[PATCH V2 13/23] xen/ioreq: Use guest_cmpxchg64() instead of cmpxchg()

[Oleksandr Tyshchenko <olekstysh@xxxxxxxxx>]

From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>

The cmpxchg() in hvm_send_buffered_ioreq() operates on memory shared
with the emulator domain (and the target domain if the legacy
interface is used).

In order to be on the safe side we need to switch
to guest_cmpxchg64() to prevent a domain to DoS Xen on Arm.

As there is no plan to support the legacy interface on Arm,
we will have a page to be mapped in a single domain at the time,
so we can use s->emulator in guest_cmpxchg64() safely.

Thankfully the only user of the legacy interface is x86 so far
and there is not concern regarding the atomics operations.

Please note, that the legacy interface *must* not be used on Arm
without revisiting the code.

Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
CC: Julien Grall <julien.grall@xxxxxxx>

---
Please note, this is a split/cleanup/hardening of Julien's PoC:
"Add support for Guest IO forwarding to a device emulator"

Changes RFC -> V1:
   - new patch

Changes V1 -> V2:
   - move earlier to avoid breaking arm32 compilation
   - add an explanation to commit description and hvm_allow_set_param()
   - pass s->emulator
---
 xen/arch/arm/hvm.c | 4 ++++
 xen/common/ioreq.c | 3 ++-
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/xen/arch/arm/hvm.c b/xen/arch/arm/hvm.c
index 8951b34..9694e5a 100644
--- a/xen/arch/arm/hvm.c
+++ b/xen/arch/arm/hvm.c
@@ -31,6 +31,10 @@
 
 #include <asm/hypercall.h>
 
+/*
+ * The legacy interface (which involves magic IOREQ pages) *must* not be used
+ * without revisiting the code.
+ */
 static int hvm_allow_set_param(const struct domain *d, unsigned int param)
 {
     switch ( param )
diff --git a/xen/common/ioreq.c b/xen/common/ioreq.c
index 98fffae..8612159 100644
--- a/xen/common/ioreq.c
+++ b/xen/common/ioreq.c
@@ -28,6 +28,7 @@
 #include <xen/trace.h>
 #include <xen/vpci.h>
 
+#include <asm/guest_atomics.h>
 #include <asm/hvm/ioreq.h>
 
 #include <public/hvm/ioreq.h>
@@ -1317,7 +1318,7 @@ static int send_buffered_ioreq(struct ioreq_server *s, 
ioreq_t *p)
 
         new.read_pointer = old.read_pointer - n * IOREQ_BUFFER_SLOT_NUM;
         new.write_pointer = old.write_pointer - n * IOREQ_BUFFER_SLOT_NUM;
-        cmpxchg(&pg->ptrs.full, old.full, new.full);
+        guest_cmpxchg64(s->emulator, &pg->ptrs.full, old.full, new.full);
     }
 
     notify_via_xen_event_channel(d, s->bufioreq_evtchn);
-- 
2.7.4