From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 95CEB336EE4 for ; Mon, 17 Nov 2025 15:02:16 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.129.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1763391738; cv=none; b=LObNIAGQZFQNzhQ5mdzSfuO6Gkmp5YcozCsQeuYHZow5NQ1mIiakSH3xKfQ8e+W3RDeJPyl/HZXs4X8HGJ59BL93wzSz9oMQEw3RN3WKAPV3Yp49UodaQidZ8tIy9zgNaxLpZLkxUBtReIuAvhyYcakYcyfG9miBnFrg0nUzuzo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1763391738; c=relaxed/simple; bh=V6fHlz8HT9iaeRZe6Hts1U21hfu7/7IRpYz4JuQDFPk=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=KlEnyfDUd01zQuYEffjwcoakw7kUve2YtdoWlncsUb6F6HhHmUrUpFk2lvki/60y1vRuzAhMQFIGJWIBGx/+hev6HUOEko9u6Xeweu8Dg5YFwzd3jx2jx1WXBQNC4HZMggxOZyKkEffZen7ZwtdmxggHL9OSiliMLeUYPMM7ZpE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=jOB9Ng97; arc=none smtp.client-ip=170.10.129.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="jOB9Ng97" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1763391735; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=s+H138e7y6HCsaCtaTqIGyoW1ghuauULhTt5/nDa2Fg=; b=jOB9Ng97HRERKBKsNC3DPpkEVVXa7vLdg2e3bY/D276d957mATbk2YAYzoDitVYSbS88Bo Bd4pBaynITr1Rk+VzlRoKmavL6rt6BVzMYs37LMGK/VTf0nC7NiDlYBWnMSQ5cELiwyMZR P6fNXTJu2kIgBqJtJ2zIwIqpaa5oWrg= Received: from mx-prod-mc-06.mail-002.prod.us-west-2.aws.redhat.com (ec2-35-165-154-97.us-west-2.compute.amazonaws.com [35.165.154.97]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-499-EAqHuR5uO7aiC1gWSDGuug-1; Mon, 17 Nov 2025 10:02:09 -0500 X-MC-Unique: EAqHuR5uO7aiC1gWSDGuug-1 X-Mimecast-MFC-AGG-ID: EAqHuR5uO7aiC1gWSDGuug_1763391715 Received: from mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.4]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 63997180123A; Mon, 17 Nov 2025 15:01:53 +0000 (UTC) Received: from fedora (unknown [10.44.32.40]) by mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with SMTP id E78D13003776; Mon, 17 Nov 2025 15:01:34 +0000 (UTC) Received: by fedora (nbSMTP-1.00) for uid 1000 oleg@redhat.com; Mon, 17 Nov 2025 16:01:52 +0100 (CET) Date: Mon, 17 Nov 2025 16:01:33 +0100 From: Oleg Nesterov To: Bernd Edlinger Cc: Christian Brauner , Alexander Viro , Alexey Dobriyan , Kees Cook , Andy Lutomirski , Will Drewry , Andrew Morton , Michal Hocko , Serge Hallyn , James Morris , Randy Dunlap , Suren Baghdasaryan , Yafang Shao , Helge Deller , "Eric W. Biederman" , Adrian Reber , Thomas Gleixner , Jens Axboe , Alexei Starovoitov , "linux-fsdevel@vger.kernel.org" , "linux-kernel@vger.kernel.org" , linux-kselftest@vger.kernel.org, linux-mm@kvack.org, linux-security-module@vger.kernel.org, tiozhang , Luis Chamberlain , "Paulo Alcantara (SUSE)" , Sergey Senozhatsky , Frederic Weisbecker , YueHaibing , Paul Moore , Aleksa Sarai , Stefan Roesch , Chao Yu , xu xin , Jeff Layton , Jan Kara , David Hildenbrand , Dave Chinner , Shuah Khan , Elena Reshetova , David Windsor , Mateusz Guzik , Ard Biesheuvel , "Joel Fernandes (Google)" , "Matthew Wilcox (Oracle)" , Hans Liljestrand , Penglei Jiang , Lorenzo Stoakes , Adrian Ratiu , Ingo Molnar , "Peter Zijlstra (Intel)" , Cyrill Gorcunov , Eric Dumazet Subject: Re: [PATCH v17] exec: Fix dead-lock in de_thread with ptrace_attach Message-ID: References: <20251105143210.GA25535@redhat.com> <20251111-ankreiden-augen-eadcf9bbdfaa@brauner> Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.4 On 11/17, Bernd Edlinger wrote: > > On 11/11/25 10:21, Christian Brauner wrote: > > On Wed, Nov 05, 2025 at 03:32:10PM +0100, Oleg Nesterov wrote: > > >> But this is minor. Why do we need "bool unsafe_execve_in_progress" ? > >> If this patch is correct, de_thread() can drop/reacquire cred_guard_mutex > >> unconditionally. > >> > > I would not like to drop the mutex when no absolutely necessary for performance reasons. OK, I won't insist... But I don't really understand how this can help to improve the performance. If nothing else, this adds another for_other_threads() loop. And again, the unsafe_execve_in_progress == T case is unlikely. I'm afraid this case (de_thread() without cred_guard_mutex) won't have enough testing. In any case, why you dislike the suggestion to add this unsafe_execve_in_progress logic in a separate patch? > >>> + if (unlikely(unsafe_execve_in_progress)) { > >>> + spin_unlock_irq(lock); > >>> + sig->exec_bprm = bprm; > >>> + mutex_unlock(&sig->cred_guard_mutex); > >>> + spin_lock_irq(lock); > >> > >> I don't think spin_unlock_irq() + spin_lock_irq() makes any sense... > >> > > Since the spin lock was acquired while holding the mutex, both should be > unlocked in reverse sequence and the spin lock re-acquired after releasing > the mutex. Why? > I'd expect the scheduler to do a task switch after the cred_guard_mutex is > unlocked, at least in the RT-linux variant, while the spin lock is not yet > unlocked. I must have missed something, but I still don't understand why this would be wrong... > >>> @@ -1114,13 +1139,31 @@ int begin_new_exec(struct linux_binprm * bprm) > >>> */ > >>> trace_sched_prepare_exec(current, bprm); > >>> > >>> + /* If the binary is not readable then enforce mm->dumpable=0 */ > >>> + would_dump(bprm, bprm->file); > >>> + if (bprm->have_execfd) > >>> + would_dump(bprm, bprm->executable); > >>> + > >>> + /* > >>> + * Figure out dumpability. Note that this checking only of current > >>> + * is wrong, but userspace depends on it. This should be testing > >>> + * bprm->secureexec instead. > >>> + */ > >>> + if (bprm->interp_flags & BINPRM_FLAGS_ENFORCE_NONDUMP || > >>> + is_dumpability_changed(current_cred(), bprm->cred) || > >>> + !(uid_eq(current_euid(), current_uid()) && > >>> + gid_eq(current_egid(), current_gid()))) > >>> + set_dumpable(bprm->mm, suid_dumpable); > >>> + else > >>> + set_dumpable(bprm->mm, SUID_DUMP_USER); > >>> + > >> > >> OK, we need to do this before de_thread() drops cred_guard_mutex. > >> But imo this too should be done in a separate patch, the changelog should > >> explain this change. > >> > > The dumpability need to be determined before de_thread, because ptrace_may_access > needs this information to determine if the tracer is allowed to ptrace. That is > part of the core of the patch, it would not work without that. Yes, > I will add more comments to make that more easy to understand. But again, why this change can't come in a separate patch? Before the patch which drops cred_guard_mutex in de_thread(). > >> int lock_current_cgm(void) > >> { > >> if (mutex_lock_interruptible(¤t->signal->cred_guard_mutex)) > >> return -ERESTARTNOINTR; > >> > >> if (!current->signal->group_exec_task) > >> return 0; > >> > >> WARN_ON(!fatal_signal_pending(current)); > >> mutex_unlock(¤t->signal->cred_guard_mutex); > >> return -ERESTARTNOINTR; > >> } > >> > >> ? > >> > > Some use mutex_lock_interruptible and some use mutex_lock_killable here, > so it wont work for all of them. I would not consider this a new kind > of dead-lock free mutex, but just an open-coded state machine, handling > the state that the tasks have whild de_thread is running. OK. and we don't have mutex_lock_state(). I think that all users could use mutex_lock_killable(), but you are right anyway, and this is minor. > >> Note that it checks ->group_exec_task, not ->exec_bprm. So this change can > >> come in a separate patch too, but I won't insist. Yes. Although this is minor too ;) > >> This is the most problematic change which I can't review... > >> > >> Firstly, it changes task->mm/real_cred for __ptrace_may_access() and this > >> looks dangerous to me. > > > > Yeah, that is not ok. This is effectively override_creds for real_cred > > and that is not a pattern I want to see us establish at all! Temporary > > credential overrides for the subjective credentials is already terrible > > but at least we have the explicit split between real_cred and cred > > expressely for that. So no, that's not an acceptable solution. > > > > Okay I understand your point. > I did this originally just to avoid to have to change the interface to all > the security engines, but instead I could add a flag PTRACE_MODE_BPRMCREDS to > the ptrace_may_access which must be handled in all security engines, to use > child->signal->exec_bprm->creds instead of __task_cred(child). Can't comment... I don't understand your idea, but this is my fault. I guess this needs more changes, in particular __ptrace_may_access_mm_cred(), but most probably I misunderstood your idea. > > >> Or. check_unsafe_exec() sets LSM_UNSAFE_PTRACE if ptrace. Is it safe to > >> ptrace the execing task after that? I have no idea what the security hooks > >> can do... > > That means the tracee is already ptraced before the execve, and SUID-bits > do not work as usual, and are more or less ignored. But in this patch > the tracee is not yet ptraced. Well. I meant that if LSM_UNSAFE_PTRACE is not set, then currently (say) security_bprm_committing_creds() has all rights to assume that the execing task is not ptraced. Yes, I don't see any potential problem right now, but still. And just in case... Lets look at this code + rcu_assign_pointer(task->real_cred, bprm->cred); + task->mm = bprm->mm; + retval = __ptrace_may_access(task, PTRACE_MODE_ATTACH_REALCREDS); + rcu_assign_pointer(task->real_cred, old_cred); + task->mm = old_mm; again. This is mostly theoretical, but what if begin_new_exec() fails after de_thread() and before exec_mmap() and/or commit_creds(bprm->cred) ? In this case the execing thread will report SIGSEGV to debugger which can (say) read old_mm. No? I am starting to think that ptrace_attach() should simply fail with -EWOULDBLOCK if it detects "unsafe_execve_in_progress" ... And perhaps this is what you already tried to do in the past, I can't recall :/ Oleg.