bcachefs: Fix sb_field_downgrade validation

- bch2_sb_downgrade_validate() wasn't checking for a downgrade entry
  extending past the end of the superblock section

- for_each_downgrade_entry() is used in to_text() and needs to work on
  malformed input; it also was missing a check for a field extending
  past the end of the section

Reported-by: syzbot+e49ccab73449180bc9be@syzkaller.appspotmail.com
Fixes: 84f1638 ("bcachefs: bch_sb_field_downgrade")
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

Author: Kent Overstreet

fs/bcachefs/sb-downgrade.c

@@ -134,15 +134,25 @@ downgrade_entry_next_c(const struct bch_sb_field_downgrade_entry *e)
 #define for_each_downgrade_entry(_d, _i)						\
 	for (const struct bch_sb_field_downgrade_entry *_i = (_d)->entries;		\
 	     (void *) _i	< vstruct_end(&(_d)->field) &&				\
-	     (void *) &_i->errors[0] < vstruct_end(&(_d)->field);			\
+	     (void *) &_i->errors[0] <= vstruct_end(&(_d)->field) &&			\
+	     (void *) downgrade_entry_next_c(_i) <= vstruct_end(&(_d)->field);		\
 	     _i = downgrade_entry_next_c(_i))
 
 static int bch2_sb_downgrade_validate(struct bch_sb *sb, struct bch_sb_field *f,
 				      enum bch_validate_flags flags, struct printbuf *err)
 {
 	struct bch_sb_field_downgrade *e = field_to_type(f, downgrade);
 
-	for_each_downgrade_entry(e, i) {
+	for (const struct bch_sb_field_downgrade_entry *i = e->entries;
+	     (void *) i	< vstruct_end(&e->field);
+	     i = downgrade_entry_next_c(i)) {
+		if (flags & BCH_VALIDATE_write &&
+		    ((void *) &i->errors[0] > vstruct_end(&e->field) ||
+		     (void *) downgrade_entry_next_c(i) > vstruct_end(&e->field))) {
+			prt_printf(err, "downgrade entry overruns end of superblock section)");
+			return -BCH_ERR_invalid_sb_downgrade;
+		}
+
 		if (BCH_VERSION_MAJOR(le16_to_cpu(i->version)) !=
 		    BCH_VERSION_MAJOR(le16_to_cpu(sb->version))) {
 			prt_printf(err, "downgrade entry with mismatched major version (%u != %u)",