Code: get_matching_model_microcode(int cpu, unsigned long start,
			     void *data, size_t size,
			     struct mc_saved_data *mc_saved_data,
			     unsigned long *mc_saved_in_initrd,
			     struct ucode_cpu_info *uci)
{
	u8 *ucode_ptr = data;
	unsigned int leftover = size;
	enum ucode_state state = UCODE_OK;
	unsigned int mc_size;
	struct microcode_header_intel *mc_header;
	struct microcode_intel *mc_saved_tmp[MAX_UCODE_COUNT];
 	unsigned int mc_saved_count = mc_saved_data->mc_saved_count;
 	int i;
 
	while (leftover) {
 		mc_header = (struct microcode_header_intel *)ucode_ptr;
 
 		mc_size = get_totalsize(mc_header);
		if (!mc_size || mc_size > leftover ||
			microcode_sanity_check(ucode_ptr, 0) < 0)
			break;

		leftover -= mc_size;

		/*
		 * Since APs with same family and model as the BSP may boot in
		 * the platform, we need to find and save microcode patches
		 * with the same family and model as the BSP.
		 */
		if (matching_model_microcode(mc_header, uci->cpu_sig.sig) !=
			 UCODE_OK) {
			ucode_ptr += mc_size;
			continue;
		}

		_save_mc(mc_saved_tmp, ucode_ptr, &mc_saved_count);

		ucode_ptr += mc_size;
	}

	if (leftover) {
		state = UCODE_ERROR;
		goto out;
	}

	if (mc_saved_count == 0) {
		state = UCODE_NFOUND;
		goto out;
	}

	for (i = 0; i < mc_saved_count; i++)
		mc_saved_in_initrd[i] = (unsigned long)mc_saved_tmp[i] - start;

	mc_saved_data->mc_saved_count = mc_saved_count;
out:
	return state;
}

Vulnerability Type: Overflow +Priv 
CWE ID: CWE-119
Summary: Stack-based buffer overflow in the get_matching_model_microcode function in arch/x86/kernel/cpu/microcode/intel_early.c in the Linux kernel before 4.0 allows context-dependent attackers to gain privileges by constructing a crafted microcode header and leveraging root privileges for write access to the initrd.
Commit Message: x86/microcode/intel: Guard against stack overflow in the loader

mc_saved_tmp is a static array allocated on the stack, we need to make
sure mc_saved_count stays within its bounds, otherwise we're overflowing
the stack in _save_mc(). A specially crafted microcode header could lead
to a kernel crash or potentially kernel execution.

Signed-off-by: Quentin Casasnovas <quentin.casasnovas@oracle.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Link: http://lkml.kernel.org/r/1422964824-22056-1-git-send-email-quentin.casasnovas@oracle.com
Signed-off-by: Borislav Petkov <bp@suse.de>