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

[Geert Uytterhoeven]

Hi Andy,

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")?

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds