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 <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

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

···
 {
 	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);