gfs2: Fix iomap buffer head reference counting bug

GFS2 passes the inode buffer head (dibh) from gfs2_iomap_begin to
gfs2_iomap_end in iomap->private.  It sets that private pointer in
gfs2_iomap_get.  Users of gfs2_iomap_get other than gfs2_iomap_begin
would have to release iomap->private, but this isn't done correctly,
leading to a leak of buffer head references.

To fix this, move the code for setting iomap->private from
gfs2_iomap_get to gfs2_iomap_begin.

Fixes: 64bc06b ("gfs2: iomap buffered write support")
Cc: stable@vger.kernel.org # v4.19+
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/gfs2/bmap.c

@@ -826,7 +826,7 @@ static int gfs2_iomap_get(struct inode *inode, loff_t pos, loff_t length,
 	ret = gfs2_meta_inode_buffer(ip, &dibh);
 	if (ret)
 		goto unlock;
-	iomap->private = dibh;
+	mp->mp_bh[0] = dibh;
 
 	if (gfs2_is_stuffed(ip)) {
 		if (flags & IOMAP_WRITE) {
@@ -863,9 +863,6 @@ static int gfs2_iomap_get(struct inode *inode, loff_t pos, loff_t length,
 	len = lblock_stop - lblock + 1;
 	iomap->length = len << inode->i_blkbits;
 
-	get_bh(dibh);
-	mp->mp_bh[0] = dibh;
-
 	height = ip->i_height;
 	while ((lblock + 1) * sdp->sd_sb.sb_bsize > sdp->sd_heightsize[height])
 		height++;
@@ -898,8 +895,6 @@ static int gfs2_iomap_get(struct inode *inode, loff_t pos, loff_t length,
 	iomap->bdev = inode->i_sb->s_bdev;
 unlock:
 	up_read(&ip->i_rw_mutex);
-	if (ret && dibh)
-		brelse(dibh);
 	return ret;
 
 do_alloc:
@@ -980,9 +975,9 @@ static void gfs2_iomap_journaled_page_done(struct inode *inode, loff_t pos,
 
 static int gfs2_iomap_begin_write(struct inode *inode, loff_t pos,
 				  loff_t length, unsigned flags,
-				  struct iomap *iomap)
+				  struct iomap *iomap,
+				  struct metapath *mp)
 {
-	struct metapath mp = { .mp_aheight = 1, };
 	struct gfs2_inode *ip = GFS2_I(inode);
 	struct gfs2_sbd *sdp = GFS2_SB(inode);
 	unsigned int data_blocks = 0, ind_blocks = 0, rblocks;
@@ -996,9 +991,9 @@ static int gfs2_iomap_begin_write(struct inode *inode, loff_t pos,
 	unstuff = gfs2_is_stuffed(ip) &&
 		  pos + length > gfs2_max_stuffed_size(ip);
 
-	ret = gfs2_iomap_get(inode, pos, length, flags, iomap, &mp);
+	ret = gfs2_iomap_get(inode, pos, length, flags, iomap, mp);
 	if (ret)
-		goto out_release;
+		goto out_unlock;
 
 	alloc_required = unstuff || iomap->type == IOMAP_HOLE;
 
@@ -1013,7 +1008,7 @@ static int gfs2_iomap_begin_write(struct inode *inode, loff_t pos,
 
 		ret = gfs2_quota_lock_check(ip, &ap);
 		if (ret)
-			goto out_release;
+			goto out_unlock;
 
 		ret = gfs2_inplace_reserve(ip, &ap);
 		if (ret)
@@ -1038,25 +1033,22 @@ static int gfs2_iomap_begin_write(struct inode *inode, loff_t pos,
 		ret = gfs2_unstuff_dinode(ip, NULL);
 		if (ret)
 			goto out_trans_end;
-		release_metapath(&mp);
-		brelse(iomap->private);
-		iomap->private = NULL;
+		release_metapath(mp);
 		ret = gfs2_iomap_get(inode, iomap->offset, iomap->length,
-				     flags, iomap, &mp);
+				     flags, iomap, mp);
 		if (ret)
 			goto out_trans_end;
 	}
 
 	if (iomap->type == IOMAP_HOLE) {
-		ret = gfs2_iomap_alloc(inode, iomap, flags, &mp);
+		ret = gfs2_iomap_alloc(inode, iomap, flags, mp);
 		if (ret) {
 			gfs2_trans_end(sdp);
 			gfs2_inplace_release(ip);
 			punch_hole(ip, iomap->offset, iomap->length);
 			goto out_qunlock;
 		}
 	}
-	release_metapath(&mp);
 	if (gfs2_is_jdata(ip))
 		iomap->page_done = gfs2_iomap_journaled_page_done;
 	return 0;
@@ -1069,10 +1061,7 @@ static int gfs2_iomap_begin_write(struct inode *inode, loff_t pos,
 out_qunlock:
 	if (alloc_required)
 		gfs2_quota_unlock(ip);
-out_release:
-	if (iomap->private)
-		brelse(iomap->private);
-	release_metapath(&mp);
+out_unlock:
 	gfs2_write_unlock(inode);
 	return ret;
 }
@@ -1088,10 +1077,10 @@ static int gfs2_iomap_begin(struct inode *inode, loff_t pos, loff_t length,
 
 	trace_gfs2_iomap_start(ip, pos, length, flags);
 	if ((flags & IOMAP_WRITE) && !(flags & IOMAP_DIRECT)) {
-		ret = gfs2_iomap_begin_write(inode, pos, length, flags, iomap);
+		ret = gfs2_iomap_begin_write(inode, pos, length, flags, iomap, &mp);
 	} else {
 		ret = gfs2_iomap_get(inode, pos, length, flags, iomap, &mp);
-		release_metapath(&mp);
+
 		/*
 		 * Silently fall back to buffered I/O for stuffed files or if
 		 * we've hot a hole (see gfs2_file_direct_write).
@@ -1100,6 +1089,11 @@ static int gfs2_iomap_begin(struct inode *inode, loff_t pos, loff_t length,
 		    iomap->type != IOMAP_MAPPED)
 			ret = -ENOTBLK;
 	}
+	if (!ret) {
+		get_bh(mp.mp_bh[0]);
+		iomap->private = mp.mp_bh[0];
+	}
+	release_metapath(&mp);
 	trace_gfs2_iomap_end(ip, iomap, ret);
 	return ret;
 }