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 899CD3081A2; Tue, 27 Jan 2026 10:33:21 +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=1769510003; cv=none; b=okwS5Vta4Cl9bv8rHcOlia1Hp9liGAa0xdP4mGvOiVbC7FeTp4ZYrgYrKmu2Pa2FJUsIRdXA8E4lo1Qi4BlDD68ppXA4nga5W5XCjJ74XHP3QRzwb01TvFsBFXmWGVBlSt4dFFugkBqMtI5FZtWBDHdeFsXeRt5q8ITdPe5EOlA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1769510003; c=relaxed/simple; bh=E+gvFyaKSOt/vHyPXu0xAYjYDIi4yhB6ba6N+nJHkIU=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=rJ5Kv7gfDNY60Kbg+GKttPgYJN/58qns2Caq6pjnMQvBgfnvHY8LKKTDUqBKv1fyM6nkn9DDWaLiDRaUIln3FoJdHLm8xvIGoZzrEE2mUO082DdbEU5Jm6Ce998+jdFA246bfMd0Zn3kErVCQgSO+e1R2KBuDAHShSEXf8jLGIc= 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 833811595; Tue, 27 Jan 2026 02:33:14 -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 3A0B33F73F; Tue, 27 Jan 2026 02:33:19 -0800 (PST) Message-ID: <62b1723a-2ff3-4cac-ad99-a0e8d388ef12@arm.com> Date: Tue, 27 Jan 2026 10:33:17 +0000 Precedence: bulk X-Mailing-List: linux-hardening@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 09/10] arm64: mm: Map the kernel data/bss read-only in the linear map 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> <20260126092630.1800589-21-ardb+git@google.com> From: Ryan Roberts In-Reply-To: <20260126092630.1800589-21-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 > > On systems where the bootloader adheres to the original arm64 boot > protocol, the placement of the kernel in the physical address space is > highly predictable, and this makes the placement of its linear alias in > the kernel virtual address space equally predictable, given the lack of > randomization of the linear map. > > The linear aliases of the kernel text and rodata regions are already > mapped read-only, but the kernel data and bss are mapped read-write in > this region. This is not needed, so map them read-only as well. > > Note that the statically allocated kernel page tables do need to be > modifiable via the linear map, so leave these mapped read-write. > > Signed-off-by: Ard Biesheuvel > --- > arch/arm64/include/asm/sections.h | 1 + > arch/arm64/mm/mmu.c | 10 ++++++++-- > 2 files changed, 9 insertions(+), 2 deletions(-) > > diff --git a/arch/arm64/include/asm/sections.h b/arch/arm64/include/asm/sections.h > index 51b0d594239e..f7fe2bcbfd03 100644 > --- a/arch/arm64/include/asm/sections.h > +++ b/arch/arm64/include/asm/sections.h > @@ -23,6 +23,7 @@ extern char __irqentry_text_start[], __irqentry_text_end[]; > extern char __mmuoff_data_start[], __mmuoff_data_end[]; > extern char __entry_tramp_text_start[], __entry_tramp_text_end[]; > extern char __relocate_new_kernel_start[], __relocate_new_kernel_end[]; > +extern char __pgdir_start[]; > > static inline size_t entry_tramp_text_size(void) > { > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c > index 18415d4743bf..fdbbb018adc5 100644 > --- a/arch/arm64/mm/mmu.c > +++ b/arch/arm64/mm/mmu.c > @@ -1122,7 +1122,9 @@ static void __init map_mem(void) > { > static const u64 direct_map_end = _PAGE_END(VA_BITS_MIN); > phys_addr_t kernel_start = __pa_symbol(_text); > - phys_addr_t kernel_end = __pa_symbol(__init_begin); > + phys_addr_t init_begin = __pa_symbol(__init_begin); > + phys_addr_t init_end = __pa_symbol(__init_end); > + phys_addr_t kernel_end = __pa_symbol(__pgdir_start); > phys_addr_t start, end; > phys_addr_t early_kfence_pool; > int flags = NO_EXEC_MAPPINGS; > @@ -1158,7 +1160,9 @@ static void __init map_mem(void) > * Note that contiguous mappings cannot be remapped in this way, > * so we should avoid them here. > */ > - __map_memblock(kernel_start, kernel_end, PAGE_KERNEL, > + __map_memblock(kernel_start, init_begin, PAGE_KERNEL, > + flags | NO_CONT_MAPPINGS); > + __map_memblock(init_end, kernel_end, PAGE_KERNEL, > flags | NO_CONT_MAPPINGS); I'm probably being dumb again... why map [init_end, kernel_end) RW here, only to remap RO below? Why not just map RO here? > > /* map all the memory banks */ > @@ -1172,6 +1176,8 @@ static void __init map_mem(void) > flags); > } > > + __map_memblock(init_end, kernel_end, PAGE_KERNEL_RO, > + flags | NO_CONT_MAPPINGS); This seems iffy since __map_memblock() doesn't flush the tlb. If you want to update an existing mapping you want to be calling update_mapping_prot() right? Thanks, Ryan > arm64_kfence_map_pool(early_kfence_pool); > } >