[RFC PATCH] ptrace: don't report syscall-exit if the tracee was killed by seccomp

[Oleg Nesterov <oleg@redhat.com>]

__seccomp_filter() does

	case SECCOMP_RET_KILL_THREAD:
	case SECCOMP_RET_KILL_PROCESS:
	...
		/* Show the original registers in the dump. */
		syscall_rollback(current, current_pt_regs());

		/* Trigger a coredump with SIGSYS */
		force_sig_seccomp(this_syscall, data, true);

syscall_rollback() does regs->ax == orig_ax. This means that
ptrace_get_syscall_info_exit() will see .is_error == 0. To the tracer,
it looks as if the aborted syscall actually succeeded and returned its
own syscall number.

And since force_sig_seccomp() uses force_coredump == true, SIGSYS won't
be reported (see the SA_IMMUTABLE check in get_signal()), so the tracee
will "silently" exit with error_code == SIGSYS after the bogus report.

Change syscall_exit_work() to avoid the bogus single-step/syscall-exit
reports if the tracee is SECCOMP_MODE_DEAD.

TODO: With or without this change, get_signal() -> ptrace_signal() may
report other !SA_IMMUTABLE pending signals before it dequeues SIGSYS.
Perhaps it makes sense to change get_signal() to check SECCOMP_MODE_DEAD
too and prioritize the fatal SIGSYS.

Reported-by: Max Ver <dudududumaxver@gmail.com>
Closes: https://lore.kernel.org/all/CABjJbFJO+p3jA1r0gjUZrCepQb1Fab3kqxYhc_PSfoqo21ypeQ@mail.gmail.com/
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 include/linux/entry-common.h | 3 +++
 include/linux/seccomp.h      | 8 ++++++++
 kernel/seccomp.c             | 3 ---
 3 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/include/linux/entry-common.h b/include/linux/entry-common.h
index f83ca0abf2cd..5c62bda9dcf9 100644
--- a/include/linux/entry-common.h
+++ b/include/linux/entry-common.h
@@ -250,6 +250,9 @@ static __always_inline void syscall_exit_work(struct pt_regs *regs, unsigned lon
 	if (work & SYSCALL_WORK_SYSCALL_TRACEPOINT)
 		trace_syscall_exit(regs, syscall_get_return_value(current, regs));
 
+	if (killed_by_seccomp(current))
+		return;
+
 	step = report_single_step(work);
 	if (step || work & SYSCALL_WORK_SYSCALL_TRACE)
 		arch_ptrace_report_syscall_exit(regs, step);
diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h
index 9b959972bf4a..e95a251955c1 100644
--- a/include/linux/seccomp.h
+++ b/include/linux/seccomp.h
@@ -22,6 +22,12 @@
 #include <linux/atomic.h>
 #include <asm/seccomp.h>
 
+/* Not exposed in uapi headers: internal use only. */
+#define SECCOMP_MODE_DEAD	(SECCOMP_MODE_FILTER + 1)
+
+#define killed_by_seccomp(task)	\
+	((task)->seccomp.mode == SECCOMP_MODE_DEAD)
+
 extern int __secure_computing(void);
 
 #ifdef CONFIG_HAVE_ARCH_SECCOMP_FILTER
@@ -49,6 +55,8 @@ static inline int seccomp_mode(struct seccomp *s)
 
 struct seccomp_data;
 
+#define killed_by_seccomp(task)	0
+
 #ifdef CONFIG_HAVE_ARCH_SECCOMP_FILTER
 static inline int secure_computing(void) { return 0; }
 #else
diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 066909393c38..461eb15c66c3 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -31,9 +31,6 @@
 
 #include <asm/syscall.h>
 
-/* Not exposed in headers: strictly internal use only. */
-#define SECCOMP_MODE_DEAD	(SECCOMP_MODE_FILTER + 1)
-
 #ifdef CONFIG_SECCOMP_FILTER
 #include <linux/file.h>
 #include <linux/filter.h>
-- 
2.52.0