From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752404AbcEKKkR (ORCPT ); Wed, 11 May 2016 06:40:17 -0400 Received: from queue01c.mail.zen.net.uk ([212.23.3.237]:33549 "EHLO queue01c.mail.zen.net.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751632AbcEKKkP (ORCPT ); Wed, 11 May 2016 06:40:15 -0400 X-Greylist: delayed 3310 seconds by postgrey-1.27 at vger.kernel.org; Wed, 11 May 2016 06:40:15 EDT Message-ID: <1462959868.2834.17.camel@linaro.org> Subject: Re: [PATCH] arm64: kernel: Fix incorrect brk randomization From: "Jon Medhurst (Tixy)" To: Kees Cook Cc: Catalin Marinas , Hector Marco , Nicolas Pitre , Will Deacon , AKASHI Takahiro , Lorenzo Pieralisi , Jisheng Zhang , Mark Rutland , James Morse , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org Date: Wed, 11 May 2016 10:44:28 +0100 In-Reply-To: <20160510175549.GA20496@www.outflux.net> References: <20160510175549.GA20496@www.outflux.net> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.12.9-1+b1 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit X-Originating-smarthost03b-IP: [82.69.122.217] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 2016-05-10 at 10:55 -0700, Kees Cook wrote: > This fixes two issues with the arm64 brk randomziation. First, the > STACK_RND_MASK was being used incorrectly. The original code was: > > unsigned long range_end = base + (STACK_RND_MASK << PAGE_SHIFT) + 1; > > STACK_RND_MASK is 0x7ff (32-bit) or 0x3ffff (64-bit), with 4K pages where > PAGE_SHIFT is 12: > > #define STACK_RND_MASK (test_thread_flag(TIF_32BIT) ? \ > 0x7ff >> (PAGE_SHIFT - 12) : \ > 0x3ffff >> (PAGE_SHIFT - 12)) > > This means the resulting offset from base would be 0x7ff0001 or 0x3ffff0001, > which is wrong since it creates an unaligned end address. It was likely > intended to be: > > unsigned long range_end = base + ((STACK_RND_MASK + 1) << PAGE_SHIFT) > > Which would result in offsets of 0x800000 (32-bit) and 0x40000000 (64-bit). > > However, even this corrected 32-bit compat offset (0x00800000) is much > smaller than native ARM's brk randomization value (0x02000000): > > unsigned long arch_randomize_brk(struct mm_struct *mm) > { > unsigned long range_end = mm->brk + 0x02000000; > return randomize_range(mm->brk, range_end, 0) ? : mm->brk; > } > > So, instead of basing arm64's brk randomization on mistaken STACK_RND_MASK > calculations, just use specific corrected values for compat (0x2000000) > and native arm64 (0x40000000). > > Signed-off-by: Kees Cook There seems to be a helper 'is_compat_task()' that does 'test_thread_flag(TIF_32BIT)' so could perhaps be used instead, but that's too nit-picky. This change makes things more consistent with arch/arm and other arches, and stops use of STACK_RND_MASK for things that aren't stack related so seems good all round to me. Reviewed-by: Jon Medhurst > --- > arch/arm64/kernel/process.c | 15 ++++++++------- > 1 file changed, 8 insertions(+), 7 deletions(-) > > diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c > index 80624829db61..0d0969bcd76d 100644 > --- a/arch/arm64/kernel/process.c > +++ b/arch/arm64/kernel/process.c > @@ -382,13 +382,14 @@ unsigned long arch_align_stack(unsigned long sp) > return sp & ~0xf; > } > > -static unsigned long randomize_base(unsigned long base) > -{ > - unsigned long range_end = base + (STACK_RND_MASK << PAGE_SHIFT) + 1; > - return randomize_range(base, range_end, 0) ? : base; > -} > - > unsigned long arch_randomize_brk(struct mm_struct *mm) > { > - return randomize_base(mm->brk); > + unsigned long range_end = mm->brk; > + > + if (test_thread_flag(TIF_32BIT)) > + range_end += 0x02000000; > + else > + range_end += 0x40000000; > + > + return randomize_range(mm->brk, range_end, 0) ? : mm->brk; > } > -- > 2.6.3 > >