From mboxrd@z Thu Jan 1 00:00:00 1970 From: Will Deacon Subject: Re: [PATCH] module/ksymtab: use 64-bit relative reference for target symbol Date: Thu, 23 May 2019 10:18:11 +0100 Message-ID: <20190523091811.GA26646@fuggles.cambridge.arm.com> References: <20190522150239.19314-1-ard.biesheuvel@arm.com> <293c9d0f-dc14-1413-e4b4-4299f0acfb9e@arm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: 8bit Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org To: Ard Biesheuvel Cc: linux-arm-kernel@lists.infradead.org, marc.zyngier@arm.com, james.morse@arm.com, guillaume.gardet@arm.com, mark.rutland@arm.com, mingo@kernel.org, jeyu@kernel.org, linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, arnd@arndb.de, x86@kernel.org List-Id: linux-arch.vger.kernel.org On Thu, May 23, 2019 at 09:41:40AM +0100, Ard Biesheuvel wrote: > > > On 5/22/19 5:28 PM, Ard Biesheuvel wrote: > > > > > > On 5/22/19 4:02 PM, Ard Biesheuvel wrote: > > > The following commit > > > > > >    7290d5809571 ("module: use relative references for __ksymtab entries") > > > > > > updated the ksymtab handling of some KASLR capable architectures > > > so that ksymtab entries are emitted as pairs of 32-bit relative > > > references. This reduces the size of the entries, but more > > > importantly, it gets rid of statically assigned absolute > > > addresses, which require fixing up at boot time if the kernel > > > is self relocating (which takes a 24 byte RELA entry for each > > > member of the ksymtab struct). > > > > > > Since ksymtab entries are always part of the same module as the > > > symbol they export (or of the core kernel), it was assumed at the > > > time that a 32-bit relative reference is always sufficient to > > > capture the offset between a ksymtab entry and its target symbol. > > > > > > Unfortunately, this is not always true: in the case of per-CPU > > > variables, a per-CPU variable's base address (which usually differs > > > from the actual address of any of its per-CPU copies) could be at > > > an arbitrary offset from the ksymtab entry, and so it may be out > > > of range for a 32-bit relative reference. > > > > > (Apologies for the 3-act monologue) Exposition, development and recapitulation ;) > This turns out to be incorrect. The symbol address of per-CPU variables > exported by modules is always in the vicinity of __per_cpu_start, and so it > is simply a matter of making sure that the core kernel is in range for > module ksymtab entries containing 32-bit relative references. > > When running the arm64 with kaslr enabled, we currently randomize the module > space based on the range of ADRP/ADD instruction pairs, which have a -/+ 4 > GB range rather than the -/+ 2 GB range of 32-bit place relative data > relocations. So we can fix this by simply reducing the randomization window > to 2 GB. Makes sense. Do you see the need for an option to disable PREL relocs altogether in case somebody wants the additional randomization range? Will From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from foss.arm.com ([217.140.101.70]:41126 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726429AbfEWJSS (ORCPT ); Thu, 23 May 2019 05:18:18 -0400 Date: Thu, 23 May 2019 10:18:11 +0100 From: Will Deacon Subject: Re: [PATCH] module/ksymtab: use 64-bit relative reference for target symbol Message-ID: <20190523091811.GA26646@fuggles.cambridge.arm.com> References: <20190522150239.19314-1-ard.biesheuvel@arm.com> <293c9d0f-dc14-1413-e4b4-4299f0acfb9e@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Sender: linux-arch-owner@vger.kernel.org List-ID: To: Ard Biesheuvel Cc: linux-arm-kernel@lists.infradead.org, marc.zyngier@arm.com, james.morse@arm.com, guillaume.gardet@arm.com, mark.rutland@arm.com, mingo@kernel.org, jeyu@kernel.org, linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, arnd@arndb.de, x86@kernel.org Message-ID: <20190523091811.xab-PnJQPv2oAyvIqf-Hw-xROyukLm6HMgDtvu-FKzs@z> On Thu, May 23, 2019 at 09:41:40AM +0100, Ard Biesheuvel wrote: > > > On 5/22/19 5:28 PM, Ard Biesheuvel wrote: > > > > > > On 5/22/19 4:02 PM, Ard Biesheuvel wrote: > > > The following commit > > > > > >    7290d5809571 ("module: use relative references for __ksymtab entries") > > > > > > updated the ksymtab handling of some KASLR capable architectures > > > so that ksymtab entries are emitted as pairs of 32-bit relative > > > references. This reduces the size of the entries, but more > > > importantly, it gets rid of statically assigned absolute > > > addresses, which require fixing up at boot time if the kernel > > > is self relocating (which takes a 24 byte RELA entry for each > > > member of the ksymtab struct). > > > > > > Since ksymtab entries are always part of the same module as the > > > symbol they export (or of the core kernel), it was assumed at the > > > time that a 32-bit relative reference is always sufficient to > > > capture the offset between a ksymtab entry and its target symbol. > > > > > > Unfortunately, this is not always true: in the case of per-CPU > > > variables, a per-CPU variable's base address (which usually differs > > > from the actual address of any of its per-CPU copies) could be at > > > an arbitrary offset from the ksymtab entry, and so it may be out > > > of range for a 32-bit relative reference. > > > > > (Apologies for the 3-act monologue) Exposition, development and recapitulation ;) > This turns out to be incorrect. The symbol address of per-CPU variables > exported by modules is always in the vicinity of __per_cpu_start, and so it > is simply a matter of making sure that the core kernel is in range for > module ksymtab entries containing 32-bit relative references. > > When running the arm64 with kaslr enabled, we currently randomize the module > space based on the range of ADRP/ADD instruction pairs, which have a -/+ 4 > GB range rather than the -/+ 2 GB range of 32-bit place relative data > relocations. So we can fix this by simply reducing the randomization window > to 2 GB. Makes sense. Do you see the need for an option to disable PREL relocs altogether in case somebody wants the additional randomization range? Will