[PATCH v5] hfsplus: fix uninit-value by validating catalog record size

[Deepanshu Kartikey]

Syzbot reported a KMSAN uninit-value issue in hfsplus_strcasecmp(). The
root cause is that hfs_brec_read() doesn't validate that the on-disk
record size matches the expected size for the record type being read.

When mounting a corrupted filesystem, hfs_brec_read() may read less data
than expected. For example, when reading a catalog thread record, the
debug output showed:

  HFSPLUS_BREC_READ: rec_len=520, fd->entrylength=26
  HFSPLUS_BREC_READ: WARNING - entrylength (26) < rec_len (520) - PARTIAL READ!

hfs_brec_read() only validates that entrylength is not greater than the
buffer size, but doesn't check if it's less than expected. It successfully
reads 26 bytes into a 520-byte structure and returns success, leaving 494
bytes uninitialized.

This uninitialized data in tmp.thread.nodeName then gets copied by
hfsplus_cat_build_key_uni() and used by hfsplus_strcasecmp(), triggering
the KMSAN warning when the uninitialized bytes are used as array indices
in case_fold().

Fix by introducing hfsplus_brec_read_cat() wrapper that:
1. Calls hfs_brec_read() to read the data
2. Validates the record size based on the type field:
   - Fixed size for folder and file records
   - Variable size for thread records (depends on string length)
3. Returns -EIO if size doesn't match expected

For thread records, check minimum size before reading nodeName.length to
avoid reading uninitialized data at call sites that don't zero-initialize
the entry structure.

Also initialize the tmp variable in hfsplus_find_cat() as defensive
programming to ensure no uninitialized data even if validation is
bypassed.

Reported-by: syzbot+d80abb5b890d39261e72@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=d80abb5b890d39261e72
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Tested-by: syzbot+d80abb5b890d39261e72@xxxxxxxxxxxxxxxxxxxxxxxxx
Reviewed-by: Viacheslav Dubeyko <slava@xxxxxxxxxxx>
Tested-by: Viacheslav Dubeyko <slava@xxxxxxxxxxx>
Link: https://lore.kernel.org/all/20260120051114.1281285-1-kartikey406@xxxxxxxxx/ [v1]
Link: https://lore.kernel.org/all/20260121063109.1830263-1-kartikey406@xxxxxxxxx/ [v2]
Link: https://lore.kernel.org/all/20260212014233.2422046-1-kartikey406@xxxxxxxxx/ [v3]
Link: https://lore.kernel.org/all/20260214002100.436125-1-kartikey406@xxxxxxxxx/T/ [v4]
Signed-off-by: Deepanshu Kartikey <kartikey406@xxxxxxxxx>
---
Changes in v5:
- Add minimum size check for thread records before reading nodeName.length
  to avoid reading uninitialized data, as suggested by Charalampos Mitrodimas

Changes in v4:
- Move hfsplus_cat_thread_size() as static inline to header file

Changes in v3:
- Introduced hfsplus_brec_read_cat() wrapper function for catalog-specific
  validation instead of modifying generic hfs_brec_read()
- Added hfsplus_cat_thread_size() helper to calculate variable-size thread
  record sizes
- Use exact size match (!=) instead of minimum size check (<)
- Use sizeof(hfsplus_unichr) instead of hardcoded value 2
- Updated all catalog record read sites to use new wrapper function

Changes in v2:
- Use structure initialization (= {0}) instead of memset()
- Improved commit message to clarify how uninitialized data is used
---
 fs/hfsplus/bfind.c      | 52 +++++++++++++++++++++++++++++++++++++++++
 fs/hfsplus/catalog.c    |  4 ++--
 fs/hfsplus/dir.c        |  2 +-
 fs/hfsplus/hfsplus_fs.h |  9 +++++++
 fs/hfsplus/super.c      |  2 +-
 5 files changed, 65 insertions(+), 4 deletions(-)

diff --git a/fs/hfsplus/bfind.c b/fs/hfsplus/bfind.c
index 336d654861c5..2b9152c3107b 100644
--- a/fs/hfsplus/bfind.c
+++ b/fs/hfsplus/bfind.c
@@ -287,3 +287,55 @@ int hfs_brec_goto(struct hfs_find_data *fd, int cnt)
 	fd->bnode = bnode;
 	return res;
 }
+
+/**
+ * hfsplus_brec_read_cat - read and validate a catalog record
+ * @fd: find data structure
+ * @entry: pointer to catalog entry to read into
+ *
+ * Reads a catalog record and validates its size matches the expected
+ * size based on the record type.
+ *
+ * Returns 0 on success, or negative error code on failure.
+ */
+int hfsplus_brec_read_cat(struct hfs_find_data *fd, hfsplus_cat_entry *entry)
+{
+	int res;
+	u32 expected_size;
+
+	res = hfs_brec_read(fd, entry, sizeof(hfsplus_cat_entry));
+	if (res)
+		return res;
+
+	/* Validate catalog record size based on type */
+	switch (be16_to_cpu(entry->type)) {
+	case HFSPLUS_FOLDER:
+		expected_size = sizeof(struct hfsplus_cat_folder);
+		break;
+	case HFSPLUS_FILE:
+		expected_size = sizeof(struct hfsplus_cat_file);
+		break;
+	case HFSPLUS_FOLDER_THREAD:
+	case HFSPLUS_FILE_THREAD:
+		/* Ensure we have at least the fixed fields before reading nodeName.length */
+		if (fd->entrylength < offsetof(struct hfsplus_cat_thread, nodeName) +
+		    offsetof(struct hfsplus_unistr, unicode)) {
+			pr_err("thread record too short (got %u)\n", fd->entrylength);
+			return -EIO;
+		}
+		expected_size = hfsplus_cat_thread_size(&entry->thread);
+		break;
+	default:
+		pr_err("unknown catalog record type %d\n",
+		       be16_to_cpu(entry->type));
+		return -EIO;
+	}
+
+	if (fd->entrylength != expected_size) {
+		pr_err("catalog record size mismatch (type %d, got %u, expected %u)\n",
+		       be16_to_cpu(entry->type), fd->entrylength, expected_size);
+		return -EIO;
+	}
+
+	return 0;
+}
diff --git a/fs/hfsplus/catalog.c b/fs/hfsplus/catalog.c
index 02c1eee4a4b8..6c8380f7208d 100644
--- a/fs/hfsplus/catalog.c
+++ b/fs/hfsplus/catalog.c
@@ -194,12 +194,12 @@ static int hfsplus_fill_cat_thread(struct super_block *sb,
 int hfsplus_find_cat(struct super_block *sb, u32 cnid,
 		     struct hfs_find_data *fd)
 {
-	hfsplus_cat_entry tmp;
+	hfsplus_cat_entry tmp = {0};
 	int err;
 	u16 type;
 
 	hfsplus_cat_build_key_with_cnid(sb, fd->search_key, cnid);
-	err = hfs_brec_read(fd, &tmp, sizeof(hfsplus_cat_entry));
+	err = hfsplus_brec_read_cat(fd, &tmp);
 	if (err)
 		return err;
 
diff --git a/fs/hfsplus/dir.c b/fs/hfsplus/dir.c
index ca5f74a140ec..8aeb861969d3 100644
--- a/fs/hfsplus/dir.c
+++ b/fs/hfsplus/dir.c
@@ -49,7 +49,7 @@ static struct dentry *hfsplus_lookup(struct inode *dir, struct dentry *dentry,
 	if (unlikely(err < 0))
 		goto fail;
 again:
-	err = hfs_brec_read(&fd, &entry, sizeof(entry));
+	err = hfsplus_brec_read_cat(&fd, &entry);
 	if (err) {
 		if (err == -ENOENT) {
 			hfs_find_exit(&fd);
diff --git a/fs/hfsplus/hfsplus_fs.h b/fs/hfsplus/hfsplus_fs.h
index 5f891b73a646..61d52091dd28 100644
--- a/fs/hfsplus/hfsplus_fs.h
+++ b/fs/hfsplus/hfsplus_fs.h
@@ -509,6 +509,15 @@ int hfsplus_submit_bio(struct super_block *sb, sector_t sector, void *buf,
 		       void **data, blk_opf_t opf);
 int hfsplus_read_wrapper(struct super_block *sb);
 
+static inline u32 hfsplus_cat_thread_size(const struct hfsplus_cat_thread *thread)
+{
+	return offsetof(struct hfsplus_cat_thread, nodeName) +
+	       offsetof(struct hfsplus_unistr, unicode) +
+	       be16_to_cpu(thread->nodeName.length) * sizeof(hfsplus_unichr);
+}
+
+int hfsplus_brec_read_cat(struct hfs_find_data *fd, hfsplus_cat_entry *entry);
+
 /*
  * time helpers: convert between 1904-base and 1970-base timestamps
  *
diff --git a/fs/hfsplus/super.c b/fs/hfsplus/super.c
index 592d8fbb748c..dcb4357aae3e 100644
--- a/fs/hfsplus/super.c
+++ b/fs/hfsplus/super.c
@@ -571,7 +571,7 @@ static int hfsplus_fill_super(struct super_block *sb, struct fs_context *fc)
 	err = hfsplus_cat_build_key(sb, fd.search_key, HFSPLUS_ROOT_CNID, &str);
 	if (unlikely(err < 0))
 		goto out_put_root;
-	if (!hfs_brec_read(&fd, &entry, sizeof(entry))) {
+	if (!hfsplus_brec_read_cat(&fd, &entry)) {
 		hfs_find_exit(&fd);
 		if (entry.type != cpu_to_be16(HFSPLUS_FOLDER)) {
 			err = -EIO;
-- 
2.43.0