Re: [PATCH] gfs2: Fix use-after-free in iomap inline data write path

Hello Deepanshu,

On Fri, Jan 30, 2026 at 10:21 AM Deepanshu Kartikey
<[email protected]> wrote:
> 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.

I agree with this analysis and fix. Some nitpicking below.

> 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.
>
> Reported-by: [email protected]
> Closes: https://syzkaller.appspot.com/bug?extid=ea1cd4aa4d1e98458a55
> Signed-off-by: Deepanshu Kartikey <[email protected]>
> ---
>  fs/gfs2/bmap.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/fs/gfs2/bmap.c b/fs/gfs2/bmap.c
> index 131091520de6..e70095809e4a 100644
> --- a/fs/gfs2/bmap.c
> +++ b/fs/gfs2/bmap.c
> @@ -887,6 +887,8 @@ static int __gfs2_iomap_get(struct inode *inode, loff_t pos, loff_t length,
>                               sizeof(struct gfs2_dinode);
>                 iomap->type = IOMAP_INLINE;
>                 iomap->inline_data = dibh->b_data + sizeof(struct gfs2_dinode);
> +               iomap->private = dibh;
> +               get_bh(dibh);
>                 goto out;
>         }
>
> @@ -1144,6 +1146,12 @@ 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);
>
> +       /* Release buffer head for inline data */
> +       if (iomap->type == IOMAP_INLINE && iomap->private) {
> +               brelse(iomap->private);
> +               iomap->private = NULL;
> +       }

We're not using iomap->private for anything else and that field is
zeroed out automatically, so this can be simplified to:

if (iomap->private)
  brelse(iomap->private);

> +
>         switch (flags & (IOMAP_WRITE | IOMAP_ZERO)) {
>         case IOMAP_WRITE:
>                 if (flags & IOMAP_DIRECT)
> --
> 2.43.0
>

Thanks,
Andreas