[PATCH] s390/debug: reject zero-length input before trimming a newline

[Pengpeng Hou]

debug_get_user_string() duplicates the userspace buffer with
memdup_user_nul() and then unconditionally looks at buffer[user_len - 1]
to strip a trailing newline.

A zero-length write reaches this helper unchanged, so the newline trim
reads before the start of the allocated buffer.

Reject empty writes before accessing the last input byte.

Fixes: 66a464dbc8e0 ("[PATCH] s390: debug feature changes")

Signed-off-by: Pengpeng Hou <pengpeng@xxxxxxxxxxx>
---
 arch/s390/kernel/debug.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/s390/kernel/debug.c b/arch/s390/kernel/debug.c
index 31430e9bcfdd..2612f634e826 100644
--- a/arch/s390/kernel/debug.c
+++ b/arch/s390/kernel/debug.c
@@ -1414,6 +1414,9 @@ static inline char *debug_get_user_string(const char __user *user_buf,
 {
 	char *buffer;
 
+	if (!user_len)
+		return ERR_PTR(-EINVAL);
+
 	buffer = memdup_user_nul(user_buf, user_len);
 	if (IS_ERR(buffer))
 		return buffer;
-- 
2.50.1 (Apple Git-155)