[PATCH] fat: check for hidden entries before overwriting end-of-directory marker

[Hengyu Liang <hengyul@cs.unc.edu>]

Since commit 6a2875517c77 ("fat: stop reading directory entries past the
end-of-directory marker"), lookup and readdir stop at a slot whose first
name byte is 0, and fat_add_entries() reports corruption when its scan
finds an allocated entry after that slot.

The scan stops as soon as it has enough free slots, though. When those
slots include the marker, the new entries overwrite it and the allocated
entries after it become visible again without being checked. A name that
lookup could not find can then appear twice in the directory, and removing
one copy leaves readdir listing a name that stat cannot find.

For example:

  mkfs.fat -F 16 -C fat.img 32768
  mount -t msdos -o loop fat.img /mnt
  touch /mnt/aaa.txt /mnt/bbb.txt /mnt/file1
  umount /mnt
  off=$(grep -obUa 'BBB     TXT' fat.img | cut -d: -f1)
  dd if=/dev/zero of=fat.img bs=1 seek=$off count=32 conv=notrunc
  mount -t msdos -o loop fat.img /mnt
  touch /mnt/file1
  ls /mnt

lists file1 twice.

Before using free slots that include the marker, scan the slots up to the
next marker and take the existing corruption path if one is allocated.

Fixes: 6a2875517c77 ("fat: stop reading directory entries past the end-of-directory marker")
Cc: stable@vger.kernel.org
Signed-off-by: Hengyu Liang <hengyul@cs.unc.edu>
---
Tested on x86-64 KVM with KASAN, comparing mainline at d89e1d186ec9 with
this patch:
- The example above: the base kernel creates a second file1, which is
  still there after a remount. With the patch, touch fails with EIO, the
  error is logged and fsck.fat only finds the dirty bit.
- vfat, three-slot create whose free run ends exactly at the marker:
  the base kernel exposes the hidden file, the patch returns EIO.
- vfat, one-slot create that leaves a zero slot after the new entry:
  succeeds on both and the hidden file stays hidden.
- Valid directories on FAT16 and FAT32, msdos and vfat (creates, deletes,
  renames and mkdirs, then filling the root): the same entry counts, ENOSPC
  on a full FAT16 root and no FAT-fs messages on both; fsck.fat -n exits 0.

 fs/fat/dir.c | 34 +++++++++++++++++++++++++++++++++-
 1 file changed, 33 insertions(+), 1 deletion(-)

diff --git a/fs/fat/dir.c b/fs/fat/dir.c
index 35bdb62944a2..ab99d53a719a 100644
--- a/fs/fat/dir.c
+++ b/fs/fat/dir.c
@@ -1318,6 +1318,32 @@ static int fat_add_new_entries(struct inode *dir, void *slots, int nr_slots,
 	return err;
 }
 
+/*
+ * The free slots chosen by fat_add_entries() include the end-of-directory
+ * marker.  Once the new entries are written over it, the slots up to the
+ * next marker become visible, so make sure that none of them is allocated.
+ */
+static int fat_check_after_eod(struct inode *dir, loff_t pos)
+{
+	struct buffer_head *bh = NULL;
+	struct msdos_dir_entry *de = NULL;
+	int err = 0;
+
+	while (fat_get_entry(dir, &pos, &bh, &de) > -1) {
+		if (de->name[0] == 0)
+			break;
+		if (!IS_FREE(de->name)) {
+			fat_fs_error_ratelimit(dir->i_sb,
+				"allocated dir entry found after end-of-directory marker (i_pos %lld)",
+				MSDOS_I(dir)->i_pos);
+			err = -EIO;
+			break;
+		}
+	}
+	brelse(bh);
+	return err;
+}
+
 int fat_add_entries(struct inode *dir, void *slots, int nr_slots,
 		    struct fat_slot_info *sinfo)
 {
@@ -1351,8 +1377,14 @@ int fat_add_entries(struct inode *dir, void *slots, int nr_slots,
 				nr_bhs++;
 			}
 			free_slots++;
-			if (free_slots == nr_slots)
+			if (free_slots == nr_slots) {
+				if (saw_eod) {
+					err = fat_check_after_eod(dir, pos);
+					if (err)
+						goto error;
+				}
 				goto found;
+			}
 		} else {
 			if (saw_eod) {
 				fat_fs_error_ratelimit(sb,

base-commit: d89e1d186ec9271bf78e62771779e9fc03bf0fc3
-- 
2.53.0