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 <l...@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