From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 91FB0366575; Mon, 28 Sep 2026 16:10:11 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790611812; cv=none; b=iY5o0Kc0Y2psCyw0iKNh9MTdMxuLosRvt1noSR+ansWr9k0cdCFJPPY46SYx4d0jzBGBd5Z/jvmZDcro6Aroakb26a5HrApqqY7uE7axKAc2iVET5+MIjnFrVecDhp2+/cKrOiTXJ1yK7+QIzGqyjssSsSmExwZmGCDsQ3asvKE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790611812; c=relaxed/simple; bh=olNMz5YZQDlY1bi55VVy7KXyye4u3+GKfiXGVCkelC4=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=fGrr92k4o8GgOJKnoBdKB08OKn7sCRNqH4vL93jHxMTg158nlKixG+DpI92OgVPmb8n89XzQH1sDU01gptkCEINACalC0+hcx3MZafvFDIoh5xL7BckcQ5Diafckowz0NIz4IKlUmI5U4gRruHNzFtDSHEDXegoqIsRrYi4XkN4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=ICso0gr+; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="ICso0gr+" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 285321F000FF; Mon, 28 Sep 2026 16:10:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1790611811; bh=39jJMoh2t7NHIz08n8lMVjXlTS/HZlmaisj6W0jHknw=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To; b=ICso0gr+BVEr54WZEqW/7K+E5OkfbRgp4i22aHPIoyftRw+URJb5lWscZabIsH5CV tGxNVK9LgA+MWq/ZH6A0hxaeB2572AP2uVCBNNsNkUCml+TWwaxZHMHK8PYO7kK3P8 d7n8gEw14cCge6LrkPZ5KDuVN5XRhfRGFHeEqpgbIp/5WEIolBflftwFX9FJ0ePtW1 DRVgNCCQqWmFKtomDH8BfuEJmOrsph/Xposbt0K3Ld7gudxlKJk8RVY9APOrSDZvNz 1Z3awJdITNWJ/xFKkSUgp4ElCIbYfFtGyAaY7PJm/XWtpUbseoV+rYxw6NokTcNU/D mmYFJ1tYkmrJQ== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id C7551CE0832; Mon, 28 Sep 2026 09:10:10 -0700 (PDT) Date: Mon, 28 Sep 2026 09:10:10 -0700 From: "Paul E. McKenney" To: "Masami Hiramatsu (Google)" Cc: Steven Rostedt , Frederic Weisbecker , Neeraj Upadhyay , Mathieu Desnoyers , Josef Bacik , linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, rcu@vger.kernel.org Subject: Re: [PATCH] fprobe: Protect fprobe_return() with guard(rcu)() Message-ID: <1c2a58be-0c8d-45b1-a803-85480e976e11@paulmck-laptop> Reply-To: paulmck@kernel.org References: <179055575009.241711.6358052647499787191.stgit@devnote2> <179055575973.241711.6618845269004184577.stgit@devnote2> 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: <179055575973.241711.6618845269004184577.stgit@devnote2> On Mon, Sep 28, 2026 at 09:35:59AM +0900, Masami Hiramatsu (Google) wrote: > From: Masami Hiramatsu (Google) > > In fprobe_return(), the shadow-stack iteration and exit_handler > invocations were protected by preempt_disable_notrace(). > However, unregister_fprobe() and unregister_fprobe_async() (used > by BPF kprobe-multi) rely on standard RCU grace periods (synchronize_rcu() > and call_rcu()) to wait until the fprobe is no longer in use before > freeing it. > > In preemptible kernels (CONFIG_PREEMPT_RCU=y), standard RCU grace > periods do not wait for pure preempt_disable_notrace() critical > sections. Consequently, an unregistered fprobe may be freed while > a concurrent CPU executing fprobe_return() is still running > fp->exit_handler(), causing a use-after-free. Actually, standard RCU grace periods wait for preemption-disabled regions of code regardless of kernel configuration. So if the original code below was broken, that indicates a bug in RCU. So do you have a reproducer for this? Thanx, Paul > To resolve this, protect fprobe_return() with guard(rcu)() matching > fprobe_fgraph_entry(). This ensures both synchronous unregister_fprobe() > and asynchronous unregister_fprobe_async() safely wait for in-flight > exit_handlers to complete via standard RCU grace periods. > > Reported-by: Sashiko > Closes: https://sashiko.dev/#/bug/linux-e46bcd68-4a56-4f19-a255-e3772980e5e3 > Fixes: 657b594b2084 ("fprobe: Fix unregister_fprobe() to wait for RCU grace period") > Cc: stable@vger.kernel.org > Assisted-by: LLM > Signed-off-by: Masami Hiramatsu (Google) > --- > kernel/trace/fprobe.c | 3 +-- > 1 file changed, 1 insertion(+), 2 deletions(-) > > diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c > index 9f2d98181779..c3e1580bc648 100644 > --- a/kernel/trace/fprobe.c > +++ b/kernel/trace/fprobe.c > @@ -671,7 +671,7 @@ static void fprobe_return(struct ftrace_graph_ret *trace, > size_words = SIZE_IN_LONG(size); > ret_ip = ftrace_regs_get_instruction_pointer(fregs); > > - preempt_disable_notrace(); > + guard(rcu)(); > > curr = 0; > while (size_words > curr) { > @@ -687,7 +687,6 @@ static void fprobe_return(struct ftrace_graph_ret *trace, > } > curr += size; > } > - preempt_enable_notrace(); > } > NOKPROBE_SYMBOL(fprobe_return); > >