From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pf1-f179.google.com (mail-pf1-f179.google.com [209.85.210.179]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id CA54A12D219 for ; Tue, 5 Mar 2024 23:33:57 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.210.179 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1709681639; cv=none; b=TESK1X8pkJmPXqUAhInkXw0Kdl5syKYlZvPzz/NU82jOt37+kglfDOLQ/wBwjBigVEbNV2XO30q0FNAMeBouRkoSOzmQiczBaq1LO5VXdIRWTzkbDeSBOT/Sa2Bo1Nz7lTgrPXzSAX6ujXglFd+8MqdXBt7IKLMsUT14DJzwe9g= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1709681639; c=relaxed/simple; bh=1SnJ59Ps7URmLCLfC/TBTkRIQ6+QZh83O+LxVECZ4eE=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=ZUyMe5lON+6H/vxT48pv9uxL0pJ6Vb3K1PTSpqEl5v1uAyGJw9wNkj6Hq+lNrNzuN9tMDJPHyHyS4IYKdw710fZdb7+z7z/1QWXDibdMn2UjTDmQC+XLY1qpMSZ61jqeRShPlQRbyTebNUa8eQapprIGKVdD4F/G7KfBpurCGaU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=chromium.org; spf=pass smtp.mailfrom=chromium.org; dkim=pass (1024-bit key) header.d=chromium.org header.i=@chromium.org header.b=YDoNyAqj; arc=none smtp.client-ip=209.85.210.179 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=chromium.org Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=chromium.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=chromium.org header.i=@chromium.org header.b="YDoNyAqj" Received: by mail-pf1-f179.google.com with SMTP id d2e1a72fcca58-6e6381df003so273172b3a.0 for ; Tue, 05 Mar 2024 15:33:57 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=chromium.org; s=google; t=1709681637; x=1710286437; darn=vger.kernel.org; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:from:to:cc:subject:date:message-id:reply-to; bh=SVqJYG4NwJpbVhIQ4ejb78hlZKAflZdt5zQRCQZdbhg=; b=YDoNyAqjT4+JsgTrVnl6OAgF1k64GJMlbCpU5D4eDqNf2O9lAGCgFz+aAhOLoIHwhd TD7DBSOjOK4eP0tZwdcV4Mv0gyUEaI4uyNQ2nrVSa7h412iV3qbbB3tAONSQjBafu044 us7oVi+DRKkVfH+7Aq5xqFtH94K1j+tykG6J4= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1709681637; x=1710286437; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to; bh=SVqJYG4NwJpbVhIQ4ejb78hlZKAflZdt5zQRCQZdbhg=; b=tO7fbsBE8tWePQyv0vSAJRzCtxT8bhPE5Y0xaTgNDoWB7sjwtdCKVLnsrZvcls8STG 5gEUonJPMcHjw/4rgu0zvizMug39LjEA2JNXrFKQTDo/yNJkojwhukWXNFDpS4trb3H9 6Kj6f32eAHDAysDXL5AV+QRHaNHT/LKY2011fcgquFU+7KFAyHAQsBzxhYhbdBy2qMlG AFHkQ1wEDl9Kj1686ijYgQ1PhjGiFix8rIObCKVEcWXfyjb7z5NcgoP++ZZxRitzop/y lyEJKsUW+8n5TxdnBgxsgxhnwX+Y9Vt0dQQ2QS7sjAtM7ibJNSiYw4MGDHhhhkQgwe45 Etgw== X-Forwarded-Encrypted: i=1; AJvYcCWcpETU4ORr+9x2CN6pyKIcGS80E/aqQN21AGavYuHIQDWQGc0t010cqJEAKrM+Om91VD4HS/k0AQv+G2zLfYMPnb5sxO0ttNM1vGEM X-Gm-Message-State: AOJu0YzSEle0M8Js+tAnPMFIjfEgn1SmdJKzcQcqdjXTUO8T/Mk4Eeex 1dkg41YvWa/6Xq5HXwEpeTxl0kFna5jHrwd9uxplnXcbuxgZgj3Wq1YaG4gm6g== X-Google-Smtp-Source: AGHT+IHQdYS5+xMryfTT1IOdZhcc6rZew5WCVjHi9VllATBYGMQVhc0/wPSb9IRr0q52NcbVnxEBdQ== X-Received: by 2002:a05:6a00:1ac7:b0:6e6:27d3:96ab with SMTP id f7-20020a056a001ac700b006e627d396abmr6963065pfv.16.1709681636969; Tue, 05 Mar 2024 15:33:56 -0800 (PST) Received: from www.outflux.net ([198.0.35.241]) by smtp.gmail.com with ESMTPSA id r5-20020aa79885000000b006e530aca55asm9524721pfl.123.2024.03.05.15.33.55 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 05 Mar 2024 15:33:55 -0800 (PST) Date: Tue, 5 Mar 2024 15:33:54 -0800 From: Kees Cook To: Jeremy Linton Cc: linux-arm-kernel@lists.infradead.org, catalin.marinas@arm.com, will@kernel.org, Jason@zx2c4.com, gustavoars@kernel.org, mark.rutland@arm.com, rostedt@goodmis.org, arnd@arndb.de, broonie@kernel.org, guohui@uniontech.com, Manoj.Iyer@arm.com, linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org, James Yang , Shiyou Huang Subject: Re: [PATCH 1/1] arm64: syscall: Direct PRNG kstack randomization Message-ID: <202403051526.0BE26F99E@keescook> References: <20240305221824.3300322-1-jeremy.linton@arm.com> <20240305221824.3300322-2-jeremy.linton@arm.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: <20240305221824.3300322-2-jeremy.linton@arm.com> On Tue, Mar 05, 2024 at 04:18:24PM -0600, Jeremy Linton wrote: > The existing arm64 stack randomization uses the kernel rng to acquire > 5 bits of address space randomization. This is problematic because it > creates non determinism in the syscall path when the rng needs to be > generated or reseeded. This shows up as large tail latencies in some > benchmarks and directly affects the minimum RT latencies as seen by > cyclictest. > > Other architectures are using timers/cycle counters for this function, > which is sketchy from a randomization perspective because it should be > possible to estimate this value from knowledge of the syscall return > time, and from reading the current value of the timer/counters. > > So, a poor rng should be better than the cycle counter if it is hard > to extract the stack offsets sufficiently to be able to detect the > PRNG's period. Lets downgrade from get_random_u16() to > prandom_u32_state() under the theory that the danger of someone > guessing the 1 in 32 per call offset, is larger than that of being > able to extract sufficient history to accurately predict future > offsets. Further it should be safer to run with prandom_u32_state than > disabling stack randomization for those subset of applications where the > difference in latency is on the order of ~5X worse. > > Reported-by: James Yang > Reported-by: Shiyou Huang > Signed-off-by: Jeremy Linton > --- > arch/arm64/kernel/syscall.c | 42 ++++++++++++++++++++++++++++++++++++- > 1 file changed, 41 insertions(+), 1 deletion(-) > > diff --git a/arch/arm64/kernel/syscall.c b/arch/arm64/kernel/syscall.c > index 9a70d9746b66..33b3ea4adff8 100644 > --- a/arch/arm64/kernel/syscall.c > +++ b/arch/arm64/kernel/syscall.c > @@ -5,6 +5,7 @@ > #include > #include > #include > +#include > #include > #include > > @@ -37,6 +38,45 @@ static long __invoke_syscall(struct pt_regs *regs, syscall_fn_t syscall_fn) > return syscall_fn(regs); > } > > +#ifdef CONFIG_RANDOMIZE_KSTACK_OFFSET > +DEFINE_PER_CPU(struct rnd_state, kstackrng); > + > +static u16 kstack_rng(void) > +{ > + u32 rng = prandom_u32_state(this_cpu_ptr(&kstackrng)); > + > + return rng & 0x1ff; > +} > + > +/* Should we reseed? */ > +static int kstack_rng_setup(unsigned int cpu) > +{ > + u32 rng_seed; > + > + /* zero should be avoided as a seed */ > + do { > + rng_seed = get_random_u32(); > + } while (!rng_seed); > + prandom_seed_state(this_cpu_ptr(&kstackrng), rng_seed); > + return 0; > +} > + > +static int kstack_init(void) > +{ > + int ret; > + > + ret = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "arm64/cpuinfo:kstackrandomize", > + kstack_rng_setup, NULL); This will run initial seeding, but don't we need to reseed this with some kind of frequency? Otherwise, seems fine to me. -- Kees Cook