gfs2: Fix use-after-free in iomap inline data write path

The inline data buffer head (dibh) is being released prematurely in
gfs2_iomap_begin() via release_metapath() while iomap->inline_data
still points to dibh->b_data. This causes a use-after-free when
iomap_write_end_inline() later attempts to write to the inline data
area.

The bug sequence:
1. gfs2_iomap_begin() calls gfs2_meta_inode_buffer() to read inode
   metadata into dibh
2. Sets iomap->inline_data = dibh->b_data + sizeof(struct gfs2_dinode)
3. Calls release_metapath() which calls brelse(dibh), dropping refcount
   to 0
4. kswapd reclaims the page (~39ms later in the syzbot report)
5. iomap_write_end_inline() tries to memcpy() to iomap->inline_data
6. KASAN detects use-after-free write to freed memory

Fix by storing dibh in iomap->private and incrementing its refcount
with get_bh() in gfs2_iomap_begin(). The buffer is then properly
released in gfs2_iomap_end() after the inline write completes,
ensuring the page stays alive for the entire iomap operation.

Note: A C reproducer is not available for this issue. The fix is based
on analysis of the KASAN report and code review showing the buffer head
is freed before use.

[agruenba: Take buffer head reference in gfs2_iomap_begin() to avoid
leaks in gfs2_iomap_get() and gfs2_iomap_alloc().]

Reported-by: syzbot+ea1cd4aa4d1e98458a55@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=ea1cd4aa4d1e98458a55
Fixes: d0a22a4 ("gfs2: Fix iomap write page reclaim deadlock")
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

Author: deepanshu406

fs/gfs2/bmap.c

@@ -1127,10 +1127,18 @@ static int gfs2_iomap_begin(struct inode *inode, loff_t pos, loff_t length,
 			goto out_unlock;
 		break;
 	default:
-		goto out_unlock;
+		goto out;
 	}
 
 	ret = gfs2_iomap_begin_write(inode, pos, length, flags, iomap, &mp);
+	if (ret)
+		goto out_unlock;
+
+out:
+	if (iomap->type == IOMAP_INLINE) {
+		iomap->private = metapath_dibh(&mp);
+		get_bh(iomap->private);
+	}
 
 out_unlock:
 	release_metapath(&mp);
@@ -1144,6 +1152,9 @@ static int gfs2_iomap_end(struct inode *inode, loff_t pos, loff_t length,
 	struct gfs2_inode *ip = GFS2_I(inode);
 	struct gfs2_sbd *sdp = GFS2_SB(inode);
 
+	if (iomap->private)
+		brelse(iomap->private);
+
 	switch (flags & (IOMAP_WRITE | IOMAP_ZERO)) {
 	case IOMAP_WRITE:
 		if (flags & IOMAP_DIRECT)