uml: don't allow processes to call into stub

Kill a process that tries to branch into a stub and execute a system
call.  There are no security implications here - a system call in a
stub is treated the same as a system call anywhere else.  But if a
process is trying to branch into a stub, either it is trying something
nasty or it has gone haywire, so it's a good idea to get rid of it in
either case.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/um/os-Linux/skas/process.c

@@ -146,6 +146,9 @@ static void handle_trap(int pid, struct uml_pt_regs *regs,
 {
 	int err, status;
 
+	if ((UPT_IP(regs) >= STUB_START) && (UPT_IP(regs) < STUB_END))
+		fatal_sigsegv();
+
 	/* Mark this as a syscall */
 	UPT_SYSCALL_NR(regs) = PT_SYSCALL_NR(regs->gp);