[Qemu-devel] [PATCH] exec: Fix non-power-of-2 sized accesses

[Alex Williamson <alex.williamson@redhat.com>]

Since commit 23326164 we align access sizes to match the alignment of
the address, but we don't align the access size itself.  This means we
let illegal access sizes (ex. 3) slip through if the address is
sufficiently aligned (ex. 4).  This results in an abort which would be
easy for a guest to trigger.  Account for aligning the access size.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Cc: qemu-stable@nongnu.org
---

In the example I saw the guest was doing a 4-byte read at I/O port
0xcd7.  We satisfy the first byte with a 1-byte read leaving 3 bytes
remaining at an 8-byte aligned address... boom.  ffs() caused weird
stack smashing errors here, so I just did a loop since it can only
run for a few iterations max.

 exec.c |    7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/exec.c b/exec.c
index 3ca9381..652fc3a 100644
--- a/exec.c
+++ b/exec.c
@@ -1924,6 +1924,13 @@ static int memory_access_size(MemoryRegion *mr, unsigned l, hwaddr addr)
         }
     }
 
+    /* Size must be a power of 2 */
+    if (l & (l - 1)) {
+        while (l & (access_size_max - 1) && access_size_max > 1) {
+            access_size_max >>= 1;
+        }
+    }
+
     /* Don't attempt accesses larger than the maximum.  */
     if (l > access_size_max) {
         l = access_size_max;