kernel: harden the signature check

kernel/Makefile

@@ -29,13 +29,13 @@ KSU_EXPECTED_SIZE := 0x033b
 endif
 
 ifndef KSU_EXPECTED_HASH
-KSU_EXPECTED_HASH := 0xb0b91415
+KSU_EXPECTED_HASH := c371061b19d8c7d7d6133c6a9bafe198fa944e50c1b31c9d8daa8d7f1fc2d2d6
 endif
 
 $(info -- KernelSU Manager signature size: $(KSU_EXPECTED_SIZE))
 $(info -- KernelSU Manager signature hash: $(KSU_EXPECTED_HASH))
 
 ccflags-y += -DEXPECTED_SIZE=$(KSU_EXPECTED_SIZE)
-ccflags-y += -DEXPECTED_HASH=$(KSU_EXPECTED_HASH)
+ccflags-y += -DEXPECTED_HASH=\"$(KSU_EXPECTED_HASH)\"
 ccflags-y += -Wno-implicit-function-declaration -Wno-strict-prototypes -Wno-int-conversion -Wno-gcc-compat
 ccflags-y += -Wno-declaration-after-statement

kernel/apk_sign.c

@@ -1,20 +1,136 @@
+#include "linux/err.h"
 #include "linux/fs.h"
+#include "linux/gfp.h"
+#include "linux/kernel.h"
 #include "linux/moduleparam.h"
 
 #include "apk_sign.h"
 #include "klog.h" // IWYU pragma: keep
 #include "kernel_compat.h"
+#include "crypto/hash.h"
+#include "linux/slab.h"
+#include "linux/version.h"
 
-static __always_inline int
-check_v2_signature(char *path, unsigned expected_size, unsigned expected_hash)
+#if LINUX_VERSION_CODE >= KERNEL_VERSION(5, 11, 0)
+#include "crypto/sha2.h"
+#else
+#include "crypto/sha.h"
+#endif
+
+struct sdesc {
+	struct shash_desc shash;
+	char ctx[];
+};
+
+static struct sdesc *init_sdesc(struct crypto_shash *alg)
+{
+	struct sdesc *sdesc;
+	int size;
+
+	size = sizeof(struct shash_desc) + crypto_shash_descsize(alg);
+	sdesc = kmalloc(size, GFP_KERNEL);
+	if (!sdesc)
+		return ERR_PTR(-ENOMEM);
+	sdesc->shash.tfm = alg;
+	return sdesc;
+}
+
+static int calc_hash(struct crypto_shash *alg, const unsigned char *data,
+		     unsigned int datalen, unsigned char *digest)
+{
+	struct sdesc *sdesc;
+	int ret;
+
+	sdesc = init_sdesc(alg);
+	if (IS_ERR(sdesc)) {
+		pr_info("can't alloc sdesc\n");
+		return PTR_ERR(sdesc);
+	}
+
+	ret = crypto_shash_digest(&sdesc->shash, data, datalen, digest);
+	kfree(sdesc);
+	return ret;
+}
+
+static int ksu_sha256(const unsigned char *data, unsigned int datalen,
+		unsigned char *digest)
+{
+	struct crypto_shash *alg;
+	char *hash_alg_name = "sha256";
+	int ret;
+
+	alg = crypto_alloc_shash(hash_alg_name, 0, 0);
+	if (IS_ERR(alg)) {
+		pr_info("can't alloc alg %s\n", hash_alg_name);
+		return PTR_ERR(alg);
+	}
+	ret = calc_hash(alg, data, datalen, digest);
+	crypto_free_shash(alg);
+	return ret;
+}
+
+static bool check_block(struct file *fp, u32 *size4, loff_t *pos, u32 *offset,
+			unsigned expected_size, const char* expected_sha256)
+{
+	ksu_kernel_read_compat(fp, size4, 0x4, pos); // signer-sequence length
+	ksu_kernel_read_compat(fp, size4, 0x4, pos); // signer length
+	ksu_kernel_read_compat(fp, size4, 0x4, pos); // signed data length
+
+	*offset += 0x4 * 3;
+
+	ksu_kernel_read_compat(fp, size4, 0x4, pos); // digests-sequence length
+
+	*pos += *size4;
+	*offset += 0x4 + *size4;
+
+	ksu_kernel_read_compat(fp, size4, 0x4, pos); // certificates length
+	ksu_kernel_read_compat(fp, size4, 0x4, pos); // certificate length
+	*offset += 0x4 * 2;
+
+	if (*size4 == expected_size) {
+		*offset += *size4;
+
+		#define CERT_MAX_LENGTH 1024
+		char cert[CERT_MAX_LENGTH];
+		if (*size4 > CERT_MAX_LENGTH) {
+			pr_info("cert length overlimit\n");
+			return false;
+		}
+		ksu_kernel_read_compat(fp, cert, *size4, pos);
+		unsigned char digest[SHA256_DIGEST_SIZE];
+		if (IS_ERR(ksu_sha256(cert, *size4, digest))) {
+			pr_info("sha256 error\n");
+			return false;
+		}
+
+		char hash_str[SHA256_DIGEST_SIZE * 2 + 1];
+		hash_str[SHA256_DIGEST_SIZE * 2] = '\0';
+
+		bin2hex(hash_str, digest, SHA256_DIGEST_SIZE);
+		pr_info("sha256: %s, expected: %s\n", hash_str, expected_sha256);
+		if (strcmp(expected_sha256, hash_str) == 0) {
+			return true;
+		}
+	}
+	return false;
+}
+
+static __always_inline bool
+check_v2_signature(char *path, unsigned expected_size, const char *expected_sha256)
 {
 	unsigned char buffer[0x11] = { 0 };
 	u32 size4;
 	u64 size8, size_of_block;
 
 	loff_t pos;
+	bool block_valid;
+
+	const int NOT_EXIST = 0;
+	const int INVALID = 1;
+	const int VALID = 2;
+	int v2_signing_status = NOT_EXIST;
+	int v3_signing_status = NOT_EXIST;
 
-	int sign = -1;
 	int i;
 	struct file *fp = ksu_filp_open_compat(path, O_RDONLY, 0);
 	if (IS_ERR(fp)) {
@@ -25,7 +141,6 @@ check_v2_signature(char *path, unsigned expected_size, unsigned expected_hash)
 	// disable inotify for this file
 	fp->f_mode |= FMODE_NONOTIFY;
 
-	sign = 1;
 	// https://en.wikipedia.org/wiki/Zip_(file_format)#End_of_central_directory_record_(EOCD)
 	for (i = 0;; ++i) {
 		unsigned short n;
@@ -64,73 +179,38 @@ check_v2_signature(char *path, unsigned expected_size, unsigned expected_hash)
 	for (;;) {
 		uint32_t id;
 		uint32_t offset;
-		ksu_kernel_read_compat(fp, &size8, 0x8, &pos); // sequence length
+		ksu_kernel_read_compat(fp, &size8, 0x8,
+				       &pos); // sequence length
 		if (size8 == size_of_block) {
 			break;
 		}
 		ksu_kernel_read_compat(fp, &id, 0x4, &pos); // id
 		offset = 4;
 		pr_info("id: 0x%08x\n", id);
-		if ((id ^ 0xdeadbeefu) == 0xafa439f5u ||
-		    (id ^ 0xdeadbeefu) == 0x2efed62f) {
-			ksu_kernel_read_compat(fp, &size4, 0x4,
-				    &pos); // signer-sequence length
-			ksu_kernel_read_compat(fp, &size4, 0x4, &pos); // signer length
-			ksu_kernel_read_compat(fp, &size4, 0x4,
-				    &pos); // signed data length
-			offset += 0x4 * 3;
-
-			ksu_kernel_read_compat(fp, &size4, 0x4,
-				    &pos); // digests-sequence length
-			pos += size4;
-			offset += 0x4 + size4;
-
-			ksu_kernel_read_compat(fp, &size4, 0x4,
-				    &pos); // certificates length
-			ksu_kernel_read_compat(fp, &size4, 0x4,
-				    &pos); // certificate length
-			offset += 0x4 * 2;
-#if 0
-			int hash = 1;
-			signed char c;
-			for (i = 0; i < size4; ++i) {
-				ksu_kernel_read_compat(fp, &c, 0x1, &pos);
-				hash = 31 * hash + c;
-			}
-			offset += size4;
-			pr_info("    size: 0x%04x, hash: 0x%08x\n", size4, ((unsigned) hash) ^ 0x14131211u);
-#else
-			if (size4 == expected_size) {
-				int hash = 1;
-				signed char c;
-				for (i = 0; i < size4; ++i) {
-					ksu_kernel_read_compat(fp, &c, 0x1, &pos);
-					hash = 31 * hash + c;
-				}
-				offset += size4;
-				if ((((unsigned)hash) ^ 0x14131211u) ==
-				    expected_hash) {
-					sign = 0;
-					break;
-				}
-			}
-			// don't try again.
-			break;
-#endif
+		if (id == 0x7109871au) {
+			block_valid = check_block(fp, &size4, &pos, &offset,
+						  expected_size, expected_sha256);
+			v2_signing_status = block_valid ? VALID : INVALID;
+		} else if (id == 0xf05368c0u) {
+			block_valid = check_block(fp, &size4, &pos, &offset,
+						  expected_size, expected_sha256);
+			v3_signing_status = block_valid ? VALID : INVALID;
 		}
 		pos += (size8 - offset);
 	}
 
 clean:
 	filp_close(fp, 0);
 
-	return sign;
+	return (v2_signing_status == NOT_EXIST && v3_signing_status == VALID) ||
+	       (v2_signing_status == VALID && v3_signing_status == NOT_EXIST) ||
+	       (v2_signing_status == VALID && v3_signing_status == VALID);
 }
 
 #ifdef CONFIG_KSU_DEBUG
 
 unsigned ksu_expected_size = EXPECTED_SIZE;
-unsigned ksu_expected_hash = EXPECTED_HASH;
+const char *ksu_expected_hash = EXPECTED_HASH;
 
 #include "manager.h"
 
@@ -144,9 +224,10 @@ static int set_expected_size(const char *val, const struct kernel_param *kp)
 
 static int set_expected_hash(const char *val, const struct kernel_param *kp)
 {
-	int rv = param_set_uint(val, kp);
+	pr_info("set_expected_hash: %s\n", val);
+	int rv = param_set_charp(val, kp);
 	ksu_invalidate_manager_uid();
-	pr_info("ksu_expected_hash set to %x\n", ksu_expected_hash);
+	pr_info("ksu_expected_hash set to %s\n", ksu_expected_hash);
 	return rv;
 }
 
@@ -157,22 +238,23 @@ static struct kernel_param_ops expected_size_ops = {
 
 static struct kernel_param_ops expected_hash_ops = {
 	.set = set_expected_hash,
-	.get = param_get_uint,
+	.get = param_get_charp,
+	.free = param_free_charp,
 };
 
 module_param_cb(ksu_expected_size, &expected_size_ops, &ksu_expected_size,
 		S_IRUSR | S_IWUSR);
 module_param_cb(ksu_expected_hash, &expected_hash_ops, &ksu_expected_hash,
 		S_IRUSR | S_IWUSR);
 
-int is_manager_apk(char *path)
+bool is_manager_apk(char *path)
 {
 	return check_v2_signature(path, ksu_expected_size, ksu_expected_hash);
 }
 
 #else
 
-int is_manager_apk(char *path)
+bool is_manager_apk(char *path)
 {
 	return check_v2_signature(path, EXPECTED_SIZE, EXPECTED_HASH);
 }

kernel/apk_sign.h

@@ -1,7 +1,8 @@
 #ifndef __KSU_H_APK_V2_SIGN
 #define __KSU_H_APK_V2_SIGN
 
-// return 0 if signature match
-int is_manager_apk(char *path);
+#include "linux/types.h"
+
+bool is_manager_apk(char *path);
 
 #endif

kernel/manager.c

@@ -70,7 +70,7 @@ bool become_manager(char *pkg)
 			pr_info("invalid pkg: %s\n", pkg);
 			continue;
 		}
-		if (is_manager_apk(cwd) == 0) {
+		if (is_manager_apk(cwd)) {
 			// check passed
 			uid_t uid = current_uid().val;
 			pr_info("manager uid: %d\n", uid);