[PATCH v4 30/39] unwind_user/deferred: Make unwind deferral requests NMI-safe

Josh Poimboeuf
2025-01-22T02:32:23Z

Make unwind_deferred_request() NMI-safe so tracers in NMI context can
call it to get the cookie immediately rather than have to do the fragile
"schedule irq work and then call unwind_deferred_request()" dance.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
---
include/linux/entry-common.h | 1 +
include/linux/unwind_deferred.h | 6 ++
include/linux/unwind_deferred_types.h | 1 +
kernel/unwind/deferred.c | 106 ++++++++++++++++++++++----
4 files changed, 98 insertions(+), 16 deletions(-)

diff --git a/include/linux/entry-common.h b/include/linux/entry-common.h
index fb2b27154fee..e9b8c145f480 100644
--- a/include/linux/entry-common.h
+++ b/include/linux/entry-common.h
@@ -363,6 +363,7 @@ static __always_inline void exit_to_user_mode(void)
lockdep_hardirqs_on_prepare();
instrumentation_end();

+ unwind_exit_to_user_mode();
user_enter_irqoff();
arch_exit_to_user_mode();
lockdep_hardirqs_on(CALLER_ADDR0);
diff --git a/include/linux/unwind_deferred.h b/include/linux/unwind_deferred.h
index 741f409f0d1f..22269f4d2392 100644
--- a/include/linux/unwind_deferred.h
+++ b/include/linux/unwind_deferred.h
@@ -30,6 +30,11 @@ static __always_inline void unwind_enter_from_user_mode(void)
current->unwind_info.cookie = 0;
}

+static __always_inline void unwind_exit_to_user_mode(void)
+{
+ current->unwind_info.cookie = 0;
+}
+
#else /* !CONFIG_UNWIND_USER */

static inline void unwind_task_init(struct task_struct *task) {}
@@ -40,6 +45,7 @@ static inline int unwind_deferred_request(struct task_struct *task, struct unwin
static inline bool unwind_deferred_cancel(struct task_struct *task, struct unwind_work *work) { return false; }

static inline void unwind_enter_from_user_mode(void) {}
+static inline void unwind_exit_to_user_mode(void) {}

#endif /* !CONFIG_UNWIND_USER */

diff --git a/include/linux/unwind_deferred_types.h b/include/linux/unwind_deferred_types.h
index 6f71a06329fb..c535cca6534b 100644
--- a/include/linux/unwind_deferred_types.h
+++ b/include/linux/unwind_deferred_types.h
@@ -11,6 +11,7 @@ struct unwind_cache {
struct unwind_task_info {
struct unwind_cache cache;
u64 cookie;
+ u64 nmi_cookie;
};

#endif /* _LINUX_UNWIND_USER_DEFERRED_TYPES_H */
diff --git a/kernel/unwind/deferred.c b/kernel/unwind/deferred.c
index 2f38055cce48..939c94abaa50 100644
--- a/kernel/unwind/deferred.c
+++ b/kernel/unwind/deferred.c
@@ -29,27 +29,49 @@ static u64 ctx_to_cookie(u64 cpu, u64 ctx)

/*
* Read the task context cookie, first initializing it if this is the first
- * call to get_cookie() since the most recent entry from user.
+ * call to get_cookie() since the most recent entry from user. This has to be
+ * done carefully to coordinate with unwind_deferred_request_nmi().
*/
static u64 get_cookie(struct unwind_task_info *info)
{
u64 ctx_ctr;
u64 cookie;
- u64 cpu;

guard(irqsave)();

- cookie = info->cookie;
+ cookie = READ_ONCE(info->cookie);
if (cookie)
return cookie;

+ ctx_ctr = __this_cpu_read(unwind_ctx_ctr);

- cpu = raw_smp_processor_id();
- ctx_ctr = __this_cpu_inc_return(unwind_ctx_ctr);
- info->cookie = ctx_to_cookie(cpu, ctx_ctr);
+ /* Read ctx_ctr before info->nmi_cookie */
+ barrier();
+
+ cookie = READ_ONCE(info->nmi_cookie);
+ if (cookie) {
+ /*
+ * This is the first call to get_cookie() since an NMI handler
+ * first wrote it to info->nmi_cookie. Sync it.
+ */
+ WRITE_ONCE(info->cookie, cookie);
+ WRITE_ONCE(info->nmi_cookie, 0);
+ return cookie;
+ }
+
+ /*
+ * Write info->cookie. It's ok to race with an NMI here. The value of
+ * the cookie is based on ctx_ctr from before the NMI could have
+ * incremented it. The result will be the same even if cookie or
+ * ctx_ctr end up getting written twice.
+ */
+ cookie = ctx_to_cookie(raw_smp_processor_id(), ctx_ctr + 1);
+ WRITE_ONCE(info->cookie, cookie);
+ WRITE_ONCE(info->nmi_cookie, 0);
+ barrier();
+ __this_cpu_write(unwind_ctx_ctr, ctx_ctr + 1);

return cookie;
-
}

static void unwind_deferred_task_work(struct callback_head *head)
@@ -100,7 +122,52 @@ static void unwind_deferred_task_work(struct callback_head *head)

do_callback:
work->func(work, &trace, cookie);
- work->pending = 0;
+ WRITE_ONCE(work->pending, 0);
+}
+
+static int unwind_deferred_request_nmi(struct unwind_work *work, u64 *cookie)
+{
+ struct unwind_task_info *info = &current->unwind_info;
+ bool inited_cookie = false;
+ int ret;
+
+ *cookie = info->cookie;
+ if (!*cookie) {
+ /*
+ * This is the first unwind request since the most recent entry
+ * from user. Initialize the task cookie.
+ *
+ * Don't write to info->cookie directly, otherwise it may get
+ * cleared if the NMI occurred in the kernel during early entry
+ * or late exit before the task work gets to run. Instead, use
+ * info->nmi_cookie which gets synced later by get_cookie().
+ */
+ if (!info->nmi_cookie) {
+ u64 cpu = raw_smp_processor_id();
+ u64 ctx_ctr;
+
+ ctx_ctr = __this_cpu_inc_return(unwind_ctx_ctr);
+ info->nmi_cookie = ctx_to_cookie(cpu, ctx_ctr);
+
+ inited_cookie = true;
+ }
+
+ *cookie = info->nmi_cookie;
+
+ } else if (work->pending) {
+ return -EEXIST;
+ }
+
+ ret = task_work_add(current, &work->work, TWA_NMI_CURRENT);
+ if (ret) {
+ if (inited_cookie)
+ info->nmi_cookie = 0;
+ return ret;
+ }
+
+ work->pending = 1;
+
+ return 0;
}

/*
@@ -131,29 +198,36 @@ static void unwind_deferred_task_work(struct callback_head *head)
int unwind_deferred_request(struct unwind_work *work, u64 *cookie)
{
struct unwind_task_info *info = &current->unwind_info;
+ int pending;
int ret;

*cookie = 0;

- if (WARN_ON_ONCE(in_nmi()))
- return -EINVAL;
-
if (!current->mm || !user_mode(task_pt_regs(current)))
return -EINVAL;

+ if (in_nmi())
+ return unwind_deferred_request_nmi(work, cookie);
+
guard(irqsave)();

*cookie = get_cookie(info);

/* callback already pending? */
- if (work->pending)
+ pending = READ_ONCE(work->pending);
+ if (pending)
return -EEXIST;

- ret = task_work_add(current, &work->work, TWA_RESUME);
- if (WARN_ON_ONCE(ret))
- return ret;
+ /* Claim the work unless an NMI just now swooped in to do so. */
+ if (!try_cmpxchg(&work->pending, &pending, 1))
+ return -EEXIST;

- work->pending = 1;
+ /* The work has been claimed, now schedule it. */
+ ret = task_work_add(current, &work->work, TWA_RESUME);
+ if (WARN_ON_ONCE(ret)) {
+ WRITE_ONCE(work->pending, 0);
+ return ret;
+ }

return 0;
}
--
2.48.1