hw/core/loader: In gunzip(), check index is in range before use, not after

hw/core/loader: In gunzip(), check index is in range before use, not after

The gunzip() function reads various fields from a passed in source
buffer in order to skip a header before passing the actual compressed
data to the zlib inflate() function.  It does check whether the
passed in buffer is too small, but unfortunately it checks that only
after reading bytes from the src buffer, so it could read off the end
of the buffer.

You can see this with valgrind:

 $ printf "%b" '\x1f\x8b' > /tmp/image
 $ valgrind qemu-system-aarch64 -display none -M virt -cpu max -kernel /tmp/image
 [...]
 ==19224== Invalid read of size 1
 ==19224==    at 0x67302E: gunzip (loader.c:558)
 ==19224==    by 0x673907: load_image_gzipped_buffer (loader.c:788)
 ==19224==    by 0xA18032: load_aarch64_image (boot.c:932)
 ==19224==    by 0xA18489: arm_setup_direct_kernel_boot (boot.c:1063)
 ==19224==    by 0xA18D90: arm_load_kernel (boot.c:1317)
 ==19224==    by 0x9F3651: machvirt_init (virt.c:2114)
 ==19224==    by 0x794B7A: machine_run_board_init (machine.c:1272)
 ==19224==    by 0xD5CAD3: qemu_init_board (vl.c:2618)
 ==19224==    by 0xD5CCA6: qmp_x_exit_preconfig (vl.c:2692)
 ==19224==    by 0xD5F32E: qemu_init (vl.c:3713)
 ==19224==    by 0x5ADDB1: main (main.c:49)
 ==19224==  Address 0x3802a873 is 0 bytes after a block of size 3 alloc'd
 ==19224==    at 0x4C31B0F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
 ==19224==    by 0x61E7657: g_file_get_contents (in /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0.5600.4)
 ==19224==    by 0x673895: load_image_gzipped_buffer (loader.c:771)
 ==19224==    by 0xA18032: load_aarch64_image (boot.c:932)
 ==19224==    by 0xA18489: arm_setup_direct_kernel_boot (boot.c:1063)
 ==19224==    by 0xA18D90: arm_load_kernel (boot.c:1317)
 ==19224==    by 0x9F3651: machvirt_init (virt.c:2114)
 ==19224==    by 0x794B7A: machine_run_board_init (machine.c:1272)
 ==19224==    by 0xD5CAD3: qemu_init_board (vl.c:2618)
 ==19224==    by 0xD5CCA6: qmp_x_exit_preconfig (vl.c:2692)
 ==19224==    by 0xD5F32E: qemu_init (vl.c:3713)
 ==19224==    by 0x5ADDB1: main (main.c:49)

Check that we have enough bytes of data to read the header bytes that
we read before we read them.

Fixes: Coverity 1458997
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20210812141803.20913-1-peter.maydell@linaro.org