minix: Add required sanity checking to minix_check_superblock()

The fs/minix implementation of the minix filesystem does not currently
support any other value for s_log_zone_size than 0. This is also the
only value supported in util-linux; see mkfs.minix.c line 511. In
addition, this patch adds some sanity checking for the other minix
superblock fields, and moves the minix_blocks_needed() checks for the
zmap and imap also to minix_check_super_block().

This also closes a related syzbot bug report.

Signed-off-by: Jori Koolstra <jkoolstra@xs4all.nl>
Link: https://patch.msgid.link/20251208153947.108343-1-jkoolstra@xs4all.nl
Reviewed-by: Jan Kara <jack@suse.cz>
Reported-by: syzbot+5ad0824204c7bf9b67f2@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=5ad0824204c7bf9b67f2
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: LATENTBOUNCE

fs/minix/inode.c

@@ -170,10 +170,38 @@ static int minix_reconfigure(struct fs_context *fc)
 static bool minix_check_superblock(struct super_block *sb)
 {
 	struct minix_sb_info *sbi = minix_sb(sb);
+	unsigned long block;
 
-	if (sbi->s_imap_blocks == 0 || sbi->s_zmap_blocks == 0)
+	if (sbi->s_log_zone_size != 0) {
+		printk("minix-fs error: zone size must equal block size. "
+		       "s_log_zone_size > 0 is not supported.\n");
+		return false;
+	}
+
+	if (sbi->s_ninodes < 1 || sbi->s_firstdatazone <= 4 ||
+	    sbi->s_firstdatazone >= sbi->s_nzones)
 		return false;
 
+	/* Apparently minix can create filesystems that allocate more blocks for
+	 * the bitmaps than needed.  We simply ignore that, but verify it didn't
+	 * create one with not enough blocks and bail out if so.
+	 */
+	block = minix_blocks_needed(sbi->s_ninodes, sb->s_blocksize);
+	if (sbi->s_imap_blocks < block) {
+		printk("MINIX-fs: file system does not have enough "
+		       "imap blocks allocated. Refusing to mount.\n");
+		return false;
+	}
+
+	block = minix_blocks_needed(
+			(sbi->s_nzones - sbi->s_firstdatazone + 1),
+			sb->s_blocksize);
+	if (sbi->s_zmap_blocks < block) {
+		printk("MINIX-fs: file system does not have enough "
+		       "zmap blocks allocated. Refusing to mount.\n");
+		return false;
+	}
+
 	/*
 	 * s_max_size must not exceed the block mapping limitation.  This check
 	 * is only needed for V1 filesystems, since V2/V3 support an extra level
@@ -293,26 +321,6 @@ static int minix_fill_super(struct super_block *s, struct fs_context *fc)
 	minix_set_bit(0,sbi->s_imap[0]->b_data);
 	minix_set_bit(0,sbi->s_zmap[0]->b_data);
 
-	/* Apparently minix can create filesystems that allocate more blocks for
-	 * the bitmaps than needed.  We simply ignore that, but verify it didn't
-	 * create one with not enough blocks and bail out if so.
-	 */
-	block = minix_blocks_needed(sbi->s_ninodes, s->s_blocksize);
-	if (sbi->s_imap_blocks < block) {
-		printk("MINIX-fs: file system does not have enough "
-				"imap blocks allocated.  Refusing to mount.\n");
-		goto out_no_bitmap;
-	}
-
-	block = minix_blocks_needed(
-			(sbi->s_nzones - sbi->s_firstdatazone + 1),
-			s->s_blocksize);
-	if (sbi->s_zmap_blocks < block) {
-		printk("MINIX-fs: file system does not have enough "
-				"zmap blocks allocated.  Refusing to mount.\n");
-		goto out_no_bitmap;
-	}
-
 	/* set up enough so that it can read an inode */
 	s->s_op = &minix_sops;
 	s->s_time_min = 0;