Re: [PATCH] auxdisplay: line-display: fix OOB read on zero-length message_store()

[Andy Shevchenko]

On Mon, May 18, 2026 at 10:08:07AM +0200, Geert Uytterhoeven wrote:
> On Fri, 15 May 2026 at 09:13, Andy Shevchenko <andy.shevchenko@xxxxxxxxx> wrote:
> > On Thu, May 14, 2026 at 8:44 PM Stepan Ionichev <sozdayvek@xxxxxxxxx> wrote:
> > > linedisp_display() unconditionally reads msg[count - 1] before
> > > checking whether count is zero, so a write of zero bytes to the
> > > message sysfs attribute hits msg[-1]:
> > >
> > >         write(fd, "", 0);
> > >
> > >         -> message_store(..., buf, count=0)
> > >            -> linedisp_display(linedisp, buf, count=0)
> > >               -> msg[count - 1] == '\n'  ; OOB read
> > >
> > > The kernfs write buffer for that store is a 1-byte allocation
> > > (kernfs_fop_write_iter() does kmalloc(len + 1) with len == 0),
> > > so msg[-1] is a 1-byte read before the slab object. On a
> > > KASAN-enabled kernel this trips an out-of-bounds report and
> > > panics; on stock kernels it silently reads adjacent slab data
> > > and, if that byte happens to be '\n', the following count--
> > > wraps ssize_t 0 to -1 and is then passed to kmemdup_nul().
> > >
> > > linedisp_display() is reached from the message_store() sysfs
> > > callback (drivers/auxdisplay/line-display.c message attribute,
> > > mode 0644) and from the in-tree initial-message setup with
> > > count == -1, so the OOB path is only userspace-triggerable via
> > > zero-byte writes;
> >
> > Isn't it also triggerable when  PANEL_BOOT_MESSAGE is left default
> > with PANEL_CHANGE_MESSAGE="y"? (However these double quotes makes me
> > wonder if this even works, as usually we compare symbols against plain
> > 'n'. 'm', or 'y' (without any quotes).
> >
> > > vfs_write() does not short-circuit on
> > > count == 0 and kernfs_fop_write_iter() dispatches the store
> > > callback regardless.
> 
> I think PANEL_BOOT_MESSAGE is the only way to trigger this, as
> writing an empty string to a device attribute is a no-op according
> to commit afcb5a811ff3ab39 ("auxdisplay: img-ascii-lcd: Fix lock-up
> when displaying empty string")? If that is still true, the issue
> was introduced by commit c8ffef985af564c1 ("auxdisplay: linedisp:
> Support configuring the boot message")?

Good points. Should I drop the patch and ask for a new commit message
(and Fixes tag)?

-- 
With Best Regards,
Andy Shevchenko