[PATCH] hw/i386/amd_iommu: Avoid undefined behavior in amdvi_setevent_bits()

[Thomas Huth <thuth@redhat.com>]

From: Thomas Huth <thuth@redhat.com>

The code in amdvi_encode_event() calls amdvi_setevent_bits() with
start = 64:

    amdvi_setevent_bits(evt, addr, 64, 64);

and amdvi_setevent_bits() then calculates:

    uint64_t mask = MAKE_64BIT_MASK(start, length);

but this MAKE_64BIT_MASK() macro shifts a value left by "start" bit
positions. Shifting left by more than 63 is undefined behavior and
could have unexpected results with different compilers / architectures.

Fix it by using "bitpos" instead, which was likely the original
intended behavior anyway. (bitpos is calculated as bitpos = start % 64).

Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/3633
Signed-off-by: Thomas Huth <thuth@redhat.com>
---
 hw/i386/amd_iommu.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/hw/i386/amd_iommu.c b/hw/i386/amd_iommu.c
index 90252c52af4..578c27ccbef 100644
--- a/hw/i386/amd_iommu.c
+++ b/hw/i386/amd_iommu.c
@@ -323,7 +323,7 @@ static void amdvi_setevent_bits(uint64_t *buffer, uint64_t value, int start,
                                 int length)
 {
     int index = start / 64, bitpos = start % 64;
-    uint64_t mask = MAKE_64BIT_MASK(start, length);
+    uint64_t mask = MAKE_64BIT_MASK(bitpos, length);
     buffer[index] &= ~mask;
     buffer[index] |= (value << bitpos) & mask;
 }
-- 
2.55.0