Re: [PATCH -next v2 4/6] ext4: avoid to re-read mmp check data get from page cache

On 2021/10/7 20:44, Jan Kara wrote:

On Sat 11-09-21 17:00:57, Ye Bin wrote:

quoted

As call read_mmp_block pass bh_check which value is NULL, then call
sb_getblk to get buffer_head. But mmp_block's buffer_head is already exist
  which also is uptodate. Lead to compare the same data.

Signed-off-by: Ye Bin <redacted>

This looks good, except that read_mmp_block() also releases bh it is passed
in case of error. So it can free buffer head which is still referenced from
EXT4_SB(sb)->s_mmp_bh and cause use-after-free issues.

I guess I would just get rid of sb_getblk() in read_mmp_block() and always
expect valid bh passed. The only place that passes NULL bh after this
patch is one case in ext4_multi_mount_protect() and that can call
sb_getblk() on its own. That way we can also simplify read_mmp_block()
prototype to:

static int read_mmp_block(struct super_block *sb, struct buffer_head *bh);

								Honza
Yeah, I will refactor 'read_mmp_block'.

quoted

---
  fs/ext4/mmp.c | 17 ++++++-----------
  1 file changed, 6 insertions(+), 11 deletions(-)

diff --git a/fs/ext4/mmp.c b/fs/ext4/mmp.c
index 4433fe7e9e86..007bde3c12b8 100644
--- a/fs/ext4/mmp.c
+++ b/fs/ext4/mmp.c

@@ -213,10 +213,7 @@ static int kmmpd(void *data)
  		 * we need to check if the MMP block is as we write it.
  		 */
  		if (jiffies - last_check_time > mmp_check_interval * HZ) {
-			struct buffer_head *bh_check = NULL;
-			struct mmp_struct *mmp_check;
-
-			retval = read_mmp_block(sb, &bh_check, mmp_block);
+			retval = read_mmp_block(sb, &bh, mmp_block);
  			if (retval) {
  				ext4_error_err(sb, -retval,
  					       "error reading MMP data: %d",

@@ -224,20 +221,18 @@ static int kmmpd(void *data)
  				goto wait_to_exit;
  			}
  
-			mmp_check = (struct mmp_struct *)(bh_check->b_data);
-			if (seq != mmp_check->mmp_seq ||
-			    memcmp(nodename, mmp_check->mmp_nodename,
-				   sizeof(mmp->mmp_nodename))) {
-				dump_mmp_msg(sb, mmp_check,
+			mmp = (struct mmp_struct *)(bh->b_data);
+			if (seq != le32_to_cpu(mmp->mmp_seq) ||
+			    memcmp(nodename, mmp->mmp_nodename,
+				    sizeof(nodename))) {
+				dump_mmp_msg(sb, mmp,
  					     "Error while updating MMP info. "
  					     "The filesystem seems to have been"
  					     " multiply mounted.");
  				ext4_error_err(sb, EBUSY, "abort");
-				put_bh(bh_check);
  				retval = -EBUSY;
  				goto wait_to_exit;
  			}
-			put_bh(bh_check);
  			last_check_time = jiffies;
  		}
  

-- 
2.31.1