-table walk on all the translation
> fault. It would be possible in the future to avoid pointless walk when
> the fault in PAR_EL1 is not a translation fault.
> 
> Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
> 
> ---
> 
> There are a couple of TODO in the code. They are clean-up and performance
> improvement (e.g when the fault cannot be handled) that could be delayed after
> the series has been merged.
> 
>     Changes in v2:
>         - Check stage-2 permission during software lookup
>         - Fix typoes
> ---
>  xen/arch/arm/p2m.c | 66 
> ++++++++++++++++++++++++++++++++++++++++++++++++------
>  1 file changed, 59 insertions(+), 7 deletions(-)
> 
> diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
> index 47b54c792e..39680eeb6e 100644
> --- a/xen/arch/arm/p2m.c
> +++ b/xen/arch/arm/p2m.c
> @@ -6,6 +6,7 @@
>  
>  #include <asm/event.h>
>  #include <asm/flushtlb.h>
> +#include <asm/guest_walk.h>
>  #include <asm/page.h>
>  
>  #define MAX_VMID_8_BIT  (1UL << 8)
> @@ -1430,6 +1431,8 @@ struct page_info *get_page_from_gva(struct vcpu *v, 
> vaddr_t va,
>      struct page_info *page = NULL;
>      paddr_t maddr = 0;
>      uint64_t par;
> +    mfn_t mfn;
> +    p2m_type_t t;
>  
>      /*
>       * XXX: To support a different vCPU, we would need to load the
> @@ -1446,8 +1449,29 @@ struct page_info *get_page_from_gva(struct vcpu *v, 
> vaddr_t va,
>      par = gvirt_to_maddr(va, &maddr, flags);
>      p2m_read_unlock(p2m);
>  
> +    /*
> +     * gvirt_to_maddr may fail if the entry does not have the valid bit
> +     * set. Fallback to the second method:
> +     *  1) Translate the VA to IPA using software lookup -> Stage-1 
> page-table
> +     *  may not be accessible because the stage-2 entries may have valid
> +     *  bit unset.
> +     *  2) Software lookup of the MFN
> +     *
> +     * Note that when memaccess is enabled, we instead call directly
> +     * p2m_mem_access_check_and_get_page(...). Because the function is a
> +     * a variant of the methods described above, it will be able to
> +     * handle entries with valid bit unset.
> +     *
> +     * TODO: Integrate more nicely memaccess with the rest of the
> +     * function.
> +     * TODO: Use the fault error in PAR_EL1 to avoid pointless
> +     *  translation.
> +     */
>      if ( par )
>      {
> +        paddr_t ipa;
> +        unsigned int s1_perms;
> +
>          /*
>           * When memaccess is enabled, the translation GVA to MADDR may
>           * have failed because of a permission fault.
> @@ -1455,20 +1479,48 @@ struct page_info *get_page_from_gva(struct vcpu *v, 
> vaddr_t va,
>          if ( p2m->mem_access_enabled )
>              return p2m_mem_access_check_and_get_page(va, flags, v);
>  
> -        dprintk(XENLOG_G_DEBUG,
> -                "%pv: gvirt_to_maddr failed va=%#"PRIvaddr" flags=0x%lx 
> par=%#"PRIx64"\n",
> -                v, va, flags, par);
> -        return NULL;
> +        /*
> +         * The software stage-1 table walk can still fail, e.g, if the
> +         * GVA is not mapped.
> +         */
> +        if ( !guest_walk_tables(v, va, &ipa, &s1_perms) )
> +        {
> +            dprintk(XENLOG_G_DEBUG,
> +                    "%pv: Failed to walk page-table va %#"PRIvaddr"\n", v, 
> va);
> +            return NULL;
> +        }
> +
> +        mfn = p2m_lookup(d, gaddr_to_gfn(ipa), &t);
> +        if ( mfn_eq(INVALID_MFN, mfn) || !p2m_is_ram(t) )
> +            return NULL;
> +
> +        /*
> +         * Check permission that are assumed by the caller. For instance
> +         * in case of guestcopy, the caller assumes that the translated
> +         * page can be accessed with the requested permissions. If this
> +         * is not the case, we should fail.
> +         *
> +         * Please note that we do not check for the GV2M_EXEC
> +         * permission. This is fine because the hardware-based translation
> +         * instruction does not test for execute permissions.
> +         */
> +        if ( (flags & GV2M_WRITE) && !(s1_perms & GV2M_WRITE) )
> +            return NULL;
> +
> +        if ( (flags & GV2M_WRITE) && t != p2m_ram_rw )
> +            return NULL;

The patch looks good enough now. One question: is it a requirement that
the page we are trying to translate is of type p2m_ram_*? Could
get_page_from_gva be genuinely called passing a page of a different
kind, such as p2m_mmio_direct_* or p2m_map_foreign? Today, it is not the
case, but I wonder if it is something we want to consider?


>      }
> +    else
> +        mfn = maddr_to_mfn(maddr);
>  
> -    if ( !mfn_valid(maddr_to_mfn(maddr)) )
> +    if ( !mfn_valid(mfn) )
>      {
>          dprintk(XENLOG_G_DEBUG, "%pv: Invalid MFN %#"PRI_mfn"\n",
> -                v, mfn_x(maddr_to_mfn(maddr)));
> +                v, mfn_x(mfn));
>          return NULL;
>      }
>  
> -    page = mfn_to_page(maddr_to_mfn(maddr));
> +    page = mfn_to_page(mfn);
>      ASSERT(page);
>  
>      if ( unlikely(!get_page(page, d)) )
> -- 
> 2.11.0
> 

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