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Thu, 27 Aug 2026 09:44:32 -0700 (PDT) Date: Thu, 27 Aug 2026 18:44:12 +0200 In-Reply-To: <20260827164409.3421848-6-ardb+git@google.com> Mime-Version: 1.0 References: <20260827164409.3421848-6-ardb+git@google.com> X-Developer-Key: i=ardb@kernel.org; a=openpgp; fpr=F43D03328115A198C90016883D200E9CA6329909 X-Developer-Signature: v=1; a=openpgp-sha256; l=5363; i=ardb@kernel.org; h=from:subject; bh=IXwvq/N/tjoj1rBZtZdDN7mLt+hNS7LMJFa0rm3k0+Y=; b=owGbwMvMwCVmkMcZplerG8N4Wi2JIWtCZkzKm9Xbn65Wvf1HVXO2m+/z5fPaL7zrsHy43dJ5w ddHJXkzO0pZGMS4GGTFFFkEZv99t/P0RKla51myMHNYmUCGMHBxCsBEjEoY/qk95OHUKhcw3B27 Rl9VrelVyptFld+fWJ62rI/wquyJf8PIsKV41pJtfLPmrCkyO/FOQzDky5EoqdVnymPfdMWcS5s Uzg4A X-Mailer: git-send-email 2.55.0.897.gb25b4bd76c-goog Message-ID: <20260827164409.3421848-8-ardb+git@google.com> Subject: [RFC PATCH v2 2/4] arm64: mm: Use fault handler to permit swapper_pg_dir updates From: Ard Biesheuvel To: linux-kernel@vger.kernel.org Cc: linux-arm-kernel@lists.infradead.org, Ard Biesheuvel Content-Type: text/plain; charset="UTF-8" X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260827_094435_641344_A8608E07 X-CRM114-Status: GOOD ( 15.66 ) 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 From: Ard Biesheuvel Instead of checking the destination of a page table descriptor update against the virtual address of swapper_pg_dir in the kernel image, and taking a dedicated path to perform such updates via a fixmap alias, rely on a fault handler to do so if the plain store faulted. This is more efficient for the general case, and makes it more straight-forward to extend this logic to other page tables that may have been allocated statically in the .rodata region of the kernel image. Signed-off-by: Ard Biesheuvel --- arch/arm64/include/asm/fixmap.h | 1 + arch/arm64/include/asm/pgtable.h | 49 +++++++++----------- arch/arm64/mm/mmu.c | 18 +++---- 3 files changed, 32 insertions(+), 36 deletions(-) diff --git a/arch/arm64/include/asm/fixmap.h b/arch/arm64/include/asm/fixmap.h index 9191125738e9..8db791dcb4cb 100644 --- a/arch/arm64/include/asm/fixmap.h +++ b/arch/arm64/include/asm/fixmap.h @@ -99,6 +99,7 @@ enum fixed_addresses { FIX_PUD, FIX_P4D, FIX_PGD, + FIX_PTVAL, __end_of_fixed_addresses }; diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h index 6000905a2e86..799cd9d52c28 100644 --- a/arch/arm64/include/asm/pgtable.h +++ b/arch/arm64/include/asm/pgtable.h @@ -35,6 +35,7 @@ #include #include #include +#include #include #include #include @@ -814,7 +815,7 @@ extern pgd_t idmap_pg_dir[]; extern pgd_t tramp_pg_dir[]; extern pgd_t reserved_pg_dir[]; -extern void set_swapper_pgd(pgd_t *pgdp, pgd_t pgd); +void try_set_readonly_ptval(ptval_t *ptvalp, ptval_t ptval); static inline bool in_swapper_pgdir(void *addr) { @@ -824,17 +825,14 @@ static inline bool in_swapper_pgdir(void *addr) static inline void set_pmd(pmd_t *pmdp, pmd_t pmd) { -#ifdef __PAGETABLE_PMD_FOLDED - if (in_swapper_pgdir(pmdp)) { - set_swapper_pgd((pgd_t *)pmdp, __pgd(pmd_val(pmd))); - return; - } -#endif /* __PAGETABLE_PMD_FOLDED */ - - WRITE_ONCE(*pmdp, pmd); + __put_kernel_nofault(pmdp, &pmd, pmd_t, fault); if (pmd_valid(pmd)) queue_pte_barriers(); + return; + +fault: + try_set_readonly_ptval(&pmd_val(*pmdp), pmd_val(pmd)); } static inline void pmd_clear(pmd_t *pmdp) @@ -890,15 +888,14 @@ static inline bool pgtable_l4_enabled(void); static inline void set_pud(pud_t *pudp, pud_t pud) { - if (!pgtable_l4_enabled() && in_swapper_pgdir(pudp)) { - set_swapper_pgd((pgd_t *)pudp, __pgd(pud_val(pud))); - return; - } - - WRITE_ONCE(*pudp, pud); + __put_kernel_nofault(pudp, &pud, pud_t, fault); if (pud_valid(pud)) queue_pte_barriers(); + return; + +fault: + try_set_readonly_ptval(&pud_val(*pudp), pud_val(pud)); } static inline void pud_clear(pud_t *pudp) @@ -971,13 +968,12 @@ static inline bool mm_pud_folded(const struct mm_struct *mm) static inline void set_p4d(p4d_t *p4dp, p4d_t p4d) { - if (in_swapper_pgdir(p4dp)) { - set_swapper_pgd((pgd_t *)p4dp, __pgd(p4d_val(p4d))); - return; - } - - WRITE_ONCE(*p4dp, p4d); + __put_kernel_nofault(p4dp, &p4d, p4d_t, fault); queue_pte_barriers(); + return; + +fault: + try_set_readonly_ptval(&p4d_val(*p4dp), p4d_val(p4d)); } static inline void p4d_clear(p4d_t *p4dp) @@ -1099,13 +1095,12 @@ static inline bool mm_p4d_folded(const struct mm_struct *mm) static inline void set_pgd(pgd_t *pgdp, pgd_t pgd) { - if (in_swapper_pgdir(pgdp)) { - set_swapper_pgd(pgdp, __pgd(pgd_val(pgd))); - return; - } - - WRITE_ONCE(*pgdp, pgd); + __put_kernel_nofault(pgdp, &pgd, pgd_t, fault); queue_pte_barriers(); + return; + +fault: + try_set_readonly_ptval(&pgd_val(*pgdp), pgd_val(pgd)); } static inline void pgd_clear(pgd_t *pgdp) diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c index 9c1aa838e9d5..b97e4bf99ca5 100644 --- a/arch/arm64/mm/mmu.c +++ b/arch/arm64/mm/mmu.c @@ -63,34 +63,34 @@ static bool rodata_is_rw __ro_after_init = true; */ long __section(".mmuoff.data.write") __early_cpu_boot_status; -static DEFINE_SPINLOCK(swapper_pgdir_lock); static DEFINE_MUTEX(fixmap_lock); -void noinstr set_swapper_pgd(pgd_t *pgdp, pgd_t pgd) +void noinstr try_set_readonly_ptval(ptval_t *ptvalp, ptval_t ptval) { - pgd_t *fixmap_pgdp; + static DEFINE_SPINLOCK(lock); + + BUG_ON(!in_swapper_pgdir(ptvalp)); /* * Don't bother with the fixmap if swapper_pg_dir is still mapped * writable in the kernel mapping. */ if (rodata_is_rw) { - WRITE_ONCE(*pgdp, pgd); + WRITE_ONCE(*ptvalp, ptval); dsb(ishst); isb(); return; } - spin_lock(&swapper_pgdir_lock); - fixmap_pgdp = pgd_set_fixmap(__pa_symbol(pgdp)); - WRITE_ONCE(*fixmap_pgdp, pgd); + guard(spinlock)(&lock); + ptvalp = (ptval_t *)set_fixmap_offset(FIX_PTVAL, __pa_symbol(ptvalp)); + WRITE_ONCE(*ptvalp, ptval); /* * We need dsb(ishst) here to ensure the page-table-walker sees * our new entry before set_p?d() returns. The fixmap's * flush_tlb_kernel_range() via clear_fixmap() does this for us. */ - pgd_clear_fixmap(); - spin_unlock(&swapper_pgdir_lock); + clear_fixmap(FIX_PTVAL); } pgprot_t phys_mem_access_prot(struct file *file, unsigned long pfn, -- 2.55.0.887.g758fc8c411-goog