signal: Replace force_fatal_sig with force_exit_sig when in doubt

Recently to prevent issues with SECCOMP_RET_KILL and similar signals
being changed before they are delivered SA_IMMUTABLE was added.

Unfortunately this broke debuggers[1][2] which reasonably expect
to be able to trap synchronous SIGTRAP and SIGSEGV even when
the target process is not configured to handle those signals.

Add force_exit_sig and use it instead of force_fatal_sig where
historically the code has directly called do_exit.  This has the
implementation benefits of going through the signal exit path
(including generating core dumps) without the danger of allowing
userspace to ignore or change these signals.

This avoids userspace regressions as older kernels exited with do_exit
which debuggers also can not intercept.

In the future is should be possible to improve the quality of
implementation of the kernel by changing some of these force_exit_sig
calls to force_fatal_sig.  That can be done where it matters on
a case-by-case basis with careful analysis.

Reported-by: Kyle Huey <me@kylehuey.com>
Reported-by: kernel test robot <oliver.sang@intel.com>
[1] https://lkml.kernel.org/r/CAP045AoMY4xf8aC_4QU_-j7obuEPYgTcnQQP3Yxk=2X90jtpjw@mail.gmail.com
[2] https://lkml.kernel.org/r/20211117150258.GB5403@xsang-OptiPlex-9020
Fixes: 00b06da29cf9 ("signal: Add SA_IMMUTABLE to ensure forced siganls do not get changed")
Fixes: a3616a3c0272 ("signal/m68k: Use force_sigsegv(SIGSEGV) in fpsp040_die")
Fixes: 83a1f27ad773 ("signal/powerpc: On swapcontext failure force SIGSEGV")
Fixes: 9bc508cf0791 ("signal/s390: Use force_sigsegv in default_trap_handler")
Fixes: 086ec444f866 ("signal/sparc32: In setup_rt_frame and setup_fram use force_fatal_sig")
Fixes: c317d306d550 ("signal/sparc32: Exit with a fatal signal when try_to_clear_window_buffer fails")
Fixes: 695dd0d634df ("signal/x86: In emulate_vsyscall force a signal instead of calling do_exit")
Fixes: 1fbd60df8a85 ("signal/vm86_32: Properly send SIGSEGV when the vm86 state cannot be saved.")
Fixes: 941edc5bf174 ("exit/syscall_user_dispatch: Send ordinary signals on failure")
Link: https://lkml.kernel.org/r/871r3dqfv8.fsf_-_@email.froward.int.ebiederm.org
Reviewed-by: Kees Cook <keescook@chromium.org>
Tested-by: Kees Cook <keescook@chromium.org>
Tested-by: Kyle Huey <khuey@kylehuey.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

11 files changed, 26 insertions, 12 deletions

diff --git a/arch/m68k/kernel/traps.c b/arch/m68k/kernel/traps.c
index 99058a6da956..34d6458340b0 100644
--- a/arch/m68k/kernel/traps.c
+++ b/arch/m68k/kernel/traps.c
@@ -1145,7 +1145,7 @@ asmlinkage void set_esp0(unsigned long ssp)
  */
 asmlinkage void fpsp040_die(void)
 {
-	force_fatal_sig(SIGSEGV);
+	force_exit_sig(SIGSEGV);
 }
 
 #ifdef CONFIG_M68KFPU_EMU
diff --git a/arch/powerpc/kernel/signal_32.c b/arch/powerpc/kernel/signal_32.c
index 00a9c9cd6d42..3e053e2fd6b6 100644
--- a/arch/powerpc/kernel/signal_32.c
+++ b/arch/powerpc/kernel/signal_32.c
@@ -1063,7 +1063,7 @@ SYSCALL_DEFINE3(swapcontext, struct ucontext __user *, old_ctx,
 	 * We kill the task with a SIGSEGV in this situation.
 	 */
 	if (do_setcontext(new_ctx, regs, 0)) {
-		force_fatal_sig(SIGSEGV);
+		force_exit_sig(SIGSEGV);
 		return -EFAULT;
 	}
 
diff --git a/arch/powerpc/kernel/signal_64.c b/arch/powerpc/kernel/signal_64.c
index ef518535d436..d1e1fc0acbea 100644
--- a/arch/powerpc/kernel/signal_64.c
+++ b/arch/powerpc/kernel/signal_64.c
@@ -704,7 +704,7 @@ SYSCALL_DEFINE3(swapcontext, struct ucontext __user *, old_ctx,
 	 */
 
 	if (__get_user_sigset(&set, &new_ctx->uc_sigmask)) {
-		force_fatal_sig(SIGSEGV);
+		force_exit_sig(SIGSEGV);
 		return -EFAULT;
 	}
 	set_current_blocked(&set);
@@ -713,7 +713,7 @@ SYSCALL_DEFINE3(swapcontext, struct ucontext __user *, old_ctx,
 		return -EFAULT;
 	if (__unsafe_restore_sigcontext(current, NULL, 0, &new_ctx->uc_mcontext)) {
 		user_read_access_end();
-		force_fatal_sig(SIGSEGV);
+		force_exit_sig(SIGSEGV);
 		return -EFAULT;
 	}
 	user_read_access_end();
diff --git a/arch/s390/kernel/traps.c b/arch/s390/kernel/traps.c
index 035705c9f23e..2b780786fc68 100644
--- a/arch/s390/kernel/traps.c
+++ b/arch/s390/kernel/traps.c
@@ -84,7 +84,7 @@ static void default_trap_handler(struct pt_regs *regs)
 {
 	if (user_mode(regs)) {
 		report_user_fault(regs, SIGSEGV, 0);
-		force_fatal_sig(SIGSEGV);
+		force_exit_sig(SIGSEGV);
 	} else
 		die(regs, "Unknown program exception");
 }
diff --git a/arch/sparc/kernel/signal_32.c b/arch/sparc/kernel/signal_32.c
index cd677bc564a7..ffab16369bea 100644
--- a/arch/sparc/kernel/signal_32.c
+++ b/arch/sparc/kernel/signal_32.c
@@ -244,7 +244,7 @@ static int setup_frame(struct ksignal *ksig, struct pt_regs *regs,
 		get_sigframe(ksig, regs, sigframe_size);
 
 	if (invalid_frame_pointer(sf, sigframe_size)) {
-		force_fatal_sig(SIGILL);
+		force_exit_sig(SIGILL);
 		return -EINVAL;
 	}
 
@@ -336,7 +336,7 @@ static int setup_rt_frame(struct ksignal *ksig, struct pt_regs *regs,
 	sf = (struct rt_signal_frame __user *)
 		get_sigframe(ksig, regs, sigframe_size);
 	if (invalid_frame_pointer(sf, sigframe_size)) {
-		force_fatal_sig(SIGILL);
+		force_exit_sig(SIGILL);
 		return -EINVAL;
 	}
 
diff --git a/arch/sparc/kernel/windows.c b/arch/sparc/kernel/windows.c
index bbbd40cc6b28..8f20862ccc83 100644
--- a/arch/sparc/kernel/windows.c
+++ b/arch/sparc/kernel/windows.c
@@ -122,7 +122,7 @@ void try_to_clear_window_buffer(struct pt_regs *regs, int who)
 		if ((sp & 7) ||
 		    copy_to_user((char __user *) sp, &tp->reg_window[window],
 				 sizeof(struct reg_window32))) {
-			force_fatal_sig(SIGILL);
+			force_exit_sig(SIGILL);
 			return;
 		}
 	}
diff --git a/arch/x86/entry/vsyscall/vsyscall_64.c b/arch/x86/entry/vsyscall/vsyscall_64.c
index 0b6b277ee050..fd2ee9408e91 100644
--- a/arch/x86/entry/vsyscall/vsyscall_64.c
+++ b/arch/x86/entry/vsyscall/vsyscall_64.c
@@ -226,7 +226,7 @@ bool emulate_vsyscall(unsigned long error_code,
 	if ((!tmp && regs->orig_ax != syscall_nr) || regs->ip != address) {
 		warn_bad_vsyscall(KERN_DEBUG, regs,
 				  "seccomp tried to change syscall nr or ip");
-		force_fatal_sig(SIGSYS);
+		force_exit_sig(SIGSYS);
 		return true;
 	}
 	regs->orig_ax = -1;
diff --git a/arch/x86/kernel/vm86_32.c b/arch/x86/kernel/vm86_32.c
index cce1c89cb7df..c21bcd668284 100644
--- a/arch/x86/kernel/vm86_32.c
+++ b/arch/x86/kernel/vm86_32.c
@@ -160,7 +160,7 @@ Efault_end:
 	user_access_end();
 Efault:
 	pr_alert("could not access userspace vm86 info\n");
-	force_fatal_sig(SIGSEGV);
+	force_exit_sig(SIGSEGV);
 	goto exit_vm86;
 }
 
diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
index 23505394ef70..33a50642cf41 100644
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -352,6 +352,7 @@ extern __must_check bool do_notify_parent(struct task_struct *, int);
 extern void __wake_up_parent(struct task_struct *p, struct task_struct *parent);
 extern void force_sig(int);
 extern void force_fatal_sig(int);
+extern void force_exit_sig(int);
 extern int send_sig(int, struct task_struct *, int);
 extern int zap_other_threads(struct task_struct *p);
 extern struct sigqueue *sigqueue_alloc(void);
diff --git a/kernel/entry/syscall_user_dispatch.c b/kernel/entry/syscall_user_dispatch.c
index 4508201847d2..0b6379adff6b 100644
--- a/kernel/entry/syscall_user_dispatch.c
+++ b/kernel/entry/syscall_user_dispatch.c
@@ -48,7 +48,7 @@ bool syscall_user_dispatch(struct pt_regs *regs)
 		 * the selector is loaded by userspace.
 		 */
 		if (unlikely(__get_user(state, sd->selector))) {
-			force_fatal_sig(SIGSEGV);
+			force_exit_sig(SIGSEGV);
 			return true;
 		}
 
@@ -56,7 +56,7 @@ bool syscall_user_dispatch(struct pt_regs *regs)
 			return false;
 
 		if (state != SYSCALL_DISPATCH_FILTER_BLOCK) {
-			force_fatal_sig(SIGSYS);
+			force_exit_sig(SIGSYS);
 			return true;
 		}
 	}
diff --git a/kernel/signal.c b/kernel/signal.c
index 7815e1bbeddc..a629b11bf3e0 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1671,6 +1671,19 @@ void force_fatal_sig(int sig)
 	force_sig_info_to_task(&info, current, HANDLER_SIG_DFL);
 }
 
+void force_exit_sig(int sig)
+{
+	struct kernel_siginfo info;
+
+	clear_siginfo(&info);
+	info.si_signo = sig;
+	info.si_errno = 0;
+	info.si_code = SI_KERNEL;
+	info.si_pid = 0;
+	info.si_uid = 0;
+	force_sig_info_to_task(&info, current, HANDLER_EXIT);
+}
+
 /*
  * When things go south during signal handling, we
  * will force a SIGSEGV. And if the signal that caused