From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752160AbbKKAX4 (ORCPT ); Tue, 10 Nov 2015 19:23:56 -0500 Received: from mga11.intel.com ([192.55.52.93]:35016 "EHLO mga11.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750950AbbKKAXz (ORCPT ); Tue, 10 Nov 2015 19:23:55 -0500 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.20,272,1444719600"; d="scan'208";a="816778818" Subject: [PATCH] x86, fpu: fix 32-bit signal frame handling To: linux-kernel@vger.kernel.org Cc: dave@sr71.net, dave.hansen@linux.intel.com, x86@kernel.org, luto@kernel.org, fenghua.yu@intel.com, yu-cheng.yu@intel.com From: Dave Hansen Date: Tue, 10 Nov 2015 16:23:54 -0800 Message-Id: <20151111002354.A0799571@viggo.jf.intel.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Dave Hansen (This should have gone to LKML originally. Sorry for the extra noise, folks on the cc.) Background: Signal frames on x86 have two formats: 1. For 32-bit executables (whether on a real 32-bit kernel or under ia32 emulation on a 64-bit kernel) we have a 'fpregset_t' that includes the "FSAVE" registers. 2. For 64-bit executables (on 64-bit kernels obviously), the 'fpregset_t' is smaller and does not contain the "FSAVE" state. When creating the signal frame, we have to be aware of whether we are running a 32 or 64-bit executable so we create the correct format signal frame. Problem: save_xstate_epilog() uses 'fx_sw_reserved_ia32' whenever it is called for a 32-bit executable. This is for real 32-bit and ia32 emulation. But, fpu__init_prepare_fx_sw_frame() only initializes 'fx_sw_reserved_ia32' when emulation is enabled, *NOT* for real 32-bit kernels. This leads to really wierd situations where 32-bit programs lose their extended state when returning from a signal handler. The kernel copies the uninitialized (zero) 'fx_sw_reserved_ia32' out to userspace in save_xstate_epilog(). But when returning from the signal, the kernel errors out in check_for_xstate() when it does not see FP_XSTATE_MAGIC1 present (because it was zeroed). This leads to the FPU/XSAVE state being initialized. For MPX, this leads to the most permissive state and means we silently lose bounds violations. I think this would also mean that we could lose *ANY* FPU/SSE/AVX state. I'm not sure why no one has spotted this bug. I believe this was broken by: 72a671ced: x86, fpu: Unify signal handling code paths for x86 and x86_64 kernels way back in 2012. Signed-off-by: Dave Hansen Cc: x86@kernel.org Cc: luto@kernel.org Cc: fenghua.yu@intel.com Cc: yu-cheng.yu@intel.com --- b/arch/x86/kernel/fpu/signal.c | 11 +++++------ 1 file changed, 5 insertions(+), 6 deletions(-) diff -puN arch/x86/kernel/fpu/signal.c~only-fix-signal-bug arch/x86/kernel/fpu/signal.c --- a/arch/x86/kernel/fpu/signal.c~only-fix-signal-bug 2015-11-10 14:30:19.910135621 -0800 +++ b/arch/x86/kernel/fpu/signal.c 2015-11-10 14:30:19.913135757 -0800 @@ -385,20 +385,19 @@ fpu__alloc_mathframe(unsigned long sp, i */ void fpu__init_prepare_fx_sw_frame(void) { - int fsave_header_size = sizeof(struct fregs_state); int size = xstate_size + FP_XSTATE_MAGIC2_SIZE; - if (config_enabled(CONFIG_X86_32)) - size += fsave_header_size; - fx_sw_reserved.magic1 = FP_XSTATE_MAGIC1; fx_sw_reserved.extended_size = size; fx_sw_reserved.xfeatures = xfeatures_mask; fx_sw_reserved.xstate_size = xstate_size; - if (config_enabled(CONFIG_IA32_EMULATION)) { + if (config_enabled(CONFIG_IA32_EMULATION) || + config_enabled(CONFIG_X86_32)) { + int fsave_header_size = sizeof(struct fregs_state); + fx_sw_reserved_ia32 = fx_sw_reserved; - fx_sw_reserved_ia32.extended_size += fsave_header_size; + fx_sw_reserved_ia32.extended_size = size + fsave_header_size; } } _