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Subject[PATCH v4 8/9] kentry: Check that syscall entries and syscall exits match
Date
If arch code calls the wrong kernel entry helpers, syscall entries and
exits can get out of sync. Add a new field to task_struct to track the
syscall state and validate that it transitions correctly.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
include/linux/sched.h | 4 ++++
init/init_task.c | 8 ++++++++
kernel/entry/common.c | 24 +++++++++++++++++++++++-
3 files changed, 35 insertions(+), 1 deletion(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 6e3a5eeec509..95d6d8686d98 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1368,6 +1368,10 @@ struct task_struct {
struct llist_head kretprobe_instances;
#endif

+#ifdef CONFIG_DEBUG_ENTRY
+ bool kentry_in_syscall;
+#endif
+
/*
* New fields for task_struct should be added above here, so that
* they are included in the randomized portion of task_struct.
diff --git a/init/init_task.c b/init/init_task.c
index 8a992d73e6fb..de4fdaac4e8b 100644
--- a/init/init_task.c
+++ b/init/init_task.c
@@ -212,6 +212,14 @@ struct task_struct init_task
#ifdef CONFIG_SECCOMP
.seccomp = { .filter_count = ATOMIC_INIT(0) },
#endif
+#ifdef CONFIG_DEBUG_ENTRY
+ /*
+ * The init task, and kernel threads in general, are considered
+ * to be "in a syscall". This way they can execve() and then exit
+ * the supposed syscall that they were in to go to user mode.
+ */
+ .kentry_in_syscall = true,
+#endif
};
EXPORT_SYMBOL(init_task);

diff --git a/kernel/entry/common.c b/kernel/entry/common.c
index 8ba774184e00..152e7546be16 100644
--- a/kernel/entry/common.c
+++ b/kernel/entry/common.c
@@ -141,11 +141,21 @@ static long syscall_trace_enter(struct pt_regs *regs, long syscall,

long kentry_syscall_begin(struct pt_regs *regs, long syscall)
{
- unsigned long work = READ_ONCE(current_thread_info()->syscall_work);
+ unsigned long work;
+
+ if (IS_ENABLED(CONFIG_DEBUG_ENTRY)) {
+ DEBUG_ENTRY_WARN_ONCE(
+ current->kentry_in_syscall,
+ "entering syscall %ld while already in a syscall",
+ syscall);
+ current->kentry_in_syscall = true;
+ }

CT_WARN_ON(ct_state() != CONTEXT_KERNEL);
lockdep_assert_irqs_enabled();

+ work = READ_ONCE(current_thread_info()->syscall_work);
+
if (work & SYSCALL_WORK_ENTER)
syscall = syscall_trace_enter(regs, syscall, work);

@@ -156,11 +166,16 @@ long kentry_syscall_begin(struct pt_regs *regs, long syscall)
static __always_inline void __exit_to_user_mode(void)
{
instrumentation_begin();
+
#ifdef CONFIG_DEBUG_ENTRY
DEBUG_ENTRY_WARN_ONCE(this_cpu_read(kentry_cpu_depth) != 1,
"__exit_to_user_mode called at wrong kentry cpu depth (%u)",
this_cpu_read(kentry_cpu_depth));
+
+ DEBUG_ENTRY_WARN_ONCE(current->kentry_in_syscall,
+ "exiting to user mode while in syscall context");
#endif
+
trace_hardirqs_on_prepare();
lockdep_hardirqs_on_prepare(CALLER_ADDR0);
instrumentation_end();
@@ -317,6 +332,13 @@ void kentry_syscall_end(struct pt_regs *regs)
*/
if (unlikely(work & SYSCALL_WORK_EXIT))
syscall_exit_work(regs, work);
+
+#ifdef CONFIG_DEBUG_ENTRY
+ DEBUG_ENTRY_WARN_ONCE(!current->kentry_in_syscall,
+ "exiting syscall %lu without entering first", nr);
+
+ current->kentry_in_syscall = 0;
+#endif
}

noinstr void kentry_enter_from_user_mode(struct pt_regs *regs)
--
2.30.2
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