From a7511dcd9dd4bc55d123f9b800c8a4ed2662e5c6 Mon Sep 17 00:00:00 2001 From: Stepan Ionichev Date: Thu, 14 May 2026 22:43:42 +0500 Subject: [PATCH] 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. Fixes: 7e76aece6f03 ("auxdisplay: Extract character line display core support") Signed-off-by: Stepan Ionichev Signed-off-by: Andy Shevchenko --- 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 fb6d9294140d9..915eb5cd96b23 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) { -- 2.47.3