From 3ac2760ead2ccd7e0c2714aff6a12f81a16b2ce7 Mon Sep 17 00:00:00 2001 From: Will Deacon Date: Wed, 6 Dec 2017 10:42:10 +0000 Subject: [PATCH] arm64: SW PAN: Point saved ttbr0 at the zero page when switching to init_mm update_saved_ttbr0 mandates that mm->pgd is not swapper, since swapper contains kernel mappings and should never be installed into ttbr0. However, this means that callers must avoid passing the init_mm to update_saved_ttbr0 which in turn can cause the saved ttbr0 value to be out-of-date in the context of the idle thread. For example, EFI runtime services may leave the saved ttbr0 pointing at the EFI page table, and kernel threads may end up with stale references to freed page tables. This patch changes update_saved_ttbr0 so that the init_mm points the saved ttbr0 value to the empty zero page, which always exists and never contains valid translations. EFI and switch can then call into update_saved_ttbr0 unconditionally. Change-Id: I664095969fbd4d0b546f30aa4c312769e708d6de Cc: Mark Rutland Cc: Ard Biesheuvel Cc: Vinayak Menon Cc: Fixes: 39bc88e5e38e9b21 ("arm64: Disable TTBR0_EL1 during normal kernel execution") Reviewed-by: Catalin Marinas Reviewed-by: Mark Rutland Reported-by: Vinayak Menon Signed-off-by: Will Deacon Git-commit: 0adbdfde8cfc9415aeed2a4955d2d17b3bd9bf13 Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git Signed-off-by: Vinayak Menon --- arch/arm64/include/asm/efi.h | 4 +--- arch/arm64/include/asm/mmu_context.h | 22 +++++++++++++--------- 2 files changed, 14 insertions(+), 12 deletions(-) diff --git a/arch/arm64/include/asm/efi.h b/arch/arm64/include/asm/efi.h index 932f5a56d1a6..b7406e05056b 100644 --- a/arch/arm64/include/asm/efi.h +++ b/arch/arm64/include/asm/efi.h @@ -87,11 +87,9 @@ static inline void efi_set_pgd(struct mm_struct *mm) * Defer the switch to the current thread's TTBR0_EL1 * until uaccess_enable(). Restore the current * thread's saved ttbr0 corresponding to its active_mm - * (if different from init_mm). */ cpu_set_reserved_ttbr0(); - if (current->active_mm != &init_mm) - update_saved_ttbr0(current, current->active_mm); + update_saved_ttbr0(current, current->active_mm); } } } diff --git a/arch/arm64/include/asm/mmu_context.h b/arch/arm64/include/asm/mmu_context.h index f926b95928ee..7e00215aa9f5 100644 --- a/arch/arm64/include/asm/mmu_context.h +++ b/arch/arm64/include/asm/mmu_context.h @@ -183,11 +183,17 @@ enter_lazy_tlb(struct mm_struct *mm, struct task_struct *tsk) static inline void update_saved_ttbr0(struct task_struct *tsk, struct mm_struct *mm) { - if (system_uses_ttbr0_pan()) { - BUG_ON(mm->pgd == swapper_pg_dir); - task_thread_info(tsk)->ttbr0 = - virt_to_phys(mm->pgd) | ASID(mm) << 48; - } + u64 ttbr; + + if (!system_uses_ttbr0_pan()) + return; + + if (mm == &init_mm) + ttbr = __pa_symbol(empty_zero_page); + else + ttbr = virt_to_phys(mm->pgd) | ASID(mm) << 48; + + task_thread_info(tsk)->ttbr0 = ttbr; } #else static inline void update_saved_ttbr0(struct task_struct *tsk, @@ -223,11 +229,9 @@ switch_mm(struct mm_struct *prev, struct mm_struct *next, * Update the saved TTBR0_EL1 of the scheduled-in task as the previous * value may have not been initialised yet (activate_mm caller) or the * ASID has changed since the last run (following the context switch - * of another thread of the same process). Avoid setting the reserved - * TTBR0_EL1 to swapper_pg_dir (init_mm; e.g. via idle_task_exit). + * of another thread of the same process). */ - if (next != &init_mm) - update_saved_ttbr0(tsk, next); + update_saved_ttbr0(tsk, next); } #define deactivate_mm(tsk,mm) do { } while (0)