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Date: Fri, 24 Jun 2022 15:07:44 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Will Deacon <will@...nel.org>
Cc: Linux ARM <linux-arm-kernel@...ts.infradead.org>,
linux-hardening@...r.kernel.org, Marc Zyngier <maz@...nel.org>,
Mark Rutland <mark.rutland@....com>,
Kees Cook <keescook@...omium.org>,
Catalin Marinas <catalin.marinas@....com>,
Mark Brown <broonie@...nel.org>,
Anshuman Khandual <anshuman.khandual@....com>
Subject: Re: [PATCH v4 17/26] arm64: head: populate kernel page tables with
MMU and caches on
On Fri, 24 Jun 2022 at 14:56, Will Deacon <will@...nel.org> wrote:
>
> On Mon, Jun 13, 2022 at 04:45:41PM +0200, Ard Biesheuvel wrote:
> > Now that we can access the entire kernel image via the ID map, we can
> > execute the page table population code with the MMU and caches enabled.
> > The only thing we need to ensure is that translations via TTBR1 remain
> > disabled while we are updating the page tables the second time around,
> > in case KASLR wants them to be randomized.
> >
> > Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
> > ---
> > arch/arm64/kernel/head.S | 62 +++++---------------
> > 1 file changed, 16 insertions(+), 46 deletions(-)
> >
> > diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> > index d704d0bd8ffc..583cbea865e1 100644
> > --- a/arch/arm64/kernel/head.S
> > +++ b/arch/arm64/kernel/head.S
> > @@ -85,8 +85,6 @@
> > * x21 primary_entry() .. start_kernel() FDT pointer passed at boot in x0
> > * x22 create_idmap() .. start_kernel() ID map VA of the DT blob
> > * x23 primary_entry() .. start_kernel() physical misalignment/KASLR offset
> > - * x28 clear_page_tables() callee preserved temp register
> > - * x19/x20 __primary_switch() callee preserved temp registers
> > * x24 __primary_switch() .. relocate_kernel() current RELR displacement
> > * x28 create_idmap() callee preserved temp register
> > */
> > @@ -96,9 +94,7 @@ SYM_CODE_START(primary_entry)
> > adrp x23, __PHYS_OFFSET
> > and x23, x23, MIN_KIMG_ALIGN - 1 // KASLR offset, defaults to 0
> > bl set_cpu_boot_mode_flag
> > - bl clear_page_tables
> > bl create_idmap
> > - bl create_kernel_mapping
> >
> > /*
> > * The following calls CPU setup code, see arch/arm64/mm/proc.S for
> > @@ -128,32 +124,14 @@ SYM_CODE_START_LOCAL(preserve_boot_args)
> > SYM_CODE_END(preserve_boot_args)
> >
> > SYM_FUNC_START_LOCAL(clear_page_tables)
> > - mov x28, lr
> > -
> > - /*
> > - * Invalidate the init page tables to avoid potential dirty cache lines
> > - * being evicted. Other page tables are allocated in rodata as part of
> > - * the kernel image, and thus are clean to the PoC per the boot
> > - * protocol.
> > - */
> > - adrp x0, init_pg_dir
> > - adrp x1, init_pg_end
> > - bl dcache_inval_poc
> > -
> > /*
> > * Clear the init page tables.
> > */
> > adrp x0, init_pg_dir
> > adrp x1, init_pg_end
> > - sub x1, x1, x0
> > -1: stp xzr, xzr, [x0], #16
> > - stp xzr, xzr, [x0], #16
> > - stp xzr, xzr, [x0], #16
> > - stp xzr, xzr, [x0], #16
> > - subs x1, x1, #64
> > - b.ne 1b
> > -
> > - ret x28
> > + sub x2, x1, x0
> > + mov x1, xzr
> > + b __pi_memset // tail call
> > SYM_FUNC_END(clear_page_tables)
> >
> > /*
> > @@ -399,16 +377,8 @@ SYM_FUNC_START_LOCAL(create_kernel_mapping)
> >
> > map_memory x0, x1, x5, x6, x7, x3, (VA_BITS - PGDIR_SHIFT), x10, x11, x12, x13, x14
> >
> > - /*
> > - * Since the page tables have been populated with non-cacheable
> > - * accesses (MMU disabled), invalidate those tables again to
> > - * remove any speculatively loaded cache lines.
> > - */
> > - dmb sy
> > -
> > - adrp x0, init_pg_dir
> > - adrp x1, init_pg_end
> > - b dcache_inval_poc // tail call
> > + dsb ishst // sync with page table walker
> > + ret
> > SYM_FUNC_END(create_kernel_mapping)
> >
> > /*
> > @@ -863,14 +833,15 @@ SYM_FUNC_END(__relocate_kernel)
> > #endif
> >
> > SYM_FUNC_START_LOCAL(__primary_switch)
> > -#ifdef CONFIG_RANDOMIZE_BASE
> > - mov x19, x0 // preserve new SCTLR_EL1 value
> > - mrs x20, sctlr_el1 // preserve old SCTLR_EL1 value
> > -#endif
> > -
> > - adrp x1, init_pg_dir
> > + adrp x1, reserved_pg_dir
> > adrp x2, init_idmap_pg_dir
> > bl __enable_mmu
> > +
> > + bl clear_page_tables
> > + bl create_kernel_mapping
> > +
> > + adrp x1, init_pg_dir
> > + load_ttbr1 x1, x1, x2
> > #ifdef CONFIG_RELOCATABLE
> > #ifdef CONFIG_RELR
> > mov x24, #0 // no RELR displacement yet
> > @@ -886,9 +857,8 @@ SYM_FUNC_START_LOCAL(__primary_switch)
> > * to take into account by discarding the current kernel mapping and
> > * creating a new one.
> > */
> > - pre_disable_mmu_workaround
> > - msr sctlr_el1, x20 // disable the MMU
> > - isb
> > + adrp x1, reserved_pg_dir // Disable translations via TTBR1
> > + load_ttbr1 x1, x1, x2
>
> I'd have thought we'd need some TLB maintenance here... is that not the
> case?
>
You mean at this particular point? We are running from the ID map with
TTBR1 translations disabled. We clear the page tables, repopulate
them, and perform a TLBI VMALLE1.
So are you saying repopulating the page tables while translations are
disabled needs to occur only after doing TLB maintenance?
> Also, it might be a tiny bit easier to clear EPD1 instead of using the
> reserved_pg_dir.
>
Right. So is there any reason in particular why it would be
appropriate here but not anywhere else? IOW, why do we have
reserved_pg_dir in the first place if we can just flick EPD1 on and
off?
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