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

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

[Stepan Ionichev]

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; vfs_write() does not short-circuit on
count == 0 and kernfs_fop_write_iter() dispatches the store
callback regardless.

Guard the trailing-newline trim with a count check. The
existing if (!count) block then takes the clear-display path
unchanged.

Affects every auxdisplay driver that registers via
linedisp_register() / linedisp_attach(): ht16k33, max6959,
img-ascii-lcd, seg-led-gpio.

Fixes: 7e76aece6f03 ("auxdisplay: Extract character line display core support")
Signed-off-by: Stepan Ionichev <sozdayvek@gmail.com>
---
 drivers/auxdisplay/line-display.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/auxdisplay/line-display.c b/drivers/auxdisplay/line-display.c
index fb6d92941..915eb5cd9 100644
--- a/drivers/auxdisplay/line-display.c
+++ b/drivers/auxdisplay/line-display.c
@@ -173,7 +173,7 @@ static int linedisp_display(struct linedisp *linedisp, const char *msg,
 		count = strlen(msg);
 
 	/* if the string ends with a newline, trim it */
-	if (msg[count - 1] == '\n')
+	if (count && msg[count - 1] == '\n')
 		count--;
 
 	if (!count) {
-- 
2.43.0

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

[Andy Shevchenko]

On Thu, May 14, 2026 at 8:44 PM Stepan Ionichev <sozdayvek@gmail.com> 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.
>
> Guard the trailing-newline trim with a count check. The
> existing if (!count) block then takes the clear-display path
> unchanged.
>
> Affects every auxdisplay driver that registers via
> linedisp_register() / linedisp_attach(): ht16k33, max6959,
> img-ascii-lcd, seg-led-gpio.

In any case this seems a legit report, I will take the change.

-- 
With Best Regards,
Andy Shevchenko

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

[Stepan Ionichev]

On Fri, May 15, 2026 at 10:12:26AM +0300, Andy Shevchenko wrote:
> 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).

Yes -- the same count guard also covers the init path: when
PANEL_BOOT_MESSAGE="" and PANEL_CHANGE_MESSAGE=y, linedisp_attach()
calls linedisp_display(linedisp, "", -1), so count = strlen("") = 0
and msg[-1] reads .rodata before the empty string literal. KASAN
catches it at boot. The patch covers both paths in one guard.

Re: depends on PANEL_CHANGE_MESSAGE="y" -- agreed, that looks odd.
Normally we'd just write "depends on PANEL_CHANGE_MESSAGE". I can
send a separate Kconfig patch if you'd like.

> In any case this seems a legit report, I will take the change.

Thanks for taking it.

Stepan