fs/adfs: validate nzones in adfs_validate_bblk()

Reject ADFS disc records with a zero zone count during boot block
validation, before the disc record is used.

When nzones is 0, adfs_read_map() passes it to kmalloc_array(0, ...)
which returns ZERO_SIZE_PTR, and adfs_map_layout() then writes to
dm[-1], causing an out-of-bounds write before the allocated buffer.

adfs_validate_dr0() already rejects nzones != 1 for old-format
images.  Add the equivalent check to adfs_validate_bblk() for
new-format images so that a crafted image with nzones == 0 is
rejected at probe time.

Found by syzkaller.

Fixes: f6f14a0d71 ("fs/adfs: map: move map-specific sb initialisation to map.c")
Signed-off-by: Bae Yeonju <iwasbaeyz@gmail.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>

fs/adfs/super.c

@@ -317,6 +317,9 @@ static int adfs_validate_bblk(struct super_block *sb, struct buffer_head *bh,
 	if (adfs_checkdiscrecord(dr))
 		return -EILSEQ;
 
+	if ((dr->nzones | dr->nzones_high << 8) == 0)
+		return -EILSEQ;
+
 	*drp = dr;
 	return 0;
 }