tracing/uprobe_event: Fix strncpy corner case

BugLink: http://bugs.launchpad.net/bugs/1774173

commit 50268a3d upstream.

Fix string fetch function to terminate with NUL.
It is OK to drop the rest of string.
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Song Liu <songliubraving@fb.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: security@kernel.org
Cc: 范龙飞 <long7573@126.com>
Fixes: 5baaa59e ("tracing/probes: Implement 'memory' fetch method for uprobes")
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

kernel/trace/trace_uprobe.c

@@ -149,6 +149,8 @@ static void FETCH_FUNC_NAME(memory, string)(struct pt_regs *regs,
 		return;
 
 	ret = strncpy_from_user(dst, src, maxlen);
+	if (ret == maxlen)
+		dst[--ret] = '\0';
 
 	if (ret < 0) {	/* Failed to fetch string */
 		((u8 *)get_rloc_data(dest))[0] = '\0';