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

On Fri, Jan 30, 2026 at 12:33 PM Andreas Gruenbacher
<[email protected]> wrote:
> 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);

And then we've also got iomap->private leaking a dibh reference in
gfs2_iomap_get() and gfs2_iomap_alloc().

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