From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 868301DFFB for ; Mon, 21 Oct 2024 13:43:10 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1729518193; cv=none; b=RSZWbwgjhspRbUoiX4JhQ6aeIuOwmSjJ0sWZI4yK9/L9jqk3no4De1EOe/VpDnlbpQRYelA3i81YuF5Q1UIN5z3zkhxRN0e7vzVkG4dx3Gtwc661XYPRHRm/VaVfcQabNsixHj5eiTAzZILI/V6isXy0cIWpxCyhl/Vgqlllp2g= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1729518193; c=relaxed/simple; bh=oVQPdJOJgOtZjCWv8MMMz2YcdRPvVM+f6I5xQyISdBg=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=DG/xyqZN1s8SD8R/WFZcS2BuQHBabqUjrah1QsnGbhx64fuyHzUENbq9RjDPUqIAyQ0u3z/eaHgOgymEDYdIGCfR6llqvCYDfyQ1ztgsQoioXgCAWnsBU/3rXKbomGNomFJCgsPEXpirp8coSTWPp/QU9UZLhOmdI9EOcpE32VY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 74CCCDA7; Mon, 21 Oct 2024 06:43:39 -0700 (PDT) Received: from e133380.arm.com (e133380.arm.com [10.1.197.69]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 703613F73B; Mon, 21 Oct 2024 06:43:07 -0700 (PDT) Date: Mon, 21 Oct 2024 14:43:04 +0100 From: Dave Martin To: Kevin Brodsky Cc: linux-arm-kernel@lists.infradead.org, akpm@linux-foundation.org, anshuman.khandual@arm.com, aruna.ramakrishna@oracle.com, broonie@kernel.org, catalin.marinas@arm.com, dave.hansen@linux.intel.com, jeffxu@chromium.org, joey.gouly@arm.com, shuah@kernel.org, will@kernel.org, linux-kselftest@vger.kernel.org, x86@kernel.org Subject: Re: [PATCH 3/5] arm64: signal: Improve POR_EL0 handling to avoid uaccess failures Message-ID: References: <20241017133909.3837547-1-kevin.brodsky@arm.com> <20241017133909.3837547-4-kevin.brodsky@arm.com> Precedence: bulk X-Mailing-List: linux-kselftest@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: On Mon, Oct 21, 2024 at 12:06:07PM +0200, Kevin Brodsky wrote: > On 17/10/2024 17:53, Dave Martin wrote: > > [...] > >> +/* > >> + * Save the unpriv access state into ua_state and reset it to disable any > >> + * restrictions. > >> + */ > >> +static void save_reset_unpriv_access_state(struct unpriv_access_state *ua_state) > > Would _user_ be more consistent naming than _unpriv_ ? > > I did ponder on the naming. I considered user_access/uaccess instead of > unpriv_access, but my concern is that it might imply that only uaccess > is concerned, while in reality loads/stores that userspace itself > executes are impacted too. I thought using the "unpriv" terminology from > the Arm ARM (used for stage 1 permissions) might avoid such > misunderstanding. I'm interested to hear opinions on this, maybe > accuracy sacrifices readability. "user_access" seemed natural to me: it parses equally as "[user access]" (i.e., uaccess) and "[user] access" (i.e., access by, to, or on behalf of user(space)). Introducing an architectural term when there is already a generic OS and Linux kernel term that means the right thing seemed not to improve readability, but I guess it's a matter of opinion. Anyway, it doesn't really matter. > > > Same elsewhere. > > > >> +{ > >> + if (system_supports_poe()) { > >> + /* > >> + * Enable all permissions in all 8 keys > >> + * (inspired by REPEAT_BYTE()) > >> + */ > >> + u64 por_enable_all = (~0u / POE_MASK) * POE_RXW; > > Yikes! > > > > Seriously though, why are we granting permissions that the signal > > handler isn't itself going to have over its own stack? > > > > I think the logical thing to do is to think of the write/read of the > > signal frame as being done on behalf of the signal handler, so the > > permissions should be those we're going to give the signal handler: > > not less, and (so far as we can approximate) not more. > > Will continue that discussion on the cover letter. > > > > >> + > >> + ua_state->por_el0 = read_sysreg_s(SYS_POR_EL0); > >> + write_sysreg_s(por_enable_all, SYS_POR_EL0); > >> + /* Ensure that any subsequent uaccess observes the updated value */ > >> + isb(); > >> + } > >> +} > >> + > >> +/* > >> + * Set the unpriv access state for invoking the signal handler. > >> + * > >> + * No uaccess should be done after that function is called. > >> + */ > >> +static void set_handler_unpriv_access_state(void) > >> +{ > >> + if (system_supports_poe()) > >> + write_sysreg_s(POR_EL0_INIT, SYS_POR_EL0); > >> + > > Spurious blank line? > > Thanks! > > >> +} > > [...] > > > >> @@ -1252,9 +1310,11 @@ static int setup_rt_frame(int usig, struct ksignal *ksig, sigset_t *set, > >> { > >> struct rt_sigframe_user_layout user; > >> struct rt_sigframe __user *frame; > >> + struct unpriv_access_state ua_state; > >> int err = 0; > >> > >> fpsimd_signal_preserve_current_state(); > >> + save_reset_unpriv_access_state(&ua_state); > > (Trivial nit: maybe put the blank line before this rather than after? > > This has nothing to do with "settling" the kernel's internal context > > switch state, and a lot to do with generaing the signal frame...) > > In fact considering the concern Catalin brought up with POR_EL0 being > reset even when we fail to deliver the signal [1], I'm realising this > call should be moved after get_sigframe(), since the latter doesn't use > uaccess and can fail. > > [1] https://lore.kernel.org/linux-arm-kernel/Zw6D2waVyIwYE7wd@arm.com/ > > >> > >> if (get_sigframe(&user, ksig, regs)) > >> return 1; > > [...] ^ Ah, good point. The save_reset_unpriv_access_state(&ua_state) call probably belong just before the first __put_user() then. > > > >> @@ -1273,6 +1333,7 @@ static int setup_rt_frame(int usig, struct ksignal *ksig, sigset_t *set, > >> regs->regs[1] = (unsigned long)&frame->info; > >> regs->regs[2] = (unsigned long)&frame->uc; > >> } > >> + set_handler_unpriv_access_state(); > > This bit feels prematurely factored? We don't have separate functions > > for the other low-level preparation done here... > > I preferred to have a consistent API for all manipulations of POR_EL0, > the idea being that if more registers are added to struct > unpriv_access_state, only the *unpriv_access* helpers need to be amended. Certainly if that struct grows more state, then the factoring will help in future. I wasn't clear on how we expect this all to evolve. Either way, this is basically a non-issue, and keeping the symmetry is probably a good idea. Cheers ---Dave