Re: KASAN: use-after-free Read in task_is_descendant
From: Oleg Nesterov <oleg@redhat.com>
Date: 2018-10-25 11:14:03
Also in:
lkml
On 10/25, Tetsuo Handa wrote:
Oleg Nesterov wrote:quoted
On 10/22, Tetsuo Handa wrote:quoted
quoted
And again, I do not know how/if yama ensures that child is rcu-protected, perhaps task_is_descendant() needs to check pid_alive(child) right after rcu_read_lock() ?Since the caller (ptrace() path) called get_task_struct(child), child itself can't be released. Do we still need pid_alive(child) ?get_task_struct(child) can only ensure that this task_struct can't be freed.The report says that it is a use-after-free read at walker = rcu_dereference(walker->real_parent); which means that walker was already released.
quite possibly I missed something, but I am not sure I understand your concerns... So again, suppose that "child" is already dead. Its task_struct can't be freed, but child->real_parent can point to the already freed memory. This means that the 1st walker = rcu_dereference(walker->real_parent) is fine, this simply reads the child->real_parent pointer, but on the second iteration walker = rcu_dereference(walker->real_parent); reads the alredy freed memory.
I wonder whether pid_alive() test helps. We can get [ 40.620318] parent or walker is dead. [ 40.624146] tracee is dead. messages using below patch and reproducer.
again, I do not understand, this all looks correct...
quoted hunk ↗ jump to hunk
----------diff --git a/kernel/ptrace.c b/kernel/ptrace.c index 99cfddd..0d9d786 100644 --- a/kernel/ptrace.c +++ b/kernel/ptrace.c@@ -385,6 +385,7 @@ static int ptrace_attach(struct task_struct *task, long request, if (mutex_lock_interruptible(&task->signal->cred_guard_mutex)) goto out; + schedule_timeout_killable(HZ); task_lock(task); retval = __ptrace_may_access(task, PTRACE_MODE_ATTACH_REALCREDS); task_unlock(task);diff --git a/security/yama/yama_lsm.c b/security/yama/yama_lsm.c index ffda91a..a231ec6 100644 --- a/security/yama/yama_lsm.c +++ b/security/yama/yama_lsm.c@@ -283,6 +283,11 @@ static int task_is_descendant(struct task_struct *parent, return 0; rcu_read_lock(); + if (!pid_alive(parent) || !pid_alive(walker)) { + rcu_read_unlock(); + printk("parent or walker is dead.\n");
This is what we need to do, except I think we should change yama_ptrace_access_check(). And iiuc parent == current, pid_alive(parent) looks unnecessary. Although we need to check ptracer_exception_found(), may be it needs some changes too. And yes, task_is_descendant() can hit the dead child, if nothing else it can be killed. This can explain the kasan report.
quoted hunk ↗ jump to hunk
@@ -315,6 +320,10 @@ static int ptracer_exception_found(struct task_struct *tracer, bool found = false; rcu_read_lock(); + if (!pid_alive(tracee)) { + printk("tracee is dead.\n"); + goto unlock;
Sure, this is possible too.
But since "child" has at least one reference, reading "child->real_parent" should be safe. Therefore, I think that bailing out due to pid_is_alive(child) == false (like above patch does) cannot avoid this problem...
Why? OK. Lets ignore ptracer_exception_found() for the moment. Why do you think the patch below can't help? Oleg.
--- x/security/yama/yama_lsm.c
+++ x/security/yama/yama_lsm.c@@ -368,7 +368,8 @@ static int yama_ptrace_access_check(stru break; case YAMA_SCOPE_RELATIONAL: rcu_read_lock(); - if (!task_is_descendant(current, child) && + if (!pid_alive(child) || + !task_is_descendant(current, child) && !ptracer_exception_found(current, child) && !ns_capable(__task_cred(child)->user_ns, CAP_SYS_PTRACE)) rc = -EPERM;