[PATCH 3/4] exfat: add boot region verification

[Tetsuhiro Kohada <kohada.t2@gmail.com>]

Add Boot-Regions verification specified in exFAT specification.
Note that the checksum type is strongly related to the raw structure,
so the'u32 'type is used to clarify the number of bits.

Signed-off-by: Tetsuhiro Kohada <kohada.t2@gmail.com>
---
 fs/exfat/exfat_fs.h |  1 +
 fs/exfat/misc.c     | 14 +++++++++++++
 fs/exfat/super.c    | 50 +++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 65 insertions(+)

diff --git a/fs/exfat/exfat_fs.h b/fs/exfat/exfat_fs.h
index b0e5b9afc56c..15817281b3c8 100644
--- a/fs/exfat/exfat_fs.h
+++ b/fs/exfat/exfat_fs.h
@@ -517,6 +517,7 @@ void exfat_set_entry_time(struct exfat_sb_info *sbi, struct timespec64 *ts,
 		u8 *tz, __le16 *time, __le16 *date, u8 *time_cs);
 unsigned short exfat_calc_chksum_2byte(void *data, int len,
 		unsigned short chksum, int type);
+u32 exfat_calc_chksum32(void *data, int len, u32 chksum, int type);
 void exfat_update_bh(struct super_block *sb, struct buffer_head *bh, int sync);
 void exfat_chain_set(struct exfat_chain *ec, unsigned int dir,
 		unsigned int size, unsigned char flags);
diff --git a/fs/exfat/misc.c b/fs/exfat/misc.c
index ab7f88b1f6d3..b82d2dd5bd7c 100644
--- a/fs/exfat/misc.c
+++ b/fs/exfat/misc.c
@@ -151,6 +151,20 @@ unsigned short exfat_calc_chksum_2byte(void *data, int len,
 	return chksum;
 }
 
+u32 exfat_calc_chksum32(void *data, int len, u32 chksum, int type)
+{
+	int i;
+	u8 *c = (u8 *)data;
+
+	for (i = 0; i < len; i++, c++) {
+		if (unlikely(type == CS_BOOT_SECTOR &&
+			     (i == 106 || i == 107 || i == 112)))
+			continue;
+		chksum = ((chksum << 31) | (chksum >> 1)) + *c;
+	}
+	return chksum;
+}
+
 void exfat_update_bh(struct super_block *sb, struct buffer_head *bh, int sync)
 {
 	set_bit(EXFAT_SB_DIRTY, &EXFAT_SB(sb)->s_state);
diff --git a/fs/exfat/super.c b/fs/exfat/super.c
index 95909b4d5e75..42b3bd3df020 100644
--- a/fs/exfat/super.c
+++ b/fs/exfat/super.c
@@ -486,6 +486,50 @@ static int exfat_read_boot_sector(struct super_block *sb)
 	return 0;
 }
 
+static int exfat_verify_boot_region(struct super_block *sb)
+{
+	struct buffer_head *bh = NULL;
+	u32 chksum = 0, *p_sig, *p_chksum;
+	int sn, i;
+
+	/* read boot sector sub-regions */
+	for (sn = 0; sn < 11; sn++) {
+		bh = sb_bread(sb, sn);
+		if (!bh)
+			return -EIO;
+
+		if (sn != 0 && sn <= 8) {
+			/* extended boot sector sub-regions */
+			p_sig = (u32 *)&bh->b_data[sb->s_blocksize - 4];
+			if (le32_to_cpu(*p_sig) != EXBOOT_SIGNATURE) {
+				exfat_err(sb, "no exboot-signature");
+				brelse(bh);
+				return -EINVAL;
+			}
+		}
+
+		chksum = exfat_calc_chksum32(bh->b_data, sb->s_blocksize,
+			chksum, sn ? CS_DEFAULT : CS_BOOT_SECTOR);
+		brelse(bh);
+	}
+
+	/* boot checksum sub-regions */
+	bh = sb_bread(sb, sn);
+	if (!bh)
+		return -EIO;
+
+	for (i = 0; i < sb->s_blocksize; i += sizeof(u32)) {
+		p_chksum = (u32 *)&bh->b_data[i];
+		if (le32_to_cpu(*p_chksum) != chksum) {
+			exfat_err(sb, "mismatch checksum");
+			brelse(bh);
+			return -EINVAL;
+		}
+	}
+	brelse(bh);
+	return 0;
+}
+
 /* mount the file system volume */
 static int __exfat_fill_super(struct super_block *sb)
 {
@@ -498,6 +542,12 @@ static int __exfat_fill_super(struct super_block *sb)
 		goto free_bh;
 	}
 
+	ret = exfat_verify_boot_region(sb);
+	if (ret) {
+		exfat_err(sb, "invalid boot region");
+		goto free_bh;
+	}
+
 	ret = exfat_create_upcase_table(sb);
 	if (ret) {
 		exfat_err(sb, "failed to load upcase table");
-- 
2.25.1