From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id ACBA5C61DC7 for ; Thu, 27 Aug 2026 10:00:40 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:Content-Transfer-Encoding: Content-Type:In-Reply-To:From:References:Cc:To:Subject:MIME-Version:Date: Message-ID:Reply-To:Content-ID:Content-Description:Resent-Date:Resent-From: Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=CkZRYqBA1mCcXz21fb+xuIP1G6kRk3AykW7MgFGu0Go=; b=xBnG+PWQnEwBLwoRDxhQrLzozR ciRPWzSb4mVGmaXX5i8BeEfT30lqlMo2KjKfdmArFcBVRwiN3xyGZehWOnaG1p28GJK1DeWpxTIhX Z9c7X3ml/mEN8PvULxNPtKwywhYZRl503qK+xd+TN7HLd91Oy91CmR+2geCSA0n5Kavd9nm01vRwf Cq+pjEK23a8SENvuS48/H4+TOWQwGfgMpyPjxltATNoVyN79zRjKtc9vIedlT01yb30rFTqz6zZW2 MmQDBbqHD4ENBhSm5fkoRZ+ds3DL1IWoYXJ5FQzC7EgYYEY3vZWHcBXjOEMJYXairhS3ox6qTpmwu I/n9pWPg==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.99.1 #2 (Red Hat Linux)) id 1wzWuL-00000003nKu-45cG; Thu, 27 Aug 2026 10:00:29 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.99.1 #2 (Red Hat Linux)) id 1wzWuI-00000003nKI-2Arj for linux-arm-kernel@lists.infradead.org; Thu, 27 Aug 2026 10:00:28 +0000 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 8A52E152B; Thu, 27 Aug 2026 03:00:20 -0700 (PDT) Received: from [10.43.20.79] (e126510-lin.lund.arm.com [10.43.20.79]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 605093F66F; Thu, 27 Aug 2026 03:00:21 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1787824824; bh=N6tRlbMdPzHcK7LXdQ66VmH7fyetGjDjgJHta0FwHo4=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=RSOBswGlGSPkd1nR7KER8o9NgcnSJCgNuGoVdWEkf1sPYoTS2FDHAijLap/iN9N8m cOhFoqQIw5zB4nKdu77GuIpS3NpyAqZ1g2OCitZmDf3fDWwc6SN0OxzA1esIUNJG6b VjsmBCAkBFTm6PCZPDUEbXI1HeypOrYPW6Yv9O6I= Message-ID: Date: Thu, 27 Aug 2026 12:00:18 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH] arm64: mm: Map fixmap PTE tables r/o in the linear map To: Ard Biesheuvel , Ard Biesheuvel , linux-arm-kernel@lists.infradead.org Cc: Will Deacon , Catalin Marinas , Mark Rutland , Ryan Roberts , Anshuman Khandual , Liz Prucka , Seth Jenkins , Kees Cook , Jann Horn , linux-hardening@vger.kernel.org References: <20260805104042.1107678-2-ardb+git@google.com> <5a3b89a6-bc42-42c3-82e8-25d6f1a2c5d2@arm.com> <1c0299ab-9f5f-4c7b-826e-a0547f7863dd@app.fastmail.com> From: Kevin Brodsky Content-Language: en-GB In-Reply-To: <1c0299ab-9f5f-4c7b-826e-a0547f7863dd@app.fastmail.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260827_030026_729966_95D933E4 X-CRM114-Status: GOOD ( 26.89 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 27/08/2026 11:04, Ard Biesheuvel wrote: > On Thu, 27 Aug 2026, at 10:55, Kevin Brodsky wrote: >> On 26/08/2026 11:07, Ard Biesheuvel wrote: >>>>> [...] >>>>> >>>>> +static int __init fixmap_remap_ro(void) >>>>> +{ >>>>> + return set_direct_map_ro((unsigned long)lm_alias(&bm_pte), NR_BM_PTE_TABLES); >>>> Should we not also remap bm_pmd and bm_pud? >>>> >>>> For that matter, do we need RW access via the linear map for any page >>>> annotated with __bss_pgtbl? I suppose that might be the case for >>>> kasan_early_shadow_* but I don't know enough about KASAN to tell for sure. >>>> >>> bm_pte[] is special because it is only ever written via the kernel mapping, >>> and never via the linear map. This is why it is being singled out in this >>> patch. >> Right I see __set_fixmap(). This is a very special case of calling >> __set_pte() on a pointer derived from a global (i.e. pointing to the >> kernel image and not the LM) so it makes sense to treat it differently. >> >> I do wonder whether there is that much value in protecting bm_pte while >> all page tables in init_pg_dir (also at a fixed offset in the LM) remain >> writeable though. >> > Yes, so those need to be taken care of as well. But init_pg_dir needs to remain writeable in the LM, right? >> Either way all this is interesting for my series protecting page tables >> with pkeys [1] - it seems that to protect the fixmap, the easiest option >> would be to change the pkey of .pgtlb in both the kernel image and LM >> (the former for bm_pte, and the latter for everything else). >> >> [1] https://lore.kernel.org/all/20260818-kpkeys-v9-0-743ad31b2c8f@arm.com/ >> > Thanks, I've been meaning to go through those but they are a bit overwhelming :-) Yes there's a fair bit to go through, sorry for that... You'll probably be most interested in patch 12, 13 and 19-21 - that's where page tables are being mapped with a special pkey. The cover letter should give enough context for them to make sense on their own. >>> Whether or not bm_pmd[] can be treated as a special case depends on the page >>> size: with 4k pages, the whole array covers a virtual region of 1G, which is >>> currently guaranteed to be shared only with the PCI I/O space (but we could >>> move that out). With 16k pages, it covers 64G, and so it is shared with the >>> vmemmap and other virtual mappings in the vmalloc region, and so the current >>> kernel mapping code expects to be able to write those entries. >>> >>> What we might do is generalize the logic that uses the fixmap to modify >>> pgd level entries in swapper_pg_dir, and use it for all modifications >>> at PMD level or higher if those tables are in .rodata >>> >>> But this is a bit more complicated than this change, so I decided to >>> present this as a separate change. >> Definitely, this is orthogonal to this patch. I wonder what the >> performance impact would be if we used the fixmap for setting all kernel >> entries at PMD and above. It would be nice to keep that logic easily >> togglable, because with the kpkeys approach I mentioned above we have a >> (most likely) much cheaper way of protecting these page tables. >> > I'm currently experimenting with using __put_kernel_nofault() to update > the descriptors at all levels (including PTE) and handling the fault > using the fixmap approach. That way, all statically allocated page > tables could move to .rodata (except bm_pte[]) and there should be no > substantial performance impact. Doesn't that still mean modifying a fixmap PTE every time we update kernel page tables? That doesn't exactly sound cheap. >>>>> +} >>>>> +late_initcall(fixmap_remap_ro); >>>> Is mark_rodata_ro() definitely too early to remap these pages RO? >>>> >>> No, we might just call this from there, afaict. >> Sounds good, could we then use update_mapping_prot()? I suppose not as >> that would require mapping bm_pte separately in map_mem()? >> > Yeah, but perhaps we should do the latter anyway, so we can remap the > region even if !can_set_direct_map(). Yep makes sense. Not having to add yet another set_direct_map_* function is definitely welcome ;) - Kevin