signal/seccomp: Refactor seccomp signal and coredump generation

Factor out force_sig_seccomp from the seccomp signal generation and
place it in kernel/signal.c.  The function force_sig_seccomp takes a
parameter force_coredump to indicate that the sigaction field should
be reset to SIGDFL so that a coredump will be generated when the
signal is delivered.

force_sig_seccomp is then used to replace both seccomp_send_sigsys
and seccomp_init_siginfo.

force_sig_info_to_task gains an extra parameter to force using
the default signal action.

With this change seccomp is no longer a special case and there
becomes exactly one place do_coredump is called from.

Further it no longer becomes necessary for __seccomp_filter
to call do_group_exit.

Acked-by: Kees Cook <keescook@chromium.org>
Link: https://lkml.kernel.org/r/87r1gr6qc4.fsf_-_@disp2133
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

include/linux/sched/signal.h

@@ -329,6 +329,7 @@ int force_sig_pkuerr(void __user *addr, u32 pkey);
 int force_sig_perf(void __user *addr, u32 type, u64 sig_data);
 
 int force_sig_ptrace_errno_trap(int errno, void __user *addr);
+int force_sig_seccomp(int syscall, int reason, bool force_coredump);
 
 extern int send_sig_info(int, struct kernel_siginfo *, struct task_struct *);
 extern void force_sigsegv(int sig);

kernel/seccomp.c

@@ -922,30 +922,6 @@ void get_seccomp_filter(struct task_struct *tsk)
 	refcount_inc(&orig->users);
 }
 
-static void seccomp_init_siginfo(kernel_siginfo_t *info, int syscall, int reason)
-{
-	clear_siginfo(info);
-	info->si_signo = SIGSYS;
-	info->si_code = SYS_SECCOMP;
-	info->si_call_addr = (void __user *)KSTK_EIP(current);
-	info->si_errno = reason;
-	info->si_arch = syscall_get_arch(current);
-	info->si_syscall = syscall;
-}
-
-/**
- * seccomp_send_sigsys - signals the task to allow in-process syscall emulation
- * @syscall: syscall number to send to userland
- * @reason: filter-supplied reason code to send to userland (via si_errno)
- *
- * Forces a SIGSYS with a code of SYS_SECCOMP and related sigsys info.
- */
-static void seccomp_send_sigsys(int syscall, int reason)
-{
-	struct kernel_siginfo info;
-	seccomp_init_siginfo(&info, syscall, reason);
-	force_sig_info(&info);
-}
 #endif	/* CONFIG_SECCOMP_FILTER */
 
 /* For use with seccomp_actions_logged */
@@ -1218,7 +1194,7 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
 		/* Show the handler the original registers. */
 		syscall_rollback(current, current_pt_regs());
 		/* Let the filter pass back 16 bits of data. */
-		seccomp_send_sigsys(this_syscall, data);
+		force_sig_seccomp(this_syscall, data, false);
 		goto skip;
 
 	case SECCOMP_RET_TRACE:
@@ -1289,18 +1265,14 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
 		/* Dump core only if this is the last remaining thread. */
 		if (action != SECCOMP_RET_KILL_THREAD ||
 		    get_nr_threads(current) == 1) {
-			kernel_siginfo_t info;
-
 			/* Show the original registers in the dump. */
 			syscall_rollback(current, current_pt_regs());
-			/* Trigger a manual coredump since do_exit skips it. */
-			seccomp_init_siginfo(&info, this_syscall, data);
-			do_coredump(&info);
-		}
-		if (action == SECCOMP_RET_KILL_THREAD)
+			/* Trigger a coredump with SIGSYS */
+			force_sig_seccomp(this_syscall, data, true);
+		} else {
 			do_exit(SIGSYS);
-		else
-			do_group_exit(SIGSYS);
+		}
+		return -1; /* skip the syscall go directly to signal handling */
 	}
 
 	unreachable();

kernel/signal.c

@@ -54,6 +54,7 @@
 #include <asm/unistd.h>
 #include <asm/siginfo.h>
 #include <asm/cacheflush.h>
+#include <asm/syscall.h>	/* for syscall_get_* */
 
 /*
  * SLAB caches for signal bits.
@@ -1322,7 +1323,7 @@ int do_send_sig_info(int sig, struct kernel_siginfo *info, struct task_struct *p
  * that is why we also clear SIGNAL_UNKILLABLE.
  */
 static int
-force_sig_info_to_task(struct kernel_siginfo *info, struct task_struct *t)
+force_sig_info_to_task(struct kernel_siginfo *info, struct task_struct *t, bool sigdfl)
 {
 	unsigned long int flags;
 	int ret, blocked, ignored;
@@ -1333,7 +1334,7 @@ force_sig_info_to_task(struct kernel_siginfo *info, struct task_struct *t)
 	action = &t->sighand->action[sig-1];
 	ignored = action->sa.sa_handler == SIG_IGN;
 	blocked = sigismember(&t->blocked, sig);
-	if (blocked || ignored) {
+	if (blocked || ignored || sigdfl) {
 		action->sa.sa_handler = SIG_DFL;
 		if (blocked) {
 			sigdelset(&t->blocked, sig);
@@ -1354,7 +1355,7 @@ force_sig_info_to_task(struct kernel_siginfo *info, struct task_struct *t)
 
 int force_sig_info(struct kernel_siginfo *info)
 {
-	return force_sig_info_to_task(info, current);
+	return force_sig_info_to_task(info, current, false);
 }
 
 /*
@@ -1685,7 +1686,7 @@ int force_sig_fault_to_task(int sig, int code, void __user *addr
 	info.si_flags = flags;
 	info.si_isr = isr;
 #endif
-	return force_sig_info_to_task(&info, t);
+	return force_sig_info_to_task(&info, t, false);
 }
 
 int force_sig_fault(int sig, int code, void __user *addr
@@ -1793,6 +1794,27 @@ int force_sig_perf(void __user *addr, u32 type, u64 sig_data)
 	return force_sig_info(&info);
 }
 
+/**
+ * force_sig_seccomp - signals the task to allow in-process syscall emulation
+ * @syscall: syscall number to send to userland
+ * @reason: filter-supplied reason code to send to userland (via si_errno)
+ *
+ * Forces a SIGSYS with a code of SYS_SECCOMP and related sigsys info.
+ */
+int force_sig_seccomp(int syscall, int reason, bool force_coredump)
+{
+	struct kernel_siginfo info;
+
+	clear_siginfo(&info);
+	info.si_signo = SIGSYS;
+	info.si_code = SYS_SECCOMP;
+	info.si_call_addr = (void __user *)KSTK_EIP(current);
+	info.si_errno = reason;
+	info.si_arch = syscall_get_arch(current);
+	info.si_syscall = syscall;
+	return force_sig_info_to_task(&info, current, force_coredump);
+}
+
 /* For the crazy architectures that include trap information in
  * the errno field, instead of an actual errno value.
  */