Having upgraded from Mesa-17.3 to Mesa-18.1 in Endless OS, many users on AMD-based platforms are now reporting that the system fails to boot into the UI. I've reproduced and confirm that Xorg is crashing very early on.

Thread 4 "si_shader:0" received signal SIGSEGV, Segmentation fault.
__memcpy_ssse3_back () at ../sysdeps/x86_64/multiarch/memcpy-ssse3-back.S:1533
Backtrace:
#0  __memcpy_ssse3_back ()
    at ../sysdeps/x86_64/multiarch/memcpy-ssse3-back.S:1533
#1  0x00007fffeeba2038 in memcpy (__len=3221880836, __src=0x7fffe4000e70, 
    __dest=<optimized out>) at /usr/include/x86_64-linux-gnu/bits/string3.h:53
#2  read_data (size=3221880836, data=<optimized out>, ptr=0x7fffe4000e70)
    at ../../../../../src/gallium/drivers/radeonsi/si_state_shaders.c:95
#3  read_chunk (ptr=0x7fffe4000e70, ptr@entry=0x7fffe4000e6c, 
    data=data@entry=0x7fffe4000998, size=size@entry=0x7fffe4000980)
    at ../../../../../src/gallium/drivers/radeonsi/si_state_shaders.c:121
#4  0x00007fffeeba21b3 in si_load_shader_binary (
    shader=shader@entry=0x7fffe40008c0, binary=binary@entry=0x7fffe4000e00)
    at ../../../../../src/gallium/drivers/radeonsi/si_state_shaders.c:187
#5  0x00007fffeeba4810 in si_shader_cache_load_shader (shader=0x7fffe40008c0, 
    ir_binary=0x7fffe4000a50, sscreen=0x555555a393a0)
    at ../../../../../src/gallium/drivers/radeonsi/si_state_shaders.c:275
#6  si_init_shader_selector_async (job=job@entry=0x555555b8dfa0, 
    thread_index=thread_index@entry=0)
    at ../../../../../src/gallium/drivers/radeonsi/si_state_shaders.c:1875
#7  0x00007fffee747a55 in util_queue_thread_func (
    input=input@entry=0x555555a39fb0) at ../../../src/util/u_queue.c:271
#8  0x00007fffee7476c7 in impl_thrd_routine (p=<optimized out>)
    at ../../../include/c11/threads_posix.h:87
#9  0x00007ffff574d494 in start_thread (arg=0x7fffebe06700)

The problem here is that the on-disk radeonsi cache format changed without consideration for this in the code. The affected codepath is si_load_shader_binary() which does:

	uint32_t size = *ptr++;
	uint32_t crc32 = *ptr++;
        [...]
	ptr = read_data(ptr, &shader->config, sizeof(shader->config));
	ptr = read_data(ptr, &shader->info, sizeof(shader->info));
	ptr = read_chunk(ptr, (void**)&shader->binary.code,
			 &shader->binary.code_size);

So, the blob format is: 4 bytes size, 4 bytes CRC, shader config, shader info, code.

In mesa-17.3 the si_shader_config was 48 bytes in size, but in Mesa-18.1 and current master, si_shader_config is 52 bytes in size, because the max_simd_wave field was added.

After upgrading mesa to 18.1, with shaders compiled and cached by mesa-17.3, now the above code will obviously not behave as intended. We enter into read_chunk() with the offsets slightly wrong:

	*size = *ptr++;
	assert(*data == NULL);
	if (!*size)
		return ptr;
	*data = malloc(*size);
	return read_data(ptr, *data, *size);

and when this code executes, *size has value 3221880836, for a shader that was only 884 bytes uncompressed. read_data then tries to memcpy this much data, and that causes the crash.

In addition to the lack of invalidation of existing disk caches after the on-disk format was changed, this code also seems rather suspect in that it does not verify that it is not reading beyond the end of the shader. As an attacker I could maliciously rewrite the size field read by the read_chunk() code above to be very large, fixup the CRC and recompress, and then I could cause other apps to crash in this way.