* [PATCH] yama: don't abuse rcu_read_lock/get_task_struct in yama_task_prctl()
@ 2025-02-19 16:14 Oleg Nesterov
2025-02-19 19:33 ` Kees Cook
2025-03-04 23:53 ` Kees Cook
0 siblings, 2 replies; 6+ messages in thread
From: Oleg Nesterov @ 2025-02-19 16:14 UTC (permalink / raw)
To: Kees Cook, Paul Moore, James Morris, Serge E. Hallyn
Cc: linux-security-module, linux-kernel
current->group_leader is stable, no need to take rcu_read_lock() and do
get/put_task_struct().
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
security/yama/yama_lsm.c | 9 ++-------
1 file changed, 2 insertions(+), 7 deletions(-)
diff --git a/security/yama/yama_lsm.c b/security/yama/yama_lsm.c
index 1971710620c1..3d064dd4e03f 100644
--- a/security/yama/yama_lsm.c
+++ b/security/yama/yama_lsm.c
@@ -222,7 +222,7 @@ static int yama_task_prctl(int option, unsigned long arg2, unsigned long arg3,
unsigned long arg4, unsigned long arg5)
{
int rc = -ENOSYS;
- struct task_struct *myself = current;
+ struct task_struct *myself;
switch (option) {
case PR_SET_PTRACER:
@@ -232,11 +232,7 @@ static int yama_task_prctl(int option, unsigned long arg2, unsigned long arg3,
* leader checking is handled later when walking the ancestry
* at the time of PTRACE_ATTACH check.
*/
- rcu_read_lock();
- if (!thread_group_leader(myself))
- myself = rcu_dereference(myself->group_leader);
- get_task_struct(myself);
- rcu_read_unlock();
+ myself = current->group_leader;
if (arg2 == 0) {
yama_ptracer_del(NULL, myself);
@@ -255,7 +251,6 @@ static int yama_task_prctl(int option, unsigned long arg2, unsigned long arg3,
}
}
- put_task_struct(myself);
break;
}
--
2.25.1.362.g51ebf55
^ permalink raw reply related [flat|nested] 6+ messages in thread
* Re: [PATCH] yama: don't abuse rcu_read_lock/get_task_struct in yama_task_prctl()
2025-02-19 16:14 [PATCH] yama: don't abuse rcu_read_lock/get_task_struct in yama_task_prctl() Oleg Nesterov
@ 2025-02-19 19:33 ` Kees Cook
2025-02-19 20:17 ` Oleg Nesterov
2025-03-04 23:53 ` Kees Cook
1 sibling, 1 reply; 6+ messages in thread
From: Kees Cook @ 2025-02-19 19:33 UTC (permalink / raw)
To: Oleg Nesterov
Cc: Paul Moore, James Morris, Serge E. Hallyn, linux-security-module,
linux-kernel
On Wed, Feb 19, 2025 at 05:14:17PM +0100, Oleg Nesterov wrote:
> current->group_leader is stable, no need to take rcu_read_lock() and do
> get/put_task_struct().
Can you explain why this is true? In trying to figure this out again,
it seems that the only way current->group_leader can vanish is if
the entire process vanishes (fork or thread exec), in which case the
"current" in this prctl can't be happening; this appears to be locked
behind tsk->sighand->siglock ?
-Kees
>
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>
> ---
> security/yama/yama_lsm.c | 9 ++-------
> 1 file changed, 2 insertions(+), 7 deletions(-)
>
> diff --git a/security/yama/yama_lsm.c b/security/yama/yama_lsm.c
> index 1971710620c1..3d064dd4e03f 100644
> --- a/security/yama/yama_lsm.c
> +++ b/security/yama/yama_lsm.c
> @@ -222,7 +222,7 @@ static int yama_task_prctl(int option, unsigned long arg2, unsigned long arg3,
> unsigned long arg4, unsigned long arg5)
> {
> int rc = -ENOSYS;
> - struct task_struct *myself = current;
> + struct task_struct *myself;
>
> switch (option) {
> case PR_SET_PTRACER:
> @@ -232,11 +232,7 @@ static int yama_task_prctl(int option, unsigned long arg2, unsigned long arg3,
> * leader checking is handled later when walking the ancestry
> * at the time of PTRACE_ATTACH check.
> */
> - rcu_read_lock();
> - if (!thread_group_leader(myself))
> - myself = rcu_dereference(myself->group_leader);
> - get_task_struct(myself);
> - rcu_read_unlock();
> + myself = current->group_leader;
>
> if (arg2 == 0) {
> yama_ptracer_del(NULL, myself);
> @@ -255,7 +251,6 @@ static int yama_task_prctl(int option, unsigned long arg2, unsigned long arg3,
> }
> }
>
> - put_task_struct(myself);
> break;
> }
>
> --
> 2.25.1.362.g51ebf55
>
>
--
Kees Cook
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH] yama: don't abuse rcu_read_lock/get_task_struct in yama_task_prctl()
2025-02-19 19:33 ` Kees Cook
@ 2025-02-19 20:17 ` Oleg Nesterov
2025-02-19 21:00 ` Oleg Nesterov
0 siblings, 1 reply; 6+ messages in thread
From: Oleg Nesterov @ 2025-02-19 20:17 UTC (permalink / raw)
To: Kees Cook
Cc: Paul Moore, James Morris, Serge E. Hallyn, linux-security-module,
linux-kernel
On 02/19, Kees Cook wrote:
>
> On Wed, Feb 19, 2025 at 05:14:17PM +0100, Oleg Nesterov wrote:
> > current->group_leader is stable, no need to take rcu_read_lock() and do
> > get/put_task_struct().
>
> Can you explain why this is true? In trying to figure this out again,
> it seems that the only way current->group_leader can vanish is if
> the entire process vanishes (fork or thread exec), in which case the
> "current" in this prctl can't be happening; this appears to be locked
> behind tsk->sighand->siglock ?
Well, almost, but this has nothing to do with tsk->sighand->siglock...
task->group_leader can only be changed by thread exec, when a non leader
thread does exec, see de_thread(). But de_thread() can't succeed and change
->group_leader until all other threads exit, see the "Kill all other threads
in the thread group" code in de_thread(). The "current" task can't exit, so
current->group_leader is stable.
Note also that we already have a lot of current->group_leader users which
don't use rcu/get_task_struct.
That said, we have a lot of buggy users of tsk->group_leader when
same_thread_group(tsk, current) != true ;) For example, sys_prlimit64().
And note that rcu_read_lock/get_task_struct can't help in this case.
I am going to send some fixes.
Oleg.
>
> -Kees
>
> >
> > Signed-off-by: Oleg Nesterov <oleg@redhat.com>
> > ---
> > security/yama/yama_lsm.c | 9 ++-------
> > 1 file changed, 2 insertions(+), 7 deletions(-)
> >
> > diff --git a/security/yama/yama_lsm.c b/security/yama/yama_lsm.c
> > index 1971710620c1..3d064dd4e03f 100644
> > --- a/security/yama/yama_lsm.c
> > +++ b/security/yama/yama_lsm.c
> > @@ -222,7 +222,7 @@ static int yama_task_prctl(int option, unsigned long arg2, unsigned long arg3,
> > unsigned long arg4, unsigned long arg5)
> > {
> > int rc = -ENOSYS;
> > - struct task_struct *myself = current;
> > + struct task_struct *myself;
> >
> > switch (option) {
> > case PR_SET_PTRACER:
> > @@ -232,11 +232,7 @@ static int yama_task_prctl(int option, unsigned long arg2, unsigned long arg3,
> > * leader checking is handled later when walking the ancestry
> > * at the time of PTRACE_ATTACH check.
> > */
> > - rcu_read_lock();
> > - if (!thread_group_leader(myself))
> > - myself = rcu_dereference(myself->group_leader);
> > - get_task_struct(myself);
> > - rcu_read_unlock();
> > + myself = current->group_leader;
> >
> > if (arg2 == 0) {
> > yama_ptracer_del(NULL, myself);
> > @@ -255,7 +251,6 @@ static int yama_task_prctl(int option, unsigned long arg2, unsigned long arg3,
> > }
> > }
> >
> > - put_task_struct(myself);
> > break;
> > }
> >
> > --
> > 2.25.1.362.g51ebf55
> >
> >
>
> --
> Kees Cook
>
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH] yama: don't abuse rcu_read_lock/get_task_struct in yama_task_prctl()
2025-02-19 20:17 ` Oleg Nesterov
@ 2025-02-19 21:00 ` Oleg Nesterov
2025-02-19 21:42 ` Oleg Nesterov
0 siblings, 1 reply; 6+ messages in thread
From: Oleg Nesterov @ 2025-02-19 21:00 UTC (permalink / raw)
To: Kees Cook
Cc: Paul Moore, James Morris, Serge E. Hallyn, linux-security-module,
linux-kernel
Forgot to say...
with or without this patch the usage of ptrace_relation->tracer/tracee
doesn't look right (safe) to me... but probably I missed something and
this is another story. Currently I am trying to audit the usage of
task->group_leader.
On 02/19, Oleg Nesterov wrote:
>
> On 02/19, Kees Cook wrote:
> >
> > On Wed, Feb 19, 2025 at 05:14:17PM +0100, Oleg Nesterov wrote:
> > > current->group_leader is stable, no need to take rcu_read_lock() and do
> > > get/put_task_struct().
> >
> > Can you explain why this is true? In trying to figure this out again,
> > it seems that the only way current->group_leader can vanish is if
> > the entire process vanishes (fork or thread exec), in which case the
> > "current" in this prctl can't be happening; this appears to be locked
> > behind tsk->sighand->siglock ?
>
> Well, almost, but this has nothing to do with tsk->sighand->siglock...
>
> task->group_leader can only be changed by thread exec, when a non leader
> thread does exec, see de_thread(). But de_thread() can't succeed and change
> ->group_leader until all other threads exit, see the "Kill all other threads
> in the thread group" code in de_thread(). The "current" task can't exit, so
> current->group_leader is stable.
>
> Note also that we already have a lot of current->group_leader users which
> don't use rcu/get_task_struct.
>
> That said, we have a lot of buggy users of tsk->group_leader when
> same_thread_group(tsk, current) != true ;) For example, sys_prlimit64().
> And note that rcu_read_lock/get_task_struct can't help in this case.
> I am going to send some fixes.
>
> Oleg.
>
> >
> > -Kees
> >
> > >
> > > Signed-off-by: Oleg Nesterov <oleg@redhat.com>
> > > ---
> > > security/yama/yama_lsm.c | 9 ++-------
> > > 1 file changed, 2 insertions(+), 7 deletions(-)
> > >
> > > diff --git a/security/yama/yama_lsm.c b/security/yama/yama_lsm.c
> > > index 1971710620c1..3d064dd4e03f 100644
> > > --- a/security/yama/yama_lsm.c
> > > +++ b/security/yama/yama_lsm.c
> > > @@ -222,7 +222,7 @@ static int yama_task_prctl(int option, unsigned long arg2, unsigned long arg3,
> > > unsigned long arg4, unsigned long arg5)
> > > {
> > > int rc = -ENOSYS;
> > > - struct task_struct *myself = current;
> > > + struct task_struct *myself;
> > >
> > > switch (option) {
> > > case PR_SET_PTRACER:
> > > @@ -232,11 +232,7 @@ static int yama_task_prctl(int option, unsigned long arg2, unsigned long arg3,
> > > * leader checking is handled later when walking the ancestry
> > > * at the time of PTRACE_ATTACH check.
> > > */
> > > - rcu_read_lock();
> > > - if (!thread_group_leader(myself))
> > > - myself = rcu_dereference(myself->group_leader);
> > > - get_task_struct(myself);
> > > - rcu_read_unlock();
> > > + myself = current->group_leader;
> > >
> > > if (arg2 == 0) {
> > > yama_ptracer_del(NULL, myself);
> > > @@ -255,7 +251,6 @@ static int yama_task_prctl(int option, unsigned long arg2, unsigned long arg3,
> > > }
> > > }
> > >
> > > - put_task_struct(myself);
> > > break;
> > > }
> > >
> > > --
> > > 2.25.1.362.g51ebf55
> > >
> > >
> >
> > --
> > Kees Cook
> >
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH] yama: don't abuse rcu_read_lock/get_task_struct in yama_task_prctl()
2025-02-19 21:00 ` Oleg Nesterov
@ 2025-02-19 21:42 ` Oleg Nesterov
0 siblings, 0 replies; 6+ messages in thread
From: Oleg Nesterov @ 2025-02-19 21:42 UTC (permalink / raw)
To: Kees Cook
Cc: Paul Moore, James Morris, Serge E. Hallyn, linux-security-module,
linux-kernel
Damn, sorry for the spam ;)
On 02/19, Oleg Nesterov wrote:
>
> Forgot to say...
>
> with or without this patch the usage of ptrace_relation->tracer/tracee
> doesn't look right (safe) to me... but probably I missed something
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Yes I did. I didn't realize that put_task_struct(tracer/tracee) calls
security_task_free() -> yama_task_free().
Sorry fo the noise.
Oleg.
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH] yama: don't abuse rcu_read_lock/get_task_struct in yama_task_prctl()
2025-02-19 16:14 [PATCH] yama: don't abuse rcu_read_lock/get_task_struct in yama_task_prctl() Oleg Nesterov
2025-02-19 19:33 ` Kees Cook
@ 2025-03-04 23:53 ` Kees Cook
1 sibling, 0 replies; 6+ messages in thread
From: Kees Cook @ 2025-03-04 23:53 UTC (permalink / raw)
To: Paul Moore, James Morris, Serge E. Hallyn, Oleg Nesterov
Cc: Kees Cook, linux-security-module, linux-kernel
On Wed, 19 Feb 2025 17:14:17 +0100, Oleg Nesterov wrote:
> current->group_leader is stable, no need to take rcu_read_lock() and do
> get/put_task_struct().
>
>
Applied to for-next/hardening, thanks!
[1/1] yama: don't abuse rcu_read_lock/get_task_struct in yama_task_prctl()
https://git.kernel.org/kees/c/c6822ed5d037
Take care,
--
Kees Cook
^ permalink raw reply [flat|nested] 6+ messages in thread
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2025-02-19 16:14 [PATCH] yama: don't abuse rcu_read_lock/get_task_struct in yama_task_prctl() Oleg Nesterov
2025-02-19 19:33 ` Kees Cook
2025-02-19 20:17 ` Oleg Nesterov
2025-02-19 21:00 ` Oleg Nesterov
2025-02-19 21:42 ` Oleg Nesterov
2025-03-04 23:53 ` Kees Cook
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