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 A93A821C18A for ; Fri, 15 Aug 2025 15:44:41 +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=1755272683; cv=none; b=pObeMfsxUvvqcJ3vbkgdxaXj9tV9Tx7JKdThhIFB9WE7bv6bYTxlQ+XL1SVzUTQAPm8F8OHL4zlGfLk2zT86UHbiSOM5YQO87rJjOlvJk++DVSUkxt6l2hM0x8Tj4fHEOuOeS480WLWgEBYh7a+Qe9pgPwivrPbxJ1bkSY+bKQw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1755272683; c=relaxed/simple; bh=8sv8BoHbyse94b4XmO8DS05qUFZUjOytFd+ZdYJk9bs=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=JcPECYN5oXYVn5yMEcLtwX41BFefG4N0W46btZnStKBha0fqGMwYJMGItSqc88U1KWzhVPxp38bx+UTRzbta31FG6YKTfRPbZKkXeQXWyDCb6P7gd2rRFj1j6olP+PhQmtPiOYlxTwF0tFf/nBRlF+fPaA5HEsx3ruD41DOcrOw= 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=BrrvFSjF; 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="BrrvFSjF" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1755272680; 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=hbw40Hej2AYRZAalYk6Uw/CWMU/5i+7IQaCvtlPJ1Ms=; b=BrrvFSjF3vABl6ysdNYBzC2mQQBQemlid2N6i2cyvBFEsKWMl2l5PkesFKI4vS90Hm2+cE A4JJdA2A5Ze9wjXjEgrIcBK/wxbcpXDSEGQhHS84E84hlzp3ziHWg2uREHp4aByY267zdF wkBsgkU29xlC/Rwzwhez/EO80UbZE5o= Received: from mx-prod-mc-04.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-685-3hZ1pwxpMSeTUja2It5zUg-1; Fri, 15 Aug 2025 11:44:37 -0400 X-MC-Unique: 3hZ1pwxpMSeTUja2It5zUg-1 X-Mimecast-MFC-AGG-ID: 3hZ1pwxpMSeTUja2It5zUg_1755272675 Received: from mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.17]) (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-04.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 576611944DE0; Fri, 15 Aug 2025 15:44:35 +0000 (UTC) Received: from dhcp-27-174.brq.redhat.com (unknown [10.44.33.47]) by mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with SMTP id D658B19A4D43; Fri, 15 Aug 2025 15:44:29 +0000 (UTC) Received: by dhcp-27-174.brq.redhat.com (nbSMTP-1.00) for uid 1000 oleg@redhat.com; Fri, 15 Aug 2025 17:43:18 +0200 (CEST) Date: Fri, 15 Aug 2025 17:43:11 +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: <20250815154311.GG11549@redhat.com> References: <20250814101435.GA17362@redhat.com> <8818b456954644ce609e07d77a65714788ef9098.camel@intel.com> <20250815130125.GD11549@redhat.com> <20250815130851.GE11549@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: User-Agent: Mutt/1.5.24 (2015-08-30) X-Scanned-By: MIMEDefang 3.0 on 10.30.177.17 On 08/15, Mark Brown wrote: > > On Fri, Aug 15, 2025 at 03:08:52PM +0200, Oleg Nesterov wrote: > > On 08/15, Oleg Nesterov wrote: > > > On 08/14, Mark Brown wrote: > > > > > 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. > > Sure, but OTOH at least for arm64 there's no cost to leaving the feature > enabled unless you actually execute userspace code so if we never return > to userspace writing the code to disable isn't really buying us anything. The fact that a kernel thread can have the pointless ARCH_SHSTK_SHSTK is the only reason I know why x86_task_fpu(PF_USER_WORKER) has to work. I'd like to make this logic consistent with PF_KTHREAD, and in the longer term change the x86 FPU code so that the kernel threads can run without without "struct fpu" attached to task_struct. Again, please see https://lore.kernel.org/all/20250813191441.GA26754@redhat.com/ Oleg.