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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
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(+),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);
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?
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.
OK.
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!
Cheers,
--
Julien Grall
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