Re: [PATCH 1/8] kcsan: Add Kernel Concurrency Sanitizer infrastructure

Mark Rutland
2019-10-16T15:16:51Z

On Wed, Oct 16, 2019 at 10:39:52AM +0200, Marco Elver wrote:
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 2c2e56bd8913..34a1d9310304 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1171,6 +1171,13 @@ struct task_struct {
> #ifdef CONFIG_KASAN
> unsigned int kasan_depth;
> #endif
> +#ifdef CONFIG_KCSAN
> + /* See comments at kernel/kcsan/core.c: struct cpu_state. */
> + int kcsan_disable;
> + int kcsan_atomic_next;
> + int kcsan_atomic_region;
> + bool kcsan_atomic_region_flat;
> +#endif

Should these be unsigned?

> +/*
> + * Per-CPU state that should be used instead of 'current' if we are not in a
> + * task.
> + */
> +struct cpu_state {
> + int disable; /* disable counter */
> + int atomic_next; /* number of following atomic ops */
> +
> + /*
> + * We use separate variables to store if we are in a nestable or flat
> + * atomic region. This helps make sure that an atomic region with
> + * nesting support is not suddenly aborted when a flat region is
> + * contained within. Effectively this allows supporting nesting flat
> + * atomic regions within an outer nestable atomic region. Support for
> + * this is required as there are cases where a seqlock reader critical
> + * section (flat atomic region) is contained within a seqlock writer
> + * critical section (nestable atomic region), and the "mismatching
> + * kcsan_end_atomic()" warning would trigger otherwise.
> + */
> + int atomic_region;
> + bool atomic_region_flat;
> +};
> +static DEFINE_PER_CPU(struct cpu_state, this_state) = {
> + .disable = 0,
> + .atomic_next = 0,
> + .atomic_region = 0,
> + .atomic_region_flat = 0,
> +};

These are the same as in task_struct, so I think it probably makes sense
to have a common structure for these, e.g.

| struct kcsan_ctx {
| int disable;
| int atomic_next;
| int atomic_region;
| bool atomic_region_flat;
| };

... which you then place within task_struct, e.g.

| #ifdef CONFIG_KCSAN
| struct kcsan_ctx kcsan_ctx;
| #endif

... and here, e.g.

| static DEFINE_PER_CPU(struct kcsan_ctx, kcsan_cpu_ctx);

That would simplify a number of cases below where you have to choose one
or the other, as you can choose the pointer, then handle the rest in a
common way.

e.g. for:

> +static inline bool is_atomic(const volatile void *ptr)
> +{
> + if (in_task()) {
> + if (unlikely(current->kcsan_atomic_next > 0)) {
> + --current->kcsan_atomic_next;
> + return true;
> + }
> + if (unlikely(current->kcsan_atomic_region > 0 ||
> + current->kcsan_atomic_region_flat))
> + return true;
> + } else { /* interrupt */
> + if (unlikely(this_cpu_read(this_state.atomic_next) > 0)) {
> + this_cpu_dec(this_state.atomic_next);
> + return true;
> + }
> + if (unlikely(this_cpu_read(this_state.atomic_region) > 0 ||
> + this_cpu_read(this_state.atomic_region_flat)))
> + return true;
> + }
> +
> + return kcsan_is_atomic(ptr);
> +}

... you could have something like:

| struct kcsan_ctx *kcsan_get_ctx(void)
| {
| return in_task() ? &current->kcsan_ctx : this_cpu_ptr(kcsan_cpu_ctx);
| }
|
| static inline bool is_atomic(const volatile void *ptr)
| {
| struct kcsan_ctx *ctx = kcsan_get_ctx();
| if (unlikely(ctx->atomic_next > 0) {
| --ctx->atomic_next;
| return true;
| }
| if (unlikely(ctx->atomic_region > 0 || ctx->atomic_region_flat))
| return true;
|
| return kcsan_is_atomic(ptr);
| }

... avoiding duplicating the checks for task/irq contexts.

It's not clear to me how either that or the original code works if a
softirq is interrupted by a hardirq. IIUC most of the fields should
remain stable over that window, since the hardirq should balance most
changes it makes before returning, but I don't think that's true for
atomic_next. Can't that be corrupted from the PoV of the softirq
handler?

[...]

> +void kcsan_begin_atomic(bool nest)
> +{
> + if (nest) {
> + if (in_task())
> + ++current->kcsan_atomic_region;
> + else
> + this_cpu_inc(this_state.atomic_region);
> + } else {
> + if (in_task())
> + current->kcsan_atomic_region_flat = true;
> + else
> + this_cpu_write(this_state.atomic_region_flat, true);
> + }
> +}

Assuming my suggestion above wasn't bogus, this can be:

| void kcsan_begin_atomic(boot nest)
| {
| struct kcsan_ctx *ctx = kcsan_get_ctx();
| if (nest)
| ctx->atomic_region++;
| else
| ctx->atomic_region_flat = true;
| }

> +void kcsan_end_atomic(bool nest)
> +{
> + if (nest) {
> + int prev =
> + in_task() ?
> + current->kcsan_atomic_region-- :
> + (this_cpu_dec_return(this_state.atomic_region) +
> + 1);
> + if (prev == 0) {
> + kcsan_begin_atomic(true); /* restore to 0 */
> + kcsan_disable_current();
> + WARN(1, "mismatching %s", __func__);
> + kcsan_enable_current();
> + }
> + } else {
> + if (in_task())
> + current->kcsan_atomic_region_flat = false;
> + else
> + this_cpu_write(this_state.atomic_region_flat, false);
> + }
> +}

... similarly:

| void kcsan_end_atomic(bool nest)
| {
| struct kcsan_ctx *ctx = kcsan_get_ctx();
|
| if (nest)
| if (ctx->kcsan_atomic_region--) {
| kcsan_begin_atomic(true); /* restore to 0 */
| kcsan_disable_current();
| WARN(1, "mismatching %s"\ __func__);
| kcsan_enable_current();
| }
| } else {
| ctx->atomic_region_flat = true;
| }
| }

> +void kcsan_atomic_next(int n)
> +{
> + if (in_task())
> + current->kcsan_atomic_next = n;
> + else
> + this_cpu_write(this_state.atomic_next, n);
> +}

... and:

| void kcsan_atomic_nextint n)
| {
| kcsan_get_ctx()->atomic_next = n;
| }

Thanks,
Mark.