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b=nm3/Up7Fte+HsSj4Zi+YOwznbJD7iPvMtnYo665KKkx6dWJcE7Gzd0pPiMSQo+jK4 jne31Wtcxx5AG8x5gUZVH9V8RjxMZw7APbTvdkucjJW3hbn/Kxv2fYWn4d4iNwidcq oxgP3xyTt8eJTpuPlhI6M7N8t55F9QFPaHEBiZMxlU5purCnr6aeQmlod1arAb7qyT TejgemR812N9nuTYeJuBNAVDkdX15SepZsM2cm3UB+os/1GK6MzazI8reQvK4XdYJx 2vglztBGG3fVOstGGeKmJzZujOc0herLstlCzqy++2TxoMjnhbtu7LcpirMLnVykhl 0WusTNZOxXBdA== Message-ID: <3d1a6b5c-f3bf-462f-879a-cdb5b60868ac@kernel.org> Date: Tue, 28 Apr 2026 21:51:09 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 02/15] mm: Make empty_zero_page __ro_after_init To: Ard Biesheuvel , linux-arm-kernel@lists.infradead.org Cc: linux-kernel@vger.kernel.org, will@kernel.org, catalin.marinas@arm.com, mark.rutland@arm.com, Ard Biesheuvel , Ryan Roberts , Anshuman Khandual , Liz Prucka , Seth Jenkins , Kees Cook , Mike Rapoport , Andrew Morton , linux-mm@kvack.org, linux-hardening@vger.kernel.org References: <20260427153416.2103979-17-ardb+git@google.com> <20260427153416.2103979-19-ardb+git@google.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 99D5F160002 X-Stat-Signature: dmuqhpcgip7aogkj6b6ub5ggu6nne8sh X-Rspam-User: X-HE-Tag: 1777405877-507372 X-HE-Meta: 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 ovZUCU99 3H+rm4nFo508onCptE6xIkEDPgzXPHiUfa7iB1l3NZYK+0OMHEbzrr9vo8tkx////js5f/GlpmbqHkwn7jz4tB67I72uIBS7DdIfpwlJRdPvvZZhj91HYidOJtcqx90kA0bMEDW2YrliZZIx9b6Q5Vxk0hcv0/VMDJNJpFG4IkaiqliYAIOgscGRCNKuITEdFKJKleHI0Y1soRegY3NSZpO9FG7uY3cdwkvxmlcjRrTnoZwpxxYttU6HNlMK0O0oaeoP/6j9j8DxnvMqaZegCDYT30s1BNNaMUNZXf4iMvFrT7Q/qQJxDdGCg/E1GvgxD4mVUSyIEVpRxClY0QmvSUda/ozyfJlrZ91jOSV7rB/rF/MFwoE2/4f8UvylzT1V+8xK7wH5sVwnXbIaBKYbsVwyZRA== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 4/27/26 17:34, Ard Biesheuvel wrote: > From: Ard Biesheuvel > > The empty zero page is used to back any kernel or user space mapping > that is supposed to remain cleared, and so the page itself is never > supposed to be modified. > > So make it __ro_after_init rather than __page_aligned_bss: on most > architectures, this ensures that both the kernel's mapping of it and any > aliases that are accessible via the kernel direct (linear) map are > mapped read-only, and cannot be used (inadvertently or maliciously) to > corrupt the contents of the zero page. > > Signed-off-by: Ard Biesheuvel > --- > mm/mm_init.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/mm/mm_init.c b/mm/mm_init.c > index f9f8e1af921c..6ca01ed2a5a4 100644 > --- a/mm/mm_init.c > +++ b/mm/mm_init.c > @@ -57,7 +57,7 @@ unsigned long zero_page_pfn __ro_after_init; > EXPORT_SYMBOL(zero_page_pfn); > > #ifndef __HAVE_COLOR_ZERO_PAGE > -uint8_t empty_zero_page[PAGE_SIZE] __page_aligned_bss; > +uint8_t empty_zero_page[PAGE_SIZE] __ro_after_init __aligned(PAGE_SIZE); > EXPORT_SYMBOL(empty_zero_page); > > struct page *__zero_page __ro_after_init; I am no expert on BSS etc, but from what I understand, we'll still get zeroed page-aligned memory. I don't know if there is any other impact on not having it in bss.page_aligned. I assume no Acked-by: David Hildenbrand (Arm) -- Cheers, David