isofs_export_get_parent() assumes valid "." and ".." entries.

A malformed image can provide an invalid length for the first entry.

Validate both records before using the first length as the ".." offset.

This keeps the NFS export get_parent path from accepting malformed records.

Signed-off-by: Yichong Chen <chenyichong@uniontech.com>
---
 fs/isofs/export.c | 30 +++++++++++++++++++++++++++++-
 1 file changed, 29 insertions(+), 1 deletion(-)

diff --git a/fs/isofs/export.c b/fs/isofs/export.c
index 78f80c1a5c54..bffec45bb274 100644
--- a/fs/isofs/export.c
+++ b/fs/isofs/export.c
@@ -16,6 +16,26 @@
 
 #include "isofs.h"
 
+static bool isofs_dir_record_valid(struct iso_directory_record *de,
+				   unsigned long offset,
+				   unsigned long bufsize)
+{
+	unsigned int len;
+	unsigned int name_len;
+	unsigned long min_len = offsetof(struct iso_directory_record, name);
+
+	if (offset > bufsize || bufsize - offset < min_len)
+		return false;
+
+	len = isonum_711(de->length);
+	name_len = isonum_711(de->name_len);
+	if (len < min_len || name_len > len - min_len)
+		return false;
+	if (len > bufsize - offset)
+		return false;
+	return true;
+}
+
 static struct dentry *
 isofs_export_iget(struct super_block *sb,
 		  unsigned long block,
@@ -83,13 +103,21 @@ static struct dentry *isofs_export_get_parent(struct dentry *child)
 
 	/* This is the "." entry. */
 	de = (struct iso_directory_record*)bh->b_data;
+	if (!isofs_dir_record_valid(de, 0, child_inode->i_sb->s_blocksize) ||
+	    isonum_711(de->name_len) != 1 || de->name[0] != 0) {
+		printk(KERN_ERR "isofs: Unable to find the \".\" directory for NFS.\n");
+		rv = ERR_PTR(-EACCES);
+		goto out;
+	}
 
 	/* The ".." entry is always the second entry. */
 	parent_offset = (unsigned long)isonum_711(de->length);
 	de = (struct iso_directory_record*)(bh->b_data + parent_offset);
 
 	/* Verify it is in fact the ".." entry. */
-	if ((isonum_711(de->name_len) != 1) || (de->name[0] != 1)) {
+	if (!isofs_dir_record_valid(de, parent_offset,
+				    child_inode->i_sb->s_blocksize) ||
+	    isonum_711(de->name_len) != 1 || de->name[0] != 1) {
 		printk(KERN_ERR "isofs: Unable to find the \"..\" "
 		       "directory for NFS.\n");
 		rv = ERR_PTR(-EACCES);
-- 
2.51.0