signal/powerpc: On swapcontext failure force SIGSEGV

If the register state may be partial and corrupted instead of calling
do_exit, call force_sigsegv(SIGSEGV).  Which properly kills the
process with SIGSEGV and does not let any more userspace code execute,
instead of just killing one thread of the process and potentially
confusing everything.

Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: linuxppc-dev@lists.ozlabs.org
History-tree: git://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git
Fixes: 756f1ae8a44e ("PPC32: Rework signal code and add a swapcontext system call.")
Fixes: 04879b04bf50 ("[PATCH] ppc64: VMX (Altivec) support & signal32 rework, from Ben Herrenschmidt")
Link: https://lkml.kernel.org/r/20211020174406.17889-7-ebiederm@xmission.comSigned-off-by: NEric W. Biederman <ebiederm@xmission.com>

arch/powerpc/kernel/signal_32.c

@@ -1062,8 +1062,10 @@ SYSCALL_DEFINE3(swapcontext, struct ucontext __user *, old_ctx,
 	 * or if another thread unmaps the region containing the context.
 	 * We kill the task with a SIGSEGV in this situation.
 	 */
-	if (do_setcontext(new_ctx, regs, 0))
-		do_exit(SIGSEGV);
+	if (do_setcontext(new_ctx, regs, 0)) {
+		force_sigsegv(SIGSEGV);
+		return -EFAULT;
+	}
 
 	set_thread_flag(TIF_RESTOREALL);
 	return 0;

arch/powerpc/kernel/signal_64.c

@@ -703,15 +703,18 @@ SYSCALL_DEFINE3(swapcontext, struct ucontext __user *, old_ctx,
 	 * We kill the task with a SIGSEGV in this situation.
 	 */
 
-	if (__get_user_sigset(&set, &new_ctx->uc_sigmask))
-		do_exit(SIGSEGV);
+	if (__get_user_sigset(&set, &new_ctx->uc_sigmask)) {
+		force_sigsegv(SIGSEGV);
+		return -EFAULT;
+	}
 	set_current_blocked(&set);
 
 	if (!user_read_access_begin(new_ctx, ctx_size))
 		return -EFAULT;
 	if (__unsafe_restore_sigcontext(current, NULL, 0, &new_ctx->uc_mcontext)) {
 		user_read_access_end();
-		do_exit(SIGSEGV);
+		force_sigsegv(SIGSEGV);
+		return -EFAULT;
 	}
 	user_read_access_end();