From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 8E13D2E1EF8; Tue, 27 Jan 2026 09:42:13 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1769506935; cv=none; b=O/So++2GxOTO4tCUjvdWFGpmM1Udy9aMaUuKMSNES/ktZkmN7bshdossaovcEk2ItDkP0FDVcqDmQpszmb6N+LL8pjIUGl8+bUfghx9Cd5A/DgcHxhLEczbHFkp1b90fdZ7xAGr+D7RsrIFLvEr/N7JbPWQrvM/M/KXxeRl16lg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1769506935; c=relaxed/simple; bh=Dc5cx4Hl/9rWDSVfiy8JhRlYwVnmfEbUfdDDWdzaYv4=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=cY55WizNrCdJgvUOd4hVNAm9KEMjjmSScMTsG65zE85LMHjkneDbTLpJB02UGKq4lMa6st+ybJIrMreu62Ka0xhzDk8cuRsypihpuecOayISFpBipsJcaIrWD8QhaZaZYxqqujWNh1fTkFLLDxA+CMlTSzHHcqvlLOL+l7v/RrM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com 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 2A34C339; Tue, 27 Jan 2026 01:42:06 -0800 (PST) Received: from [10.57.94.246] (unknown [10.57.94.246]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id D870A3F632; Tue, 27 Jan 2026 01:42:10 -0800 (PST) Message-ID: <823c6406-5b70-490d-a318-25ac48950ec6@arm.com> Date: Tue, 27 Jan 2026 09:42:09 +0000 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 02/10] arm64: Move fixmap page tables to end of kernel image Content-Language: en-GB To: Ard Biesheuvel , linux-kernel@vger.kernel.org Cc: linux-arm-kernel@lists.infradead.org, will@kernel.org, catalin.marinas@arm.com, mark.rutland@arm.com, Ard Biesheuvel , Anshuman Khandual , Liz Prucka , Seth Jenkins , Kees Cook , linux-hardening@vger.kernel.org References: <20260126092630.1800589-12-ardb+git@google.com> <20260126092630.1800589-14-ardb+git@google.com> From: Ryan Roberts In-Reply-To: <20260126092630.1800589-14-ardb+git@google.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 26/01/2026 09:26, Ard Biesheuvel wrote: > From: Ard Biesheuvel > > Move the fixmap page tables out of the BSS section, and place them at > the end of the image, right before the init_pg_dir section where some of > the other statically allocated page tables live. > > These page tables are currently the only data objects in vmlinux that > are meant to be accessed via the kernel image's linear alias, and so > placing them together allows the remainder of the data/bss section to be > remapped read-only or unmapped entirely. > > Signed-off-by: Ard Biesheuvel Reviewed-by: Ryan Roberts > --- > arch/arm64/kernel/vmlinux.lds.S | 5 +++++ > arch/arm64/mm/fixmap.c | 7 ++++--- > 2 files changed, 9 insertions(+), 3 deletions(-) > > diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S > index b2a093f5b3fc..c089b83308a5 100644 > --- a/arch/arm64/kernel/vmlinux.lds.S > +++ b/arch/arm64/kernel/vmlinux.lds.S > @@ -335,6 +335,11 @@ SECTIONS > __pi___bss_start = __bss_start; > > . = ALIGN(PAGE_SIZE); > + .pgdir : { > + __pgdir_start = .; > + *(.fixmap_bss) > + } Observing that this still gets zero-initialized: /* Clear BSS and the initial page tables */ memset(__bss_start, 0, (char *)init_pg_end - (char *)__bss_start); Thanks, Ryan > + > __pi_init_pg_dir = .; > . += INIT_DIR_SIZE; > __pi_init_pg_end = .; > diff --git a/arch/arm64/mm/fixmap.c b/arch/arm64/mm/fixmap.c > index c5c5425791da..b649ea1a46e4 100644 > --- a/arch/arm64/mm/fixmap.c > +++ b/arch/arm64/mm/fixmap.c > @@ -31,9 +31,10 @@ static_assert(NR_BM_PMD_TABLES == 1); > > #define BM_PTE_TABLE_IDX(addr) __BM_TABLE_IDX(addr, PMD_SHIFT) > > -static pte_t bm_pte[NR_BM_PTE_TABLES][PTRS_PER_PTE] __page_aligned_bss; > -static pmd_t bm_pmd[PTRS_PER_PMD] __page_aligned_bss __maybe_unused; > -static pud_t bm_pud[PTRS_PER_PUD] __page_aligned_bss __maybe_unused; > +#define __fixmap_bss __section(".fixmap_bss") __aligned(PAGE_SIZE) > +static pte_t bm_pte[NR_BM_PTE_TABLES][PTRS_PER_PTE] __fixmap_bss; > +static pmd_t bm_pmd[PTRS_PER_PMD] __fixmap_bss __maybe_unused; > +static pud_t bm_pud[PTRS_PER_PUD] __fixmap_bss __maybe_unused; > > static inline pte_t *fixmap_pte(unsigned long addr) > {