[PATCH] x86/microcode: Refactor Intel microcode loading

[Jann Horn]

This changes generic_load_microcode() to use the iov_iter API instead of
an open-coded version. This allows us to avoid explicitly casting between
user and kernel pointers.

Because the iov_iter API makes it hard to read the same location twice, as
a side effect, this also fixes a double-read of the microcode header (which
could e.g. lead to out-of-bounds reads in microcode_sanity_check()).
Not that it matters much, only root can do this anyway...

Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
---
I have tested that with this patch applied, microcode loading still works
both via "iucode-tool -k" and via
/sys/devices/system/cpu/microcode/reload.

 arch/x86/kernel/cpu/microcode/intel.c | 64 +++++++++++++--------------
 1 file changed, 32 insertions(+), 32 deletions(-)

diff --git a/arch/x86/kernel/cpu/microcode/intel.c b/arch/x86/kernel/cpu/microcode/intel.c
index 16936a24795c..cb976e8ef94d 100644
--- a/arch/x86/kernel/cpu/microcode/intel.c
+++ b/arch/x86/kernel/cpu/microcode/intel.c
@@ -31,6 +31,7 @@
 #include <linux/kernel.h>
 #include <linux/slab.h>
 #include <linux/cpu.h>
+#include <linux/uio.h>
 #include <linux/mm.h>
 
 #include <asm/microcode_intel.h>
@@ -861,32 +862,33 @@ static enum ucode_state apply_microcode_intel(int cpu)
 	return ret;
 }
 
-static enum ucode_state generic_load_microcode(int cpu, void *data, size_t size,
-				int (*get_ucode_data)(void *, const void *, size_t))
+static enum ucode_state generic_load_microcode(int cpu, struct iov_iter *iter)
 {
 	struct ucode_cpu_info *uci = ucode_cpu_info + cpu;
-	u8 *ucode_ptr = data, *new_mc = NULL, *mc = NULL;
+	u8 *new_mc = NULL, *mc = NULL;
 	int new_rev = uci->cpu_sig.rev;
-	unsigned int leftover = size;
 	unsigned int curr_mc_size = 0, new_mc_size = 0;
 	unsigned int csig, cpf;
 	enum ucode_state ret = UCODE_OK;
 
-	while (leftover) {
+	while (iov_iter_count(iter)) {
 		struct microcode_header_intel mc_header;
-		unsigned int mc_size;
+		unsigned int mc_size, data_size;
+		u8 *data;
 
-		if (leftover < sizeof(mc_header)) {
-			pr_err("error! Truncated header in microcode data file\n");
+		if (!copy_from_iter_full(&mc_header, sizeof(mc_header), iter)) {
+			pr_err("error! Truncated or inaccessible header in microcode data\n");
 			break;
 		}
 
-		if (get_ucode_data(&mc_header, ucode_ptr, sizeof(mc_header)))
-			break;
-
 		mc_size = get_totalsize(&mc_header);
-		if (!mc_size || mc_size > leftover) {
-			pr_err("error! Bad data in microcode data file\n");
+		if (mc_size < sizeof(mc_header)) {
+			pr_err("error! Bad data in microcode data file (totalsize too small)\n");
+			break;
+		}
+		data_size = mc_size - sizeof(mc_header);
+		if (data_size > iov_iter_count(iter)) {
+			pr_err("error! Bad data in microcode data file (truncated file?)\n");
 			break;
 		}
 
@@ -899,7 +901,9 @@ static enum ucode_state generic_load_microcode(int cpu, void *data, size_t size,
 			curr_mc_size = mc_size;
 		}
 
-		if (get_ucode_data(mc, ucode_ptr, mc_size) ||
+		memcpy(mc, &mc_header, sizeof(mc_header));
+		data = mc + sizeof(mc_header);
+		if (!copy_from_iter_full(data, data_size, iter) ||
 		    microcode_sanity_check(mc, 1) < 0) {
 			break;
 		}
@@ -914,14 +918,11 @@ static enum ucode_state generic_load_microcode(int cpu, void *data, size_t size,
 			mc = NULL;	/* trigger new vmalloc */
 			ret = UCODE_NEW;
 		}
-
-		ucode_ptr += mc_size;
-		leftover  -= mc_size;
 	}
 
 	vfree(mc);
 
-	if (leftover) {
+	if (iov_iter_count(iter)) {
 		vfree(new_mc);
 		return UCODE_ERROR;
 	}
@@ -945,12 +946,6 @@ static enum ucode_state generic_load_microcode(int cpu, void *data, size_t size,
 	return ret;
 }
 
-static int get_ucode_fw(void *to, const void *from, size_t n)
-{
-	memcpy(to, from, n);
-	return 0;
-}
-
 static bool is_blacklisted(unsigned int cpu)
 {
 	struct cpuinfo_x86 *c = &cpu_data(cpu);
@@ -981,6 +976,8 @@ static enum ucode_state request_microcode_fw(int cpu, struct device *device,
 	struct cpuinfo_x86 *c = &cpu_data(cpu);
 	const struct firmware *firmware;
 	enum ucode_state ret;
+	struct kvec kvec;
+	struct iov_iter iter;
 
 	if (is_blacklisted(cpu))
 		return UCODE_NFOUND;
@@ -993,26 +990,29 @@ static enum ucode_state request_microcode_fw(int cpu, struct device *device,
 		return UCODE_NFOUND;
 	}
 
-	ret = generic_load_microcode(cpu, (void *)firmware->data,
-				     firmware->size, &get_ucode_fw);
+	kvec.iov_base = (void *)firmware->data;
+	kvec.iov_len = firmware->size;
+	iov_iter_kvec(&iter, WRITE, &kvec, 1, firmware->size);
+	ret = generic_load_microcode(cpu, &iter);
 
 	release_firmware(firmware);
 
 	return ret;
 }
 
-static int get_ucode_user(void *to, const void *from, size_t n)
-{
-	return copy_from_user(to, from, n);
-}
-
 static enum ucode_state
 request_microcode_user(int cpu, const void __user *buf, size_t size)
 {
+	struct iovec iov;
+	struct iov_iter iter;
+
 	if (is_blacklisted(cpu))
 		return UCODE_NFOUND;
 
-	return generic_load_microcode(cpu, (void *)buf, size, &get_ucode_user);
+	iov.iov_base = (void __user *)buf;
+	iov.iov_len = size;
+	iov_iter_init(&iter, WRITE, &iov, 1, size);
+	return generic_load_microcode(cpu, &iter);
 }
 
 static struct microcode_ops microcode_intel_ops = {
-- 
2.21.0.392.gf8f6787159e-goog