[PATCH 1/3] quota: Check next/prev free block number after reading from quota file

[Zhihao Cheng]

Following process:
 Init: v2_read_file_info: <3> dqi_free_blk 0 dqi_free_entry 5 dqi_blks 6

 Step 1. chown bin f_a -> dquot_acquire -> v2_write_dquot:
  qtree_write_dquot
   do_insert_tree
    find_free_dqentry
     get_free_dqblk
      write_blk(info->dqi_blocks) // info->dqi_blocks = 6, failure. The
	   content in physical block (corresponding to blk 6) is random.

 Step 2. chown root f_a -> dquot_transfer -> dqput_all -> dqput ->
         ext4_release_dquot -> v2_release_dquot -> qtree_delete_dquot:
  dquot_release
   remove_tree
    free_dqentry
     put_free_dqblk(6)
      info->dqi_free_blk = blk    // info->dqi_free_blk = 6

 Step 3. drop cache (buffer head for block 6 is released)

 Step 4. chown bin f_b -> dquot_acquire -> commit_dqblk -> v2_write_dquot:
  qtree_write_dquot
   do_insert_tree
    find_free_dqentry
     get_free_dqblk
      dh = (struct qt_disk_dqdbheader *)buf
      blk = info->dqi_free_blk     // 6
      ret = read_blk(info, blk, buf)  // The content of buf is random
      info->dqi_free_blk = le32_to_cpu(dh->dqdh_next_free)  // random blk

 Step 5. chown bin f_c -> notify_change -> ext4_setattr -> dquot_transfer:
  dquot = dqget -> acquire_dquot -> ext4_acquire_dquot -> dquot_acquire ->
          commit_dqblk -> v2_write_dquot -> dq_insert_tree:
   do_insert_tree
    find_free_dqentry
     get_free_dqblk
      blk = info->dqi_free_blk    // If blk < 0 and blk is not an error
				     code, it will be returned as dquot

  transfer_to[USRQUOTA] = dquot  // A random negative value
  __dquot_transfer(transfer_to)
   dquot_add_inodes(transfer_to[cnt])
    spin_lock(&dquot->dq_dqb_lock)  // page fault

, which will lead to kernel page fault:
 Quota error (device sda): qtree_write_dquot: Error -8000 occurred
 while creating quota
 BUG: unable to handle page fault for address: ffffffffffffe120
 #PF: supervisor write access in kernel mode
 #PF: error_code(0x0002) - not-present page
 Oops: 0002 [#1] PREEMPT SMP
 CPU: 0 PID: 5974 Comm: chown Not tainted 6.0.0-rc1-00004
 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996)
 RIP: 0010:_raw_spin_lock+0x3a/0x90
 Call Trace:
  dquot_add_inodes+0x28/0x270
  __dquot_transfer+0x377/0x840
  dquot_transfer+0xde/0x540
  ext4_setattr+0x405/0x14d0
  notify_change+0x68e/0x9f0
  chown_common+0x300/0x430
  __x64_sys_fchownat+0x29/0x40

In order to avoid accessing invalid quota memory address, this patch adds
block number checking of next/prev free block read from quota file.

Fetch a reproducer in [Link].

Link: https://bugzilla.kernel.org/show_bug.cgi?id=216372
Fixes: 1da177e4c3f4152 ("Linux-2.6.12-rc2")
Signed-off-by: Zhihao Cheng <chengzhihao1@xxxxxxxxxx>
---
 fs/quota/quota_tree.c | 38 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 38 insertions(+)

diff --git a/fs/quota/quota_tree.c b/fs/quota/quota_tree.c
index 5f2405994280..f2e7f5b869a4 100644
--- a/fs/quota/quota_tree.c
+++ b/fs/quota/quota_tree.c
@@ -71,6 +71,35 @@ static ssize_t write_blk(struct qtree_mem_dqinfo *info, uint blk, char *buf)
 	return ret;
 }
 
+static inline int do_check_range(struct super_block *sb, uint val, uint max_val)
+{
+	if (val >= max_val) {
+		quota_error(sb, "Getting block too big (%u >= %u)",
+			    val, max_val);
+		return -EUCLEAN;
+	}
+
+	return 0;
+}
+
+static int check_free_block(struct qtree_mem_dqinfo *info,
+			    struct qt_disk_dqdbheader *dh)
+{
+	int err = 0;
+	uint nextblk, prevblk;
+
+	nextblk = le32_to_cpu(dh->dqdh_next_free);
+	err = do_check_range(info->dqi_sb, nextblk, info->dqi_blocks);
+	if (err)
+		return err;
+	prevblk = le32_to_cpu(dh->dqdh_prev_free);
+	err = do_check_range(info->dqi_sb, prevblk, info->dqi_blocks);
+	if (err)
+		return err;
+
+	return err;
+}
+
 /* Remove empty block from list and return it */
 static int get_free_dqblk(struct qtree_mem_dqinfo *info)
 {
@@ -85,6 +114,9 @@ static int get_free_dqblk(struct qtree_mem_dqinfo *info)
 		ret = read_blk(info, blk, buf);
 		if (ret < 0)
 			goto out_buf;
+		ret = check_free_block(info, dh);
+		if (ret)
+			goto out_buf;
 		info->dqi_free_blk = le32_to_cpu(dh->dqdh_next_free);
 	}
 	else {
@@ -232,6 +264,9 @@ static uint find_free_dqentry(struct qtree_mem_dqinfo *info,
 		*err = read_blk(info, blk, buf);
 		if (*err < 0)
 			goto out_buf;
+		*err = check_free_block(info, dh);
+		if (*err)
+			goto out_buf;
 	} else {
 		blk = get_free_dqblk(info);
 		if ((int)blk < 0) {
@@ -424,6 +459,9 @@ static int free_dqentry(struct qtree_mem_dqinfo *info, struct dquot *dquot,
 		goto out_buf;
 	}
 	dh = (struct qt_disk_dqdbheader *)buf;
+	ret = check_free_block(info, dh);
+	if (ret)
+		goto out_buf;
 	le16_add_cpu(&dh->dqdh_entries, -1);
 	if (!le16_to_cpu(dh->dqdh_entries)) {	/* Block got free? */
 		ret = remove_free_dqentry(info, buf, blk);
-- 
2.31.1