Re: [PATCH] ptrace: restore smp_rmb() in __ptrace_may_access()

[Kees Cook]

On Wed, May 29, 2019 at 01:31:57PM +0200, Jann Horn wrote:
> Restore the read memory barrier in __ptrace_may_access() that was deleted
> a couple years ago. Also add comments on this barrier and the one it pairs
> with to explain why they're there (as far as I understand).
> 
> Fixes: bfedb589252c ("mm: Add a user_ns owner to mm_struct and fix ptrace permission checks")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>

Ah, thanks! Nice find.

Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>

(And, yeah, Eric, I say snag it if you've got stuff queued up...)

-Kees

> ---
> (I have no clue whatsoever what the relevant tree for this is, but I
> guess Oleg is the relevant maintainer?)
> 
>  kernel/cred.c   |  9 +++++++++
>  kernel/ptrace.c | 10 ++++++++++
>  2 files changed, 19 insertions(+)
> 
> diff --git a/kernel/cred.c b/kernel/cred.c
> index 45d77284aed0..07e069d00696 100644
> --- a/kernel/cred.c
> +++ b/kernel/cred.c
> @@ -450,6 +450,15 @@ int commit_creds(struct cred *new)
>  		if (task->mm)
>  			set_dumpable(task->mm, suid_dumpable);
>  		task->pdeath_signal = 0;
> +		/*
> +		 * If a task drops privileges and becomes nondumpable,
> +		 * the dumpability change must become visible before
> +		 * the credential change; otherwise, a __ptrace_may_access()
> +		 * racing with this change may be able to attach to a task it
> +		 * shouldn't be able to attach to (as if the task had dropped
> +		 * privileges without becoming nondumpable).
> +		 * Pairs with a read barrier in __ptrace_may_access().
> +		 */
>  		smp_wmb();
>  	}
>  
> diff --git a/kernel/ptrace.c b/kernel/ptrace.c
> index 5710d07e67cf..e54452c2954b 100644
> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -324,6 +324,16 @@ static int __ptrace_may_access(struct task_struct *task, unsigned int mode)
>  	return -EPERM;
>  ok:
>  	rcu_read_unlock();
> +	/*
> +	 * If a task drops privileges and becomes nondumpable (through a syscall
> +	 * like setresuid()) while we are trying to access it, we must ensure
> +	 * that the dumpability is read after the credentials; otherwise,
> +	 * we may be able to attach to a task that we shouldn't be able to
> +	 * attach to (as if the task had dropped privileges without becoming
> +	 * nondumpable).
> +	 * Pairs with a write barrier in commit_creds().
> +	 */
> +	smp_rmb();
>  	mm = task->mm;
>  	if (mm &&
>  	    ((get_dumpable(mm) != SUID_DUMP_USER) &&
> -- 
> 2.22.0.rc1.257.g3120a18244-goog
> 

-- 
Kees Cook