* [PATCH v3 2/2] perf: Refactor permissions check into perf_check_permission()
2021-07-05 8:44 [PATCH v3 1/2] perf: Fix required permissions if sigtrap is requested Marco Elver
@ 2021-07-05 8:44 ` Marco Elver
2021-07-06 6:16 ` Dmitry Vyukov
2021-07-06 6:16 ` [PATCH v3 1/2] perf: Fix required permissions if sigtrap is requested Dmitry Vyukov
2021-07-12 10:32 ` Marco Elver
2 siblings, 1 reply; 6+ messages in thread
From: Marco Elver @ 2021-07-05 8:44 UTC (permalink / raw)
To: elver, peterz
Cc: tglx, mingo, dvyukov, glider, kasan-dev, linux-kernel, mingo,
acme, mark.rutland, alexander.shishkin, jolsa, namhyung,
linux-perf-users, ebiederm, omosnace, serge,
linux-security-module
Refactor the permission check in perf_event_open() into a helper
perf_check_permission(). This makes the permission check logic more
readable (because we no longer have a negated disjunction). Add a
comment mentioning the ptrace check also checks the uid.
No functional change intended.
Signed-off-by: Marco Elver <elver@google.com>
---
v3:
* Introduce this patch to refactor the permissions checking logic to
make it more readable (reported by Eric W. Biederman).
---
kernel/events/core.c | 58 ++++++++++++++++++++++++--------------------
1 file changed, 32 insertions(+), 26 deletions(-)
diff --git a/kernel/events/core.c b/kernel/events/core.c
index f79ee82e644a..3008b986994b 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -11912,6 +11912,37 @@ __perf_event_ctx_lock_double(struct perf_event *group_leader,
return gctx;
}
+static bool
+perf_check_permission(struct perf_event_attr *attr, struct task_struct *task)
+{
+ unsigned int ptrace_mode = PTRACE_MODE_READ_REALCREDS;
+ bool is_capable = perfmon_capable();
+
+ if (attr->sigtrap) {
+ /*
+ * perf_event_attr::sigtrap sends signals to the other task.
+ * Require the current task to also have CAP_KILL.
+ */
+ rcu_read_lock();
+ is_capable &= ns_capable(__task_cred(task)->user_ns, CAP_KILL);
+ rcu_read_unlock();
+
+ /*
+ * If the required capabilities aren't available, checks for
+ * ptrace permissions: upgrade to ATTACH, since sending signals
+ * can effectively change the target task.
+ */
+ ptrace_mode = PTRACE_MODE_ATTACH_REALCREDS;
+ }
+
+ /*
+ * Preserve ptrace permission check for backwards compatibility. The
+ * ptrace check also includes checks that the current task and other
+ * task have matching uids, and is therefore not done here explicitly.
+ */
+ return is_capable || ptrace_may_access(task, ptrace_mode);
+}
+
/**
* sys_perf_event_open - open a performance event, associate it to a task/cpu
*
@@ -12152,43 +12183,18 @@ SYSCALL_DEFINE5(perf_event_open,
}
if (task) {
- unsigned int ptrace_mode = PTRACE_MODE_READ_REALCREDS;
- bool is_capable;
-
err = down_read_interruptible(&task->signal->exec_update_lock);
if (err)
goto err_file;
- is_capable = perfmon_capable();
- if (attr.sigtrap) {
- /*
- * perf_event_attr::sigtrap sends signals to the other
- * task. Require the current task to also have
- * CAP_KILL.
- */
- rcu_read_lock();
- is_capable &= ns_capable(__task_cred(task)->user_ns, CAP_KILL);
- rcu_read_unlock();
-
- /*
- * If the required capabilities aren't available, checks
- * for ptrace permissions: upgrade to ATTACH, since
- * sending signals can effectively change the target
- * task.
- */
- ptrace_mode = PTRACE_MODE_ATTACH_REALCREDS;
- }
-
/*
- * Preserve ptrace permission check for backwards compatibility.
- *
* We must hold exec_update_lock across this and any potential
* perf_install_in_context() call for this new event to
* serialize against exec() altering our credentials (and the
* perf_event_exit_task() that could imply).
*/
err = -EACCES;
- if (!is_capable && !ptrace_may_access(task, ptrace_mode))
+ if (!perf_check_permission(&attr, task))
goto err_cred;
}
--
2.32.0.93.g670b81a890-goog
^ permalink raw reply related [flat|nested] 6+ messages in thread* Re: [PATCH v3 2/2] perf: Refactor permissions check into perf_check_permission()
2021-07-05 8:44 ` [PATCH v3 2/2] perf: Refactor permissions check into perf_check_permission() Marco Elver
@ 2021-07-06 6:16 ` Dmitry Vyukov
0 siblings, 0 replies; 6+ messages in thread
From: Dmitry Vyukov @ 2021-07-06 6:16 UTC (permalink / raw)
To: Marco Elver
Cc: peterz, tglx, mingo, glider, kasan-dev, linux-kernel, mingo,
acme, mark.rutland, alexander.shishkin, jolsa, namhyung,
linux-perf-users, ebiederm, omosnace, serge,
linux-security-module
On Mon, Jul 5, 2021 at 10:45 AM Marco Elver <elver@google.com> wrote:
>
> Refactor the permission check in perf_event_open() into a helper
> perf_check_permission(). This makes the permission check logic more
> readable (because we no longer have a negated disjunction). Add a
> comment mentioning the ptrace check also checks the uid.
>
> No functional change intended.
>
> Signed-off-by: Marco Elver <elver@google.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
> ---
> v3:
> * Introduce this patch to refactor the permissions checking logic to
> make it more readable (reported by Eric W. Biederman).
> ---
> kernel/events/core.c | 58 ++++++++++++++++++++++++--------------------
> 1 file changed, 32 insertions(+), 26 deletions(-)
>
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index f79ee82e644a..3008b986994b 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -11912,6 +11912,37 @@ __perf_event_ctx_lock_double(struct perf_event *group_leader,
> return gctx;
> }
>
> +static bool
> +perf_check_permission(struct perf_event_attr *attr, struct task_struct *task)
> +{
> + unsigned int ptrace_mode = PTRACE_MODE_READ_REALCREDS;
> + bool is_capable = perfmon_capable();
> +
> + if (attr->sigtrap) {
> + /*
> + * perf_event_attr::sigtrap sends signals to the other task.
> + * Require the current task to also have CAP_KILL.
> + */
> + rcu_read_lock();
> + is_capable &= ns_capable(__task_cred(task)->user_ns, CAP_KILL);
> + rcu_read_unlock();
> +
> + /*
> + * If the required capabilities aren't available, checks for
> + * ptrace permissions: upgrade to ATTACH, since sending signals
> + * can effectively change the target task.
> + */
> + ptrace_mode = PTRACE_MODE_ATTACH_REALCREDS;
> + }
> +
> + /*
> + * Preserve ptrace permission check for backwards compatibility. The
> + * ptrace check also includes checks that the current task and other
> + * task have matching uids, and is therefore not done here explicitly.
> + */
> + return is_capable || ptrace_may_access(task, ptrace_mode);
> +}
> +
> /**
> * sys_perf_event_open - open a performance event, associate it to a task/cpu
> *
> @@ -12152,43 +12183,18 @@ SYSCALL_DEFINE5(perf_event_open,
> }
>
> if (task) {
> - unsigned int ptrace_mode = PTRACE_MODE_READ_REALCREDS;
> - bool is_capable;
> -
> err = down_read_interruptible(&task->signal->exec_update_lock);
> if (err)
> goto err_file;
>
> - is_capable = perfmon_capable();
> - if (attr.sigtrap) {
> - /*
> - * perf_event_attr::sigtrap sends signals to the other
> - * task. Require the current task to also have
> - * CAP_KILL.
> - */
> - rcu_read_lock();
> - is_capable &= ns_capable(__task_cred(task)->user_ns, CAP_KILL);
> - rcu_read_unlock();
> -
> - /*
> - * If the required capabilities aren't available, checks
> - * for ptrace permissions: upgrade to ATTACH, since
> - * sending signals can effectively change the target
> - * task.
> - */
> - ptrace_mode = PTRACE_MODE_ATTACH_REALCREDS;
> - }
> -
> /*
> - * Preserve ptrace permission check for backwards compatibility.
> - *
> * We must hold exec_update_lock across this and any potential
> * perf_install_in_context() call for this new event to
> * serialize against exec() altering our credentials (and the
> * perf_event_exit_task() that could imply).
> */
> err = -EACCES;
> - if (!is_capable && !ptrace_may_access(task, ptrace_mode))
> + if (!perf_check_permission(&attr, task))
> goto err_cred;
> }
>
> --
> 2.32.0.93.g670b81a890-goog
>
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH v3 1/2] perf: Fix required permissions if sigtrap is requested
2021-07-05 8:44 [PATCH v3 1/2] perf: Fix required permissions if sigtrap is requested Marco Elver
2021-07-05 8:44 ` [PATCH v3 2/2] perf: Refactor permissions check into perf_check_permission() Marco Elver
@ 2021-07-06 6:16 ` Dmitry Vyukov
2021-07-12 10:32 ` Marco Elver
2 siblings, 0 replies; 6+ messages in thread
From: Dmitry Vyukov @ 2021-07-06 6:16 UTC (permalink / raw)
To: Marco Elver
Cc: peterz, tglx, mingo, glider, kasan-dev, linux-kernel, mingo,
acme, mark.rutland, alexander.shishkin, jolsa, namhyung,
linux-perf-users, ebiederm, omosnace, serge,
linux-security-module, stable
On Mon, Jul 5, 2021 at 10:45 AM Marco Elver <elver@google.com> wrote:
>
> If perf_event_open() is called with another task as target and
> perf_event_attr::sigtrap is set, and the target task's user does not
> match the calling user, also require the CAP_KILL capability or
> PTRACE_MODE_ATTACH permissions.
>
> Otherwise, with the CAP_PERFMON capability alone it would be possible
> for a user to send SIGTRAP signals via perf events to another user's
> tasks. This could potentially result in those tasks being terminated if
> they cannot handle SIGTRAP signals.
>
> Note: The check complements the existing capability check, but is not
> supposed to supersede the ptrace_may_access() check. At a high level we
> now have:
>
> capable of CAP_PERFMON and (CAP_KILL if sigtrap)
> OR
> ptrace_may_access(...) // also checks for same thread-group and uid
>
> Fixes: 97ba62b27867 ("perf: Add support for SIGTRAP on perf events")
> Cc: <stable@vger.kernel.org> # 5.13+
> Reported-by: Dmitry Vyukov <dvyukov@google.com>
> Signed-off-by: Marco Elver <elver@google.com>
Acked-by: Dmitry Vyukov <dvyukov@google.com>
> ---
> v3:
> * Upgrade ptrace mode check to ATTACH if attr.sigtrap, otherwise it's
> possible to change the target task (send signal) even if only read
> ptrace permissions were granted (reported by Eric W. Biederman).
>
> v2: https://lkml.kernel.org/r/20210701083842.580466-1-elver@google.com
> * Drop kill_capable() and just check CAP_KILL (reported by Ondrej Mosnacek).
> * Use ns_capable(__task_cred(task)->user_ns, CAP_KILL) to check for
> capability in target task's ns (reported by Ondrej Mosnacek).
>
> v1: https://lkml.kernel.org/r/20210630093709.3612997-1-elver@google.com
> ---
> kernel/events/core.c | 25 ++++++++++++++++++++++++-
> 1 file changed, 24 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index fe88d6eea3c2..f79ee82e644a 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -12152,10 +12152,33 @@ SYSCALL_DEFINE5(perf_event_open,
> }
>
> if (task) {
> + unsigned int ptrace_mode = PTRACE_MODE_READ_REALCREDS;
> + bool is_capable;
> +
> err = down_read_interruptible(&task->signal->exec_update_lock);
> if (err)
> goto err_file;
>
> + is_capable = perfmon_capable();
> + if (attr.sigtrap) {
> + /*
> + * perf_event_attr::sigtrap sends signals to the other
> + * task. Require the current task to also have
> + * CAP_KILL.
> + */
> + rcu_read_lock();
> + is_capable &= ns_capable(__task_cred(task)->user_ns, CAP_KILL);
> + rcu_read_unlock();
> +
> + /*
> + * If the required capabilities aren't available, checks
> + * for ptrace permissions: upgrade to ATTACH, since
> + * sending signals can effectively change the target
> + * task.
> + */
> + ptrace_mode = PTRACE_MODE_ATTACH_REALCREDS;
> + }
> +
> /*
> * Preserve ptrace permission check for backwards compatibility.
> *
> @@ -12165,7 +12188,7 @@ SYSCALL_DEFINE5(perf_event_open,
> * perf_event_exit_task() that could imply).
> */
> err = -EACCES;
> - if (!perfmon_capable() && !ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS))
> + if (!is_capable && !ptrace_may_access(task, ptrace_mode))
> goto err_cred;
> }
>
> --
> 2.32.0.93.g670b81a890-goog
>
^ permalink raw reply [flat|nested] 6+ messages in thread* Re: [PATCH v3 1/2] perf: Fix required permissions if sigtrap is requested
2021-07-05 8:44 [PATCH v3 1/2] perf: Fix required permissions if sigtrap is requested Marco Elver
2021-07-05 8:44 ` [PATCH v3 2/2] perf: Refactor permissions check into perf_check_permission() Marco Elver
2021-07-06 6:16 ` [PATCH v3 1/2] perf: Fix required permissions if sigtrap is requested Dmitry Vyukov
@ 2021-07-12 10:32 ` Marco Elver
2021-07-13 9:48 ` Peter Zijlstra
2 siblings, 1 reply; 6+ messages in thread
From: Marco Elver @ 2021-07-12 10:32 UTC (permalink / raw)
To: elver, peterz
Cc: tglx, mingo, dvyukov, glider, kasan-dev, linux-kernel, mingo,
acme, mark.rutland, alexander.shishkin, jolsa, namhyung,
linux-perf-users, ebiederm, omosnace, serge,
linux-security-module, stable
It'd be good to get this sorted -- please take another look.
Many thanks,
-- Marco
On Mon, 5 Jul 2021 at 10:45, Marco Elver <elver@google.com> wrote:
> If perf_event_open() is called with another task as target and
> perf_event_attr::sigtrap is set, and the target task's user does not
> match the calling user, also require the CAP_KILL capability or
> PTRACE_MODE_ATTACH permissions.
>
> Otherwise, with the CAP_PERFMON capability alone it would be possible
> for a user to send SIGTRAP signals via perf events to another user's
> tasks. This could potentially result in those tasks being terminated if
> they cannot handle SIGTRAP signals.
>
> Note: The check complements the existing capability check, but is not
> supposed to supersede the ptrace_may_access() check. At a high level we
> now have:
>
> capable of CAP_PERFMON and (CAP_KILL if sigtrap)
> OR
> ptrace_may_access(...) // also checks for same thread-group and uid
>
> Fixes: 97ba62b27867 ("perf: Add support for SIGTRAP on perf events")
> Cc: <stable@vger.kernel.org> # 5.13+
> Reported-by: Dmitry Vyukov <dvyukov@google.com>
> Signed-off-by: Marco Elver <elver@google.com>
> ---
> v3:
> * Upgrade ptrace mode check to ATTACH if attr.sigtrap, otherwise it's
> possible to change the target task (send signal) even if only read
> ptrace permissions were granted (reported by Eric W. Biederman).
>
> v2: https://lkml.kernel.org/r/20210701083842.580466-1-elver@google.com
> * Drop kill_capable() and just check CAP_KILL (reported by Ondrej Mosnacek).
> * Use ns_capable(__task_cred(task)->user_ns, CAP_KILL) to check for
> capability in target task's ns (reported by Ondrej Mosnacek).
>
> v1: https://lkml.kernel.org/r/20210630093709.3612997-1-elver@google.com
> ---
> kernel/events/core.c | 25 ++++++++++++++++++++++++-
> 1 file changed, 24 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index fe88d6eea3c2..f79ee82e644a 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -12152,10 +12152,33 @@ SYSCALL_DEFINE5(perf_event_open,
> }
>
> if (task) {
> + unsigned int ptrace_mode = PTRACE_MODE_READ_REALCREDS;
> + bool is_capable;
> +
> err = down_read_interruptible(&task->signal->exec_update_lock);
> if (err)
> goto err_file;
>
> + is_capable = perfmon_capable();
> + if (attr.sigtrap) {
> + /*
> + * perf_event_attr::sigtrap sends signals to the other
> + * task. Require the current task to also have
> + * CAP_KILL.
> + */
> + rcu_read_lock();
> + is_capable &= ns_capable(__task_cred(task)->user_ns, CAP_KILL);
> + rcu_read_unlock();
> +
> + /*
> + * If the required capabilities aren't available, checks
> + * for ptrace permissions: upgrade to ATTACH, since
> + * sending signals can effectively change the target
> + * task.
> + */
> + ptrace_mode = PTRACE_MODE_ATTACH_REALCREDS;
> + }
> +
> /*
> * Preserve ptrace permission check for backwards compatibility.
> *
> @@ -12165,7 +12188,7 @@ SYSCALL_DEFINE5(perf_event_open,
> * perf_event_exit_task() that could imply).
> */
> err = -EACCES;
> - if (!perfmon_capable() && !ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS))
> + if (!is_capable && !ptrace_may_access(task, ptrace_mode))
> goto err_cred;
> }
>
> --
> 2.32.0.93.g670b81a890-goog
>
^ permalink raw reply [flat|nested] 6+ messages in thread* Re: [PATCH v3 1/2] perf: Fix required permissions if sigtrap is requested
2021-07-12 10:32 ` Marco Elver
@ 2021-07-13 9:48 ` Peter Zijlstra
0 siblings, 0 replies; 6+ messages in thread
From: Peter Zijlstra @ 2021-07-13 9:48 UTC (permalink / raw)
To: Marco Elver
Cc: tglx, mingo, dvyukov, glider, kasan-dev, linux-kernel, mingo,
acme, mark.rutland, alexander.shishkin, jolsa, namhyung,
linux-perf-users, ebiederm, omosnace, serge,
linux-security-module, stable
On Mon, Jul 12, 2021 at 12:32:33PM +0200, Marco Elver wrote:
> It'd be good to get this sorted -- please take another look.
Thanks!
I'll queue them into perf/urgent.
^ permalink raw reply [flat|nested] 6+ messages in thread