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[PATCH V4 13/24] xen/ioreq: Use guest_cmpxchg64() instead of cmpxchg()

From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>

The cmpxchg() in ioreq_send_buffered() 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>
Acked-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
CC: Julien Grall <julien.grall@xxxxxxx>
[On Arm only]
Tested-by: Wei Chen <Wei.Chen@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

Changes V2 -> V3:
   - update patch description

Changes V3 -> V4:
   - add Stefano's A-b
   - drop comment from arm/hvm.c
 xen/common/ioreq.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/xen/common/ioreq.c b/xen/common/ioreq.c
index d233a49..d5f4dd3 100644
--- a/xen/common/ioreq.c
+++ b/xen/common/ioreq.c
@@ -29,6 +29,7 @@
 #include <xen/trace.h>
 #include <xen/vpci.h>
+#include <asm/guest_atomics.h>
 #include <asm/hvm/ioreq.h>
 #include <public/hvm/ioreq.h>
@@ -1185,7 +1186,7 @@ static int ioreq_send_buffered(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);



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