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Wed, 12 Aug 2020 14:16:02 +0000 Date: Wed, 12 Aug 2020 15:15:59 +0100 From: Szabolcs Nagy To: Will Deacon Cc: peterz@infradead.org, Ard Biesheuvel , Jessica Yu , Mauro Carvalho Chehab , Linux Kernel Mailing List , Thomas Gleixner , Kees Cook , Josh Poimboeuf , Miroslav Benes , Mark Rutland , nd@arm.com Subject: Re: [PATCH v2] module: Harden STRICT_MODULE_RWX Message-ID: <20200812141557.GQ14398@arm.com> References: <20200808101222.5103093e@coco.lan> <20200810092523.GA8612@linux-8ccs> <20200810150647.GB8612@linux-8ccs> <20200811163427.6edbf343@coco.lan> <20200811145524.GE2674@hirez.programming.kicks-ass.net> <20200811172738.2d632a09@coco.lan> <20200811160134.GA13652@linux-8ccs> <20200812104005.GN2674@hirez.programming.kicks-ass.net> <20200812125645.GA8675@willie-the-truck> Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20200812125645.GA8675@willie-the-truck> User-Agent: Mutt/1.9.4 (2018-02-28) X-ClientProxiedBy: LO2P265CA0458.GBRP265.PROD.OUTLOOK.COM (2603:10a6:600:a2::14) To AM6PR08MB3047.eurprd08.prod.outlook.com (2603:10a6:209:4c::23) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from arm.com (217.140.106.55) by LO2P265CA0458.GBRP265.PROD.OUTLOOK.COM (2603:10a6:600:a2::14) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3283.15 via Frontend Transport; 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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 12 Aug 2020 14:16:41.5311 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 5a3d6801-fb47-4413-061c-08d83eca55c4 X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d;Ip=[63.35.35.123];Helo=[64aa7808-outbound-1.mta.getcheckrecipient.com] X-MS-Exchange-CrossTenant-AuthSource: AM5EUR03FT010.eop-EUR03.prod.protection.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: DB6PR08MB2648 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org The 08/12/2020 13:56, Will Deacon wrote: > On Wed, Aug 12, 2020 at 12:40:05PM +0200, peterz@infradead.org wrote: > > On Wed, Aug 12, 2020 at 10:56:56AM +0200, Ard Biesheuvel wrote: > > > The module .lds has BYTE(0) in the section contents to prevent the > > > linker from pruning them entirely. The (NOLOAD) is there to ensure > > > that this byte does not end up in the .ko, which is more a matter of > > > principle than anything else, so we can happily drop that if it helps. > > > > > > However, this should only affect the PROGBITS vs NOBITS designation, > > > and so I am not sure whether it makes a difference. > > > > > > Depending on where the w^x check occurs, we might simply override the > > > permissions of these sections, and strip the writable permission if it > > > is set in the PLT handling init code, which manipulates the metadata > > > of all these 3 sections before the module space is vmalloc'ed. > > > > What's curious is that this seems the result of some recent binutils > > change. Every build with binutils-2.34 (or older) does not seem to > > generate these as WAX, but has the much more sensible WA. > > > > I suppose we can change the kernel check and 'allow' W^X for 0 sized > > sections, but I think we should still figure out why binutils-2.35 is > > now generating WAX sections all of a sudden, it might come bite us > > elsewhere. > > Agreed, I think it's important to figure out what's going on here before we > try to bodge around it. > > Adding Szabolcs, in case he has any ideas. > > To save him reading the whole thread, here's a summary: > > AArch64 kernel modules built with binutils 2.35 end up with a couple of > ELF sections marked as SHF_WRITE | SHF_ALLOC | SHF_EXECINSTR: > > [ 5] .plt PROGBITS 0000000000000388 01d000 000008 00 WAX 0 0 1 > [ 6] .init.plt NOBITS 0000000000000390 01d008 000008 00 WA 0 0 1 > [ 7] .text.ftrace_trampoline PROGBITS 0000000000000398 01d008 000008 00 WAX 0 0 1 > > This results in the module being rejected by our loader, because we don't > permit writable, executable mappings. > > Our linker script for these entries uses NOLOAD, so it's odd to see PROGBITS > appearing in the readelf output above (and older binutils emits NOBITS > sections). Anyway, here's the linker script: > > SECTIONS { > .plt (NOLOAD) : { BYTE(0) } > .init.plt (NOLOAD) : { BYTE(0) } > .text.ftrace_trampoline (NOLOAD) : { BYTE(0) } > } > > It appears that the name of the section influences the behaviour, as > Jessica observed [1] that sections named .text.* end up with PROGBITS, > whereas random naming such as ".test" ends up with NOBITS, as before. > > We've looked at the changelog between binutils 2.34 and 2.35, but nothing > stands out. Any clues? Is this intentional binutils behaviour? for me it bisects to https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=8c803a2dd7d3d742a3d0071914f557ef465afe71 i will have to investigate further what's going on. > > Thanks, > > Will > > [1] https://lore.kernel.org/r/20200812114127.GA10824@linux-8ccs.fritz.box