On Mon, 12 Oct 2020 at 17:25, Andrey Konovalov <andreyknvl@xxxxxxxxxx> wrote:
>
> Currently there's a KCOV remote coverage collection section in
> __usb_hcd_giveback_urb(). Initially that section was added based on the
> assumption that usb_hcd_giveback_urb() can only be called in interrupt
> context as indicated by a comment before it. This is what happens when
> syzkaller is fuzzing the USB stack via the dummy_hcd driver.
>
> As it turns out, it's actually valid to call usb_hcd_giveback_urb() in task
> context, provided that the caller turned off the interrupts; USB/IP does
> exactly that. This can lead to a nested KCOV remote coverage collection
> sections both trying to collect coverage in task context. This isn't
> supported by KCOV, and leads to a WARNING.
>
> The approach this patch takes is to add another set of kcov_remote_*()
> callbacks that specify the context they are supposed to be executed in.
> If the current context doesn't match the mask provided to a callback,
> that callback is ignored. KCOV currently only supports collecting remote
> coverage in two contexts: task and softirq. This patch constraints KCOV to
> only collect coverage from __usb_hcd_giveback_urb() when it's executed in
> softirq context.
>
> As the result, the coverage from USB/IP related usb_hcd_giveback_urb() calls
> won't be collected, but the WARNING is fixed.
>
> A potential future improvement would be to support nested remote coverage
> collection sections, but this patch doesn't address that.
>
> Signed-off-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
> ---
>
> Changes v2->v3:
> - Keep behavoir of existing callbacks the same except for the
> __usb_hcd_giveback_urb() one.
> - Fix build error with KOV disabled.
>
> ---
> Documentation/dev-tools/kcov.rst | 6 ++++++
> drivers/usb/core/hcd.c | 4 ++--
> include/linux/kcov.h | 31 +++++++++++++++++++++++++++++--
> kernel/kcov.c | 26 +++++++++++++++++++-------
> 4 files changed, 56 insertions(+), 11 deletions(-)
>
> diff --git a/Documentation/dev-tools/kcov.rst b/Documentation/dev-tools/kcov.rst
> index 8548b0b04e43..2c0f58988512 100644
> --- a/Documentation/dev-tools/kcov.rst
> +++ b/Documentation/dev-tools/kcov.rst
> @@ -235,6 +235,12 @@ saved to the kcov_handle field in the current task_struct and needs to be
> passed to the newly spawned threads via custom annotations. Those threads
> should in turn be annotated with kcov_remote_start()/kcov_remote_stop().
>
> +Besides the annotations that only accept a handle, there are also
> +kcov_remote_start_context()/kcov_remote_stop_context() that accept a
> +context mask. This mask describes the contexts in which these annotations
> +should be applied. E.g. specifying KCOV_CONTEXT_SOFTIRQ will result in the
> +corresponding annotations being ignored in any context other than softirq.
> +
> Internally kcov stores handles as u64 integers. The top byte of a handle
> is used to denote the id of a subsystem that this handle belongs to, and
> the lower 4 bytes are used to denote the id of a thread instance within
> diff --git a/drivers/usb/core/hcd.c b/drivers/usb/core/hcd.c
> index a33b849e8beb..ea93d9ebcb2e 100644
> --- a/drivers/usb/core/hcd.c
> +++ b/drivers/usb/core/hcd.c
> @@ -1646,9 +1646,9 @@ static void __usb_hcd_giveback_urb(struct urb *urb)
>
> /* pass ownership to the completion handler */
> urb->status = status;
> - kcov_remote_start_usb((u64)urb->dev->bus->busnum);
> + kcov_remote_start_usb_softirq((u64)urb->dev->bus->busnum);
> urb->complete(urb);
> - kcov_remote_stop();
> + kcov_remote_stop_softirq();
>
> usb_anchor_resume_wakeups(anchor);
> atomic_dec(&urb->use_count);
> diff --git a/include/linux/kcov.h b/include/linux/kcov.h
> index a10e84707d82..9e31b71ee3f9 100644
> --- a/include/linux/kcov.h
> +++ b/include/linux/kcov.h
> @@ -22,6 +22,10 @@ enum kcov_mode {
> KCOV_MODE_TRACE_CMP = 3,
> };
>
> +#define KCOV_CONTEXT_TASK (1u << 0)
> +#define KCOV_CONTEXT_SOFTIRQ (1u << 1)
> +#define KCOV_CONTEXT_MASK (KCOV_CONTEXT_TASK | KCOV_CONTEXT_SOFTIRQ)
> +
> #define KCOV_IN_CTXSW (1 << 30)
>
> void kcov_task_init(struct task_struct *t);
> @@ -38,10 +42,21 @@ do { \
> } while (0)
>
> /* See Documentation/dev-tools/kcov.rst for usage details. */
> -void kcov_remote_start(u64 handle);
> -void kcov_remote_stop(void);
> +
> +void kcov_remote_start_context(u64 handle, unsigned int context);
> +void kcov_remote_stop_context(unsigned int context);
> u64 kcov_common_handle(void);
>
> +static inline void kcov_remote_start(u64 handle)
> +{
> + return kcov_remote_start_context(handle, KCOV_CONTEXT_MASK);
> +}
> +
> +static inline void kcov_remote_stop(void)
> +{
> + return kcov_remote_stop_context(KCOV_CONTEXT_MASK);
> +}
These are returning void whereas kcov_remote_start_usb_softirq isn't.
Was it intentional?
Other than that,
Acked-by: Marco Elver <elver@xxxxxxxxxx>
Thank you!
> static inline void kcov_remote_start_common(u64 id)
> {
> kcov_remote_start(kcov_remote_handle(KCOV_SUBSYSTEM_COMMON, id));
> @@ -52,6 +67,16 @@ static inline void kcov_remote_start_usb(u64 id)
> kcov_remote_start(kcov_remote_handle(KCOV_SUBSYSTEM_USB, id));
> }
>
> +static inline void kcov_remote_start_usb_softirq(u64 id)
> +{
> + kcov_remote_start_context(kcov_remote_handle(KCOV_SUBSYSTEM_USB, id), KCOV_CONTEXT_SOFTIRQ);
> +}
> +
> +static inline void kcov_remote_stop_softirq(void)
> +{
> + return kcov_remote_stop_context(KCOV_CONTEXT_SOFTIRQ);
> +}
> +
> #else
>
> static inline void kcov_task_init(struct task_struct *t) {}
> @@ -66,6 +91,8 @@ static inline u64 kcov_common_handle(void)
> }
> static inline void kcov_remote_start_common(u64 id) {}
> static inline void kcov_remote_start_usb(u64 id) {}
> +static inline void kcov_remote_start_usb_softirq(u64 id) {}
> +static inline void kcov_remote_stop_softirq(void) {}
>
> #endif /* CONFIG_KCOV */
> #endif /* _LINUX_KCOV_H */
> diff --git a/kernel/kcov.c b/kernel/kcov.c
> index 6b8368be89c8..3ccdbe060f47 100644
> --- a/kernel/kcov.c
> +++ b/kernel/kcov.c
> @@ -808,7 +808,8 @@ static void kcov_remote_softirq_stop(struct task_struct *t)
> }
> }
>
> -void kcov_remote_start(u64 handle)
> +/* Also see kcov_remote_start() defined in include/linux/kcov.h. */
> +void kcov_remote_start_context(u64 handle, unsigned int context)
> {
> struct task_struct *t = current;
> struct kcov_remote *remote;
> @@ -821,7 +822,11 @@ void kcov_remote_start(u64 handle)
>
> if (WARN_ON(!kcov_check_handle(handle, true, true, true)))
> return;
> - if (!in_task() && !in_serving_softirq())
> + if (WARN_ON((context & ~KCOV_CONTEXT_MASK) || !context))
> + return;
> + if (in_task() && !(context & KCOV_CONTEXT_TASK))
> + return;
> + if (in_serving_softirq() && !(context & KCOV_CONTEXT_SOFTIRQ))
> return;
>
> local_irq_save(flags);
> @@ -894,7 +899,7 @@ void kcov_remote_start(u64 handle)
> local_irq_restore(flags);
>
> }
> -EXPORT_SYMBOL(kcov_remote_start);
> +EXPORT_SYMBOL(kcov_remote_start_context);
>
> static void kcov_move_area(enum kcov_mode mode, void *dst_area,
> unsigned int dst_area_size, void *src_area)
> @@ -951,8 +956,11 @@ static void kcov_move_area(enum kcov_mode mode, void *dst_area,
> }
> }
>
> -/* See the comment before kcov_remote_start() for usage details. */
> -void kcov_remote_stop(void)
> +/*
> + * Also see kcov_remote_stop() defined in include/linux/kcov.h.
> + * See the comment before kcov_remote_start_context() for usage details.
> + */
> +void kcov_remote_stop_context(unsigned int context)
> {
> struct task_struct *t = current;
> struct kcov *kcov;
> @@ -962,7 +970,11 @@ void kcov_remote_stop(void)
> int sequence;
> unsigned long flags;
>
> - if (!in_task() && !in_serving_softirq())
> + if (WARN_ON((context & ~KCOV_CONTEXT_MASK) || !context))
> + return;
> + if (in_task() && !(context & KCOV_CONTEXT_TASK))
> + return;
> + if (in_serving_softirq() && !(context & KCOV_CONTEXT_SOFTIRQ))
> return;
>
> local_irq_save(flags);
> @@ -1018,7 +1030,7 @@ void kcov_remote_stop(void)
> /* Get in kcov_remote_start(). */
> kcov_put(kcov);
> }
> -EXPORT_SYMBOL(kcov_remote_stop);
> +EXPORT_SYMBOL(kcov_remote_stop_context);
>
> /* See the comment before kcov_remote_start() for usage details. */
> u64 kcov_common_handle(void)
> --
> 2.28.0.1011.ga647a8990f-goog
>
