[PATCH V4 20/24] xen/arm: io: Harden sign extension check

[Oleksandr Tyshchenko <olekstysh@xxxxxxxxx>]

From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>

In the ideal world we would never get an undefined behavior when
propagating the sign bit since that bit can only be set for access
size smaller than the register size (i.e byte/half-word for aarch32,
byte/half-word/word for aarch64).

In the real world we need to care for *possible* hardware bug such as
advertising a sign extension for either 64-bit (or 32-bit) on Arm64
(resp. Arm32).

So harden a bit more the code to prevent undefined behavior when
propagating the sign bit in case of buggy hardware.

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 V3 -> V4:
   - new patch
---
 xen/include/asm-arm/traps.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/xen/include/asm-arm/traps.h b/xen/include/asm-arm/traps.h
index e301c44..992d537 100644
--- a/xen/include/asm-arm/traps.h
+++ b/xen/include/asm-arm/traps.h
@@ -93,7 +93,8 @@ static inline register_t sign_extend(const struct hsr_dabt 
dabt, register_t r)
      * Note that we expect the read handler to have zeroed the bits
      * outside the requested access size.
      */
-    if ( dabt.sign && (r & (1UL << (size - 1))) )
+    if ( dabt.sign && (size < sizeof(register_t) * 8) &&
+         (r & (1UL << (size - 1))) )
     {
         /*
          * We are relying on register_t using the same as
-- 
2.7.4