[PATCH v2] fs/squashfs: sqfs_read: Prevent arbitrary code execution

[Miquel Raynal <miquel.raynal@bootlin.com>]

Following Jincheng's report, an out-of-band write leading to arbitrary
code execution is possible because on one side the squashfs logic
accepts directory names up to 65535 bytes (u16), while U-Boot fs logic
accepts directory names up to 255 bytes long.

Prevent such an exploit from happening by capping directory name sizes
to 255. Use a define for this purpose so that developers can link the
limitation to its source and eventually kill it some day by dynamically
allocating this array (if ever desired).

Link: https://lore.kernel.org/all/CALO=DHFB+yBoXxVr5KcsK0iFdg+e7ywko4-e+72kjbcS8JBfPw@mail.gmail.com
Reported-by: Jincheng Wang <jc.w4ng@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
---

Changes in v2:
* Jincheng reported in private that there was a problem with small name
  sizes, the last byte was lost. The reason is, dirs->entry->name_size
  contains the length of the string minus one (and excluding the
  trailing '\0'). The previous implementation had this handled correctly
  but my initial fix did not kept the "+ 1" in place because it felt
  wrong but is actually necessary. This information is actually
  available in a comment a bit above in this file.
 

 fs/squashfs/sqfs.c | 8 +++++---
 include/fs.h       | 4 +++-
 2 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/fs/squashfs/sqfs.c b/fs/squashfs/sqfs.c
index b4484fa17f5..3f1030057c4 100644
--- a/fs/squashfs/sqfs.c
+++ b/fs/squashfs/sqfs.c
@@ -976,6 +976,7 @@ int sqfs_readdir(struct fs_dir_stream *fs_dirs, struct fs_dirent **dentp)
 	int i_number, offset = 0, ret;
 	struct fs_dirent *dent;
 	unsigned char *ipos;
+	u16 name_size;
 
 	dirs = (struct squashfs_dir_stream *)fs_dirs;
 	if (!dirs->size) {
@@ -1058,9 +1059,10 @@ int sqfs_readdir(struct fs_dir_stream *fs_dirs, struct fs_dirent **dentp)
 		return -SQFS_STOP_READDIR;
 	}
 
-	/* Set entry name */
-	strncpy(dent->name, dirs->entry->name, dirs->entry->name_size + 1);
-	dent->name[dirs->entry->name_size + 1] = '\0';
+	/* Set entry name (capped at FS_DIRENT_NAME_LEN which is a U-Boot limitation) */
+	name_size = min_t(u16, dirs->entry->name_size + 1, FS_DIRENT_NAME_LEN - 1);
+	strncpy(dent->name, dirs->entry->name, name_size);
+	dent->name[name_size] = '\0';
 
 	offset = dirs->entry->name_size + 1 + SQFS_ENTRY_BASE_LENGTH;
 	dirs->entry_count--;
diff --git a/include/fs.h b/include/fs.h
index b43f16a692f..2195dc172ec 100644
--- a/include/fs.h
+++ b/include/fs.h
@@ -174,6 +174,8 @@ int fs_write(const char *filename, ulong addr, loff_t offset, loff_t len,
 #define FS_DT_REG  8         /* regular file */
 #define FS_DT_LNK  10        /* symbolic link */
 
+#define FS_DIRENT_NAME_LEN 256
+
 /**
  * struct fs_dirent - directory entry
  *
@@ -194,7 +196,7 @@ struct fs_dirent {
 	/** change_time:	time of last modification */
 	struct rtc_time change_time;
 	/** name:		file name */
-	char name[256];
+	char name[FS_DIRENT_NAME_LEN];
 };
 
 /* Note: fs_dir_stream should be treated as opaque to the user of fs layer */
-- 
2.34.1