CVE-2026-63949auxdisplay: line-display: fix OOB read on zero-length message_store()

Official description

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 (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.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/ca5b0781946d5083ceafa752141f47f085853620
• https://git.kernel.org/stable/c/8776032fe989a9b5fc77f2de5e03e4adb44c630e
• https://git.kernel.org/stable/c/3859960daeb9b7b39b9847b5b0113bc6081eb735
• https://git.kernel.org/stable/c/197476b126010bac1b3199833c6966cd6f54c2a9
• https://git.kernel.org/stable/c/6ad4f75ef9f3372fce8cad494e789ac6a5507bef
• https://git.kernel.org/stable/c/a7511dcd9dd4bc55d123f9b800c8a4ed2662e5c6