[PATCH v2] Yama: access task_struct->comm directly

[PATCH v2] Yama: access task_struct->comm directly

[Kees Cook]

The core ptrace access checking routine holds a task lock, and when
reporting a failure, Yama takes a separate task lock. To avoid a
potential deadlock with two ptracers taking the opposite locks, do not
use get_task_comm() and just use ->comm directly since accuracy is not
important for the report.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Suggested-by: Oleg Nesterov <oleg@redhat.com>
CC: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: John Johansen <john.johansen@canonical.com>

---
v2: update with newly added traceme report.
---
 security/yama/yama_lsm.c |   10 ++--------
 1 files changed, 2 insertions(+), 8 deletions(-)

diff --git a/security/yama/yama_lsm.c b/security/yama/yama_lsm.c
index d51b7c7..0cc99a3 100644
--- a/security/yama/yama_lsm.c
+++ b/security/yama/yama_lsm.c
@@ -279,12 +279,9 @@ static int yama_ptrace_access_check(struct task_struct *child,
 	}
 
 	if (rc) {
-		char name[sizeof(current->comm)];
 		printk_ratelimited(KERN_NOTICE
 			"ptrace of pid %d was attempted by: %s (pid %d)\n",
-			child->pid,
-			get_task_comm(name, current),
-			current->pid);
+			child->pid, current->comm, current->pid);
 	}
 
 	return rc;
@@ -319,12 +316,9 @@ static int yama_ptrace_traceme(struct task_struct *parent)
 	}
 
 	if (rc) {
-		char name[sizeof(current->comm)];
 		printk_ratelimited(KERN_NOTICE
 			"ptraceme of pid %d was attempted by: %s (pid %d)\n",
-			current->pid,
-			get_task_comm(name, parent),
-			parent->pid);
+			current->pid, parent->comm, parent->pid);
 	}
 
 	return rc;
-- 
1.7.0.4


-- 
Kees Cook
Chrome OS Security

Re: [PATCH v2] Yama: access task_struct->comm directly

[Oleg Nesterov]

On 08/15, Kees Cook wrote:
>
> v2: update with newly added traceme report.

...

> @@ -319,12 +316,9 @@ static int yama_ptrace_traceme(struct task_struct *parent)
>  	}
>  
>  	if (rc) {
> -		char name[sizeof(current->comm)];
>  		printk_ratelimited(KERN_NOTICE
>  			"ptraceme of pid %d was attempted by: %s (pid %d)\n",
> -			current->pid,
> -			get_task_comm(name, parent),
> -			parent->pid);
> +			current->pid, parent->comm, parent->pid);

Hmm, yes. This change is even more important. This is called under
write_lock_irq(tasklist_lock), task_lock() can deadlock if we race
with another CPU doing task_lock()->interrupt->read_lock(tasklist).

Oleg.