CVE-2026-63949

In the Linux kernel, the following vulnerability has been resolved:

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

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 (kernfsfopwriteiter() 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 ssizet 0 to -1 and is then passed to kmemdup_nul().

linedispdisplay() is reached from the messagestore() 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; vfswrite() does not short-circuit on count == 0 and kernfsfopwriteiter() 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 linedispregister() / linedispattach(): ht16k33, max6959, img-ascii-lcd, seg-led-gpio.