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

Hi Stefano,

On 06/12/2018 22:04, Stefano Stabellini wrote:
On Wed, 5 Dec 2018, Julien Grall wrote:
On 04/12/2018 23:59, Stefano Stabellini wrote:
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
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(+),should  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);
   +    /*
+     * 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
+     *  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
-                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
+         * 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?

This function can only possibly work with p2m_ram_* because of the
get_page(...) below, indeed the page should belong to the domain.

Effectively this function will only be used for hypercall as you use a virtual
address. I question the value of allowing a guest to do a hypercall with the
data backed in any other memories than guest RAM. For the foreign mapping,
this could potentially end up with a leakage.


I can probably add a few more dprintk in the error paths to help the developer to diagnostics the problem if that's ever happen. What do you think?

Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>

Thank you!


Julien Grall

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