From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) (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 666B82737FD for ; Fri, 15 Aug 2025 13:10:23 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.133.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1755263424; cv=none; b=ig1dfm3FlxwGPZ1D2jrmH2IsLrVL7cED7Sz0a4YoSqehGPt+dRSW8/2uUr3EgdD9mQPPzzfDnHEnTIMyhqxhrCq4iPm2W/Vrz2hs8iEaVPdWqdLFDc8TOtyLV5k4AdU3oycci9ZeTo1fwwBWZ/lKREb88ExGwxoPX7Iw69JBw9U= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1755263424; c=relaxed/simple; bh=ouhTtUdeLnWLdIw/kT8b0m42gt4skQY0uLukSVHGSl8=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=PZ79GybGv+dwjmg2Pjhuidb1ouBpgnx0fQCQg2cx2MPBFqATwGdRdtMvBgQpdsCrxDR7F3gRhCu/yr9U9oYzNjpaz3tlRo4+Gy2YlPSPNTDO0hYkFLcExv+T6lNP/gqFNMT/kcqOvgkx1CfsNZlXuIS98Hb8bpJ5OkPaZB8uqVk= 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=hs9m90XG; arc=none smtp.client-ip=170.10.133.124 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="hs9m90XG" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1755263422; 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=xLihiNWOsBcPy5wz/AbaCFEnala49xsz/G/E/MUFAgQ=; b=hs9m90XGnNUk1YaT9JcBR5NYuMGg/MYb5uPOQDNtO3WN1DvMhnMz+UgjxHsnJnVzhNRzJC 61NADZW682EUIfdF4se0AAkTyRO+jbh87F7eW5Ios3mk4c1aoEPTpNvJRU3DOqXfIq9ZcU r+NAiOfwI+Ji9YVdcIpZZS4gquBcCoI= Received: from mx-prod-mc-05.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-512-cuyCy1G6PD2vCWQDtMFHUg-1; Fri, 15 Aug 2025 09:10:18 -0400 X-MC-Unique: cuyCy1G6PD2vCWQDtMFHUg-1 X-Mimecast-MFC-AGG-ID: cuyCy1G6PD2vCWQDtMFHUg_1755263416 Received: from mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.12]) (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-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 8AB4F19775AD; Fri, 15 Aug 2025 13:10:14 +0000 (UTC) Received: from dhcp-27-174.brq.redhat.com (unknown [10.44.33.47]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with SMTP id DD9ED1954B06; Fri, 15 Aug 2025 13:10:09 +0000 (UTC) Received: by dhcp-27-174.brq.redhat.com (nbSMTP-1.00) for uid 1000 oleg@redhat.com; Fri, 15 Aug 2025 15:08:57 +0200 (CEST) Date: Fri, 15 Aug 2025 15:08:52 +0200 From: Oleg Nesterov To: Mark Brown Cc: "Edgecombe, Rick P" , "mingo@kernel.org" , "dave.hansen@linux.intel.com" , "bp@alien8.de" , "peterz@infradead.org" , "hpa@zytor.com" , "axboe@kernel.dk" , "tglx@linutronix.de" , "Mehta, Sohil" , "linux-kernel@vger.kernel.org" , "x86@kernel.org" , "debug@rivosinc.com" Subject: Re: [PATCH 5/6] x86/shstk: don't create the shadow stack for PF_USER_WORKERs Message-ID: <20250815130851.GE11549@redhat.com> References: <20250814101435.GA17362@redhat.com> <8818b456954644ce609e07d77a65714788ef9098.camel@intel.com> <20250815130125.GD11549@redhat.com> Precedence: bulk X-Mailing-List: linux-kernel@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: <20250815130125.GD11549@redhat.com> User-Agent: Mutt/1.5.24 (2015-08-30) X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 On 08/15, Oleg Nesterov wrote: > > On 08/14, Mark Brown wrote: > > > > On Thu, Aug 14, 2025 at 05:03:36PM +0000, Edgecombe, Rick P wrote: > > > On Thu, 2025-08-14 at 12:14 +0200, Oleg Nesterov wrote: > > > > > > If a features_enabled(ARCH_SHSTK_SHSTK) userspace thread creates a > > > > PF_USER_WORKER thread, shstk_alloc_thread_stack() allocates the shadow > > > > stack for no reason, the new (kernel) thread will never return to usermode. > > > > > I agree we don't need to allocate a shadow stack in this case, but I'm not sure > > > it is right to fully disable shadow stack in thread.features. First of all, > > > disabling it from shstk_alloc_thread_stack() seems weird. It just handles > > > allocating shadow stacks. > > > > I agree that it's better to leave userspace shadow stacks enabled, given > > that the reason we're not allocating the shadow stack is that we don't > > expect to ever return to userspace then it should be fine to leave the > > feature turned on for userspace. If we mess up and do somehow return to > > userspace > > But a PF_USER_WORKER task can never return to userspace. It doesn't differ > from PF_KTHREAD in this respect. ... of course unless it does exec. Oleg.