From 98602942c143075ab7464f917e0fc5d31ce28c3f Mon Sep 17 00:00:00 2001 From: Olivier Fourdan Date: Wed, 27 Nov 2024 14:41:45 +0100 Subject: [PATCH xserver] xkb: Fix buffer overflow in XkbVModMaskText() The code in XkbVModMaskText() allocates a fixed sized buffer on the stack and copies the virtual mod name. There's actually two issues in the code that can lead to a buffer overflow. First, the bound check mixes pointers and integers using misplaced parenthesis, defeating the bound check. But even though, if the check fails, the data is still copied, so the stack overflow will occur regardless. Change the logic to skip the copy entirely if the bound check fails. CVE-2025-26595, ZDI-CAN-25545 This vulnerability was discovered by: Jan-Niklas Sohn working with Trend Micro Zero Day Initiative Signed-off-by: Olivier Fourdan Reviewed-by: Peter Hutterer --- xkb/xkbtext.c | 16 ++++++++-------- 1 file changed, 8 insertions(+), 8 deletions(-) diff --git a/xkb/xkbtext.c b/xkb/xkbtext.c index 018466420..93262528b 100644 --- a/xkb/xkbtext.c +++ b/xkb/xkbtext.c @@ -173,14 +173,14 @@ XkbVModMaskText(XkbDescPtr xkb, len = strlen(tmp) + 1 + (str == buf ? 0 : 1); if (format == XkbCFile) len += 4; - if ((str - (buf + len)) <= VMOD_BUFFER_SIZE) { - if (str != buf) { - if (format == XkbCFile) - *str++ = '|'; - else - *str++ = '+'; - len--; - } + if ((str - buf) + len > VMOD_BUFFER_SIZE) + continue; /* Skip */ + if (str != buf) { + if (format == XkbCFile) + *str++ = '|'; + else + *str++ = '+'; + len--; } if (format == XkbCFile) sprintf(str, "%sMask", tmp); -- 2.48.1