Re: [PATCH resend] powerpc/code-patching: Avoid r/w mapping of the zero page
From: "Christophe Leroy (CS GROUP)" <chleroy@kernel.org>
Date: 2026-05-20 09:36:47
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Le 20/05/2026 à 10:54, Ard Biesheuvel a écrit :
quoted hunk ↗ jump to hunk
The only remaining use of map_patch_area() is mapping the zero page, and immediately unmapping it again so that the intermediate page table levels are all guaranteed to be populated. The use of the zero page here is completely arbitrary, and not harmful per se, but currently, it creates a writable mapping, and does so in a manner that requires that the empty_zero_page[] symbol is not const-qualified. Given that this is about to change, and that map_patch_area() now never maps anything other than the zero page, let's simplify the code and - take the PA of empty_zero_page directly - create a read-only temporary mapping. This allows empty_zero_page[] to be repainted as const u8[] in a subsequent patch, without making substantial changes to this code patching logic. Cc: Madhavan Srinivasan <maddy@linux.ibm.com> Cc: Michael Ellerman <mpe@ellerman.id.au> Cc: Nicholas Piggin <npiggin@gmail.com> Cc: "Christophe Leroy (CS GROUP)" <chleroy@kernel.org> Signed-off-by: Ard Biesheuvel <ardb@kernel.org> --- Build tested only (Clang) Resending from my non-Google email. Apologies if you are receiving this twice. arch/powerpc/lib/code-patching.c | 11 +++++------ 1 file changed, 5 insertions(+), 6 deletions(-)diff --git a/arch/powerpc/lib/code-patching.c b/arch/powerpc/lib/code-patching.c index f84e0337cc02..13a8acf851f1 100644 --- a/arch/powerpc/lib/code-patching.c +++ b/arch/powerpc/lib/code-patching.c@@ -60,7 +60,7 @@ struct patch_context { static DEFINE_PER_CPU(struct patch_context, cpu_patching_context); -static int map_patch_area(void *addr, unsigned long text_poke_addr); +static int map_patch_area(unsigned long text_poke_addr); static void unmap_patch_area(unsigned long addr); static bool mm_patch_enabled(void)@@ -117,7 +117,7 @@ static int text_area_cpu_up(unsigned int cpu) // Map/unmap the area to ensure all page tables are pre-allocated addr = (unsigned long)area->addr; - err = map_patch_area(empty_zero_page, addr); + err = map_patch_area(addr);
I would get rid of map_patch_area() completely and just do:
err = map_kernel_page(addr, __pa_symbol(empty_zero_page), PAGE_KERNEL_RO);
By the way I don't know how vital it is to map in read-only but in case
it is see commit da3a3b0a0e38 ("powerpc/32s: map kasan zero shadow with
PAGE_READONLY instead of PAGE_KERNEL_RO")
quoted hunk ↗ jump to hunk
if (err) return err;@@ -236,11 +236,10 @@ static unsigned long get_patch_pfn(void *addr) /* * This can be called for kernel text or a module. */ -static int map_patch_area(void *addr, unsigned long text_poke_addr) +static int map_patch_area(unsigned long text_poke_addr) { - unsigned long pfn = get_patch_pfn(addr); - - return map_kernel_page(text_poke_addr, (pfn << PAGE_SHIFT), PAGE_KERNEL); + return map_kernel_page(text_poke_addr, __pa_symbol(empty_zero_page), + PAGE_KERNEL_RO); } static void unmap_patch_area(unsigned long addr)