Commit faddeb84 authored by Deepanshu Kartikey's avatar Deepanshu Kartikey Committed by Andreas Gruenbacher
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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: default avatar <syzbot+ea1cd4aa4d1e98458a55@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=ea1cd4aa4d1e98458a55


Fixes: d0a22a4b ("gfs2: Fix iomap write page reclaim deadlock")
Signed-off-by: default avatarDeepanshu Kartikey <kartikey406@gmail.com>
Signed-off-by: default avatarAndreas Gruenbacher <agruenba@redhat.com>
parent 22150a7d

fs/gfs2/bmap.c

+12 −1
Original line number Diff line number Diff line
@@ -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)