arm: avoid heap-buffer-overflow in load_aarch64_image

Spotted by ASAN:

elmarco@boraha:~/src/qemu/build (master *%)$ QTEST_QEMU_BINARY=aarch64-softmmu/qemu-system-aarch64 tests/boot-serial-test
/aarch64/boot-serial/virt: ** (process:19740): DEBUG: 18:39:30.275: foo /tmp/qtest-boot-serial-cXaS94D
=================================================================
==19740==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x603000069648 at pc 0x7f1d2201cc54 bp 0x7fff331f6a40 sp 0x7fff331f61e8
READ of size 4 at 0x603000069648 thread T0
    #0 0x7f1d2201cc53  (/lib64/libasan.so.4+0xafc53)
    #1 0x55bc86685ee3 in load_aarch64_image /home/elmarco/src/qemu/hw/arm/boot.c:894
    #2 0x55bc86687217 in arm_load_kernel_notify /home/elmarco/src/qemu/hw/arm/boot.c:1047
    #3 0x55bc877363b5 in notifier_list_notify /home/elmarco/src/qemu/util/notify.c:40
    #4 0x55bc869331ea in qemu_run_machine_init_done_notifiers /home/elmarco/src/qemu/vl.c:2716
    #5 0x55bc8693bc39 in main /home/elmarco/src/qemu/vl.c:4679
    #6 0x7f1d1652c009 in __libc_start_main (/lib64/libc.so.6+0x21009)
    #7 0x55bc86255cc9 in _start (/home/elmarco/src/qemu/build/aarch64-softmmu/qemu-system-aarch64+0x1ae5cc9)

Signed-off-by: Marc-André Lureau <[email protected]>
Reviewed-by: Peter Maydell <[email protected]>
Signed-off-by: Peter Maydell <[email protected]>

hw/arm/boot.c

@@ -891,7 +891,8 @@ static uint64_t load_aarch64_image(const char *filename, hwaddr mem_base,
     }
 
     /* check the arm64 magic header value -- very old kernels may not have it */
-    if (memcmp(buffer + ARM64_MAGIC_OFFSET, "ARM\x64", 4) == 0) {
+    if (size > ARM64_MAGIC_OFFSET + 4 &&
+        memcmp(buffer + ARM64_MAGIC_OFFSET, "ARM\x64", 4) == 0) {
         uint64_t hdrvals[2];
 
         /* The arm64 Image header has text_offset and image_size fields at 8 and