Re: [PATCH v3 2/2] arm64: mm: install KPTI nG mappings with MMU enabled

[Ard Biesheuvel <ardb@kernel.org>]

On Fri, 20 May 2022 at 15:00, Mark Rutland <mark.rutland@arm.com> wrote:
>
> Hi Ard,
>
> On Thu, Apr 21, 2022 at 04:03:39PM +0200, Ard Biesheuvel wrote:
> > In cases where we unmap the kernel while running in user space, we rely
> > on ASIDs to distinguish the minimal trampoline from the full kernel
> > mapping, and this means we must use non-global attributes for those
> > mappings, to ensure they are scoped by ASID and will not hit in the TLB
> > inadvertently.
> >
> > We only do this when needed, as this is generally more costly in terms
> > of TLB pressure, and so we boot without these non-global attributes, and
> > apply them to all existing kernel mappings once all CPUs are up and we
> > know whether or not the non-global attributes are needed. At this point,
> > we cannot simply unmap and remap the entire address space, so we have to
> > update all existing block and page descriptors in place.
> >
> > Currently, we go through a lot of trouble to perform these updates with
> > the MMU and caches off, to avoid violating break before make (BBM) rules
> > imposed by the architecture. Since we make changes to page tables that
> > are not covered by the ID map, we gain access to those descriptors by
> > disabling translations altogether. This means that the stores to memory
> > are issued with device attributes, and require extra care in terms of
> > coherency, which is costly. We also rely on the ID map to access a
> > shared flag, which requires the ID map to be executable and writable at
> > the same time, which is another thing we'd prefer to avoid.
> >
> > So let's switch to an approach where we replace the kernel mapping with
> > a minimal mapping of a few pages that can be used for the shared flag,
> > as well as a minimal, ad-hoc fixmap that we can use to map each page
> > table in turn as we traverse the hierarchy. This requires one PTE per
> > level, and an associated page worth of VA space in the temporary
> > mapping.
>
> I really like the high-level idea!
>

Thanks!

> Overall this looks good. I'm not fond of the way we synchronize between
> primary/secondary CPUs, and would prefer if we could split that differently --
> I've given more detail on that below (and a branch with a proprosal).
>
> > Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> > ---
> >  arch/arm64/include/asm/mmu.h   |   4 +
> >  arch/arm64/kernel/cpufeature.c |  65 +++++++++++-
> >  arch/arm64/mm/mmu.c            |   8 +-
> >  arch/arm64/mm/proc.S           | 107 ++++++++++++--------
> >  4 files changed, 134 insertions(+), 50 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/mmu.h b/arch/arm64/include/asm/mmu.h
> > index 48f8466a4be9..b896f0ac4985 100644
> > --- a/arch/arm64/include/asm/mmu.h
> > +++ b/arch/arm64/include/asm/mmu.h
> > @@ -68,6 +68,10 @@ extern void init_mem_pgprot(void);
> >  extern void create_pgd_mapping(struct mm_struct *mm, phys_addr_t phys,
> >                              unsigned long virt, phys_addr_t size,
> >                              pgprot_t prot, bool page_mappings_only);
> > +extern void __create_pgd_mapping(pgd_t *pgdir, phys_addr_t phys,
> > +                              unsigned long virt, phys_addr_t size,
> > +                              pgprot_t prot,
> > +                              phys_addr_t (*pgtable_alloc)(int), int flags);
> >  extern void *fixmap_remap_fdt(phys_addr_t dt_phys, int *size, pgprot_t prot);
> >  extern void mark_linear_text_alias_ro(void);
> >  extern bool kaslr_requires_kpti(void);
> > diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> > index d72c4b4d389c..f0688e812e19 100644
> > --- a/arch/arm64/kernel/cpufeature.c
> > +++ b/arch/arm64/kernel/cpufeature.c
> > @@ -1596,14 +1596,31 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
> >  }
> >
> >  #ifdef CONFIG_UNMAP_KERNEL_AT_EL0
> > +#define KPTI_NG_TEMP_VA              (-(1UL << PMD_SHIFT))
> > +
> > +static phys_addr_t kpti_ng_temp_alloc;
> > +
> > +static phys_addr_t kpti_ng_pgd_alloc(int shift)
> > +{
> > +     kpti_ng_temp_alloc -= PAGE_SIZE;
> > +     return kpti_ng_temp_alloc;
> > +}
> > +
> >  static void __nocfi
> >  kpti_install_ng_mappings(const struct arm64_cpu_capabilities *__unused)
> >  {
> > -     typedef void (kpti_remap_fn)(int, int, phys_addr_t);
> > +     static atomic_t flag = ATOMIC_INIT(0);
> > +     static pgd_t *kpti_ng_temp_pgd;
> > +     static u64 alloc;
> > +
> > +     typedef void (kpti_remap_fn)(int, int, phys_addr_t, unsigned long);
> >       extern kpti_remap_fn idmap_kpti_install_ng_mappings;
> >       kpti_remap_fn *remap_fn;
> >
> > -     int cpu = smp_processor_id();
> > +     int levels = CONFIG_PGTABLE_LEVELS;
> > +     int order = order_base_2(levels + 1);
> > +     int num_cpus = num_online_cpus();
> > +     int primary = 0;
> >
> >       if (__this_cpu_read(this_cpu_vector) == vectors) {
> >               const char *v = arm64_get_bp_hardening_vector(EL1_VECTOR_KPTI);
> > @@ -1619,14 +1636,54 @@ kpti_install_ng_mappings(const struct arm64_cpu_capabilities *__unused)
> >       if (arm64_use_ng_mappings)
> >               return;
> >
> > +     // First CPU to arrive here gets the job
> > +     if (atomic_inc_return(&flag) == 1) {
>
> I'm not too keen on the way this is split, because (aside from the flag), the
> secondaries have no need to have anything mapped in TTBR1. If we leave the flag
> in the idmap, and have the secondaries all use the reserved TTBR1 page, we can
> consistently have the boot CPU do all the hard work (and we could avoid
> broadcast maintenance/barriers during bulk of the rewrite, which could make it
> faster).
>
> I had a go at refactoring things that way (with this rebased atop), at:
>
>   https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/log/?h=arm64/kpti/rework
>
>
>
> The only downside is that we have to keep the idmap RWX, but that was already
> the case (and we could rework the idmap to have separate ROX/RW code/data pages
> in future).
>

This was kind of the point - I stumbled onto this while working on my
WXN changes, which broke this code.

> I can post that to the list if you don't have major objections to that
> approach.
>

Not at all. We can revisit the ID map RWX issue later, but I'm eager
to get that fixed at some point.

> > +             alloc =  __get_free_pages(GFP_ATOMIC | __GFP_ZERO, order);
> > +             kpti_ng_temp_pgd = (pgd_t *)(alloc + levels * PAGE_SIZE);
> > +             kpti_ng_temp_alloc = __pa(kpti_ng_temp_pgd);
> > +             primary = 1;
> > +
> > +             //
> > +             // Create a minimal page table hierarchy that permits us to
> > +             // store a shared variable that secondaries will poll, and to
> > +             // map the swapper page tables temporarily as we traverse them.
> > +             //
> > +             // The physical pages are laid out as follows:
> > +             //
> > +             // +---------+--------+-/-------+-/------ +-\\--------+
> > +             // :  data   :  PTE[] : | PMD[] : | PUD[] : || PGD[]  :
> > +             // +---------+--------+-\-------+-\------ +-//--------+
> > +             //      ^         ^
> > +             // The first two pages are mapped consecutively into this
> > +             // hierarchy at a PMD_SHIFT aligned virtual address, so that we
> > +             // have a place to store the shared variable, and so that we
> > +             // can manipulate the PTE level entries while the mapping is
> > +             // active.  The first two entries cover the data page and the
> > +             // PTE[] page itself, the remaining entries are free to be used
> > +             // as a ad-hoc fixmap.
> > +             //
> > +             __create_pgd_mapping(kpti_ng_temp_pgd, __pa(alloc),
> > +                                  KPTI_NG_TEMP_VA, 2 * PAGE_SIZE,
> > +                                  PAGE_KERNEL, kpti_ng_pgd_alloc, 0);
>
> It took me a while to realise this was relying on the order in which
> __create_pgd_mapping() calls kpti_ng_pgd_alloc(). I had a look at building the
> tables explicitly to make this clearer, but that didn't seem all that clearer,
> so this is fine as-is.
>

Yeah. On the one hand, it is kind of dodgy to rely on this but on the
other hand, it is hard to imagine how changes to the page table
routines would result in these pages to be allocated in a different
order.

We could add a comment, I suppose.

> > +
> > +             // Increment flag again to signal other CPUs to proceed as well
> > +             atomic_inc_return_release(&flag);
> > +     } else {
> > +             // Wait for the primary CPU to set up the temporary page tables
> > +             while (atomic_read(&flag) <= num_cpus)
> > +                     cpu_relax();
> > +     }
> >       remap_fn = (void *)__pa_symbol(function_nocfi(idmap_kpti_install_ng_mappings));
> >
> >       cpu_install_idmap();
> > -     remap_fn(cpu, num_online_cpus(), __pa_symbol(swapper_pg_dir));
> > +     remap_fn(!primary, num_cpus - 1, __pa(kpti_ng_temp_pgd), KPTI_NG_TEMP_VA);
> >       cpu_uninstall_idmap();
> >
> > -     if (!cpu)
> > +     // Last CPU to leave frees the pages
> > +     if (atomic_dec_return(&flag) == 1) {
> > +             free_pages(alloc, order);
> >               arm64_use_ng_mappings = true;
> > +     }
> >  }
> >  #else
> >  static void
> > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> > index 626ec32873c6..1c7299dfaa84 100644
> > --- a/arch/arm64/mm/mmu.c
> > +++ b/arch/arm64/mm/mmu.c
> > @@ -360,11 +360,9 @@ static void alloc_init_pud(pgd_t *pgdp, unsigned long addr, unsigned long end,
> >               mutex_unlock(&fixmap_lock);
> >  }
> >
> > -static void __create_pgd_mapping(pgd_t *pgdir, phys_addr_t phys,
> > -                              unsigned long virt, phys_addr_t size,
> > -                              pgprot_t prot,
> > -                              phys_addr_t (*pgtable_alloc)(int),
> > -                              int flags)
> > +void __create_pgd_mapping(pgd_t *pgdir, phys_addr_t phys, unsigned long virt,
> > +                       phys_addr_t size, pgprot_t prot,
> > +                       phys_addr_t (*pgtable_alloc)(int), int flags)
> >  {
> >       unsigned long addr, end, next;
> >       pgd_t *pgdp = pgd_offset_pgd(pgdir, virt);
> > diff --git a/arch/arm64/mm/proc.S b/arch/arm64/mm/proc.S
> > index 5619c00f8cd4..20d726207db5 100644
> > --- a/arch/arm64/mm/proc.S
> > +++ b/arch/arm64/mm/proc.S
> > @@ -14,6 +14,7 @@
> >  #include <asm/asm-offsets.h>
> >  #include <asm/asm_pointer_auth.h>
> >  #include <asm/hwcap.h>
> > +#include <asm/kernel-pgtable.h>
> >  #include <asm/pgtable-hwdef.h>
> >  #include <asm/cpufeature.h>
> >  #include <asm/alternative.h>
> > @@ -200,20 +201,21 @@ SYM_FUNC_END(idmap_cpu_replace_ttbr1)
> >       .popsection
> >
> >  #ifdef CONFIG_UNMAP_KERNEL_AT_EL0
> > +
> > +#define KPTI_NG_PTE_FLAGS    (PTE_ATTRINDX(MT_NORMAL) | SWAPPER_PTE_FLAGS)
> > +
> >       .pushsection ".idmap.text", "awx"
> >
> >       .macro  kpti_mk_tbl_ng, type, num_entries
> >       add     end_\type\()p, cur_\type\()p, #\num_entries * 8
> >  .Ldo_\type:
> > -     dc      cvac, cur_\()\type\()p          // Ensure any existing dirty
> > -     dmb     sy                              // lines are written back before
> > -     ldr     \type, [cur_\type\()p], #8      // loading the entry
> > -     tbz     \type, #0, .Lnext_\type         // Skip invalid and
> > -     tbnz    \type, #11, .Lnext_\type        // non-global entries
> > -     orr     \type, \type, #PTE_NG           // Same bit for blocks and pages
> > -     str     \type, [cur_\type\()p, #-8]     // Update the entry and ensure
> > -     dmb     sy                              // that it is visible to all
> > -     dc      civac, cur_\()\type\()p         // CPUs.
> > +     ldr     \type, [cur_\type\()p], #8      // Load the entry
> > +     .ifnc   \type, pte
> > +     tbnz    \type, #11, .Lnext_\type        // Skip visited entries
> > +     .endif
> > +     and     valid, \type, #1
> > +     orr     \type, \type, valid, lsl #11    // nG |= valid
> > +     str     \type, [cur_\type\()p, #-8]     // Update the entry
>
> IMO it was clearer to skip invalid entries than to extract the valid bit value
> and ORR that back into the nG position. Was that change intended as an
> optimization?
>

More or less. Due to some misinterpreted performance numbers, during
development I ended up with a SIMD version for level 3 that updated 8
entries at a time, and so ORRing the valid bits into position #11 was
necessary.

I kept it because the bulk of the work (and therefore the reason for
the huge delay) is the linear map being mapped down to pages, and so
virtually all entries are valid. Omitting the conditional branch is
unlikely to make a measurable difference in performance, though, so we
can reinstate it if you prefer.

> >       .ifnc   \type, pte
> >       tbnz    \type, #1, .Lderef_\type
> >       .endif
>
> As with the last patch, I'd prfeer is we had an explicit ADD for the increment
> in the `.Lnext_\type`. Even with the CMOs removed, I reckon that's clearer.
>

Fine with me.

> > @@ -222,19 +224,42 @@ SYM_FUNC_END(idmap_cpu_replace_ttbr1)
> >       b.ne    .Ldo_\type
> >       .endm
> >
> > +     /*
> > +      * Dereference the current table entry and map it into the temporary
> > +      * fixmap slot associated with the current level. The ad-hoc fixmap
> > +      * is a set of PTEs that are located above the PTEs that cover the
> > +      * level 3 page table and the scratch page that precedes it.
> > +      */
> > +     .macro  kpti_map_pgtbl, type, level
> > +     str     xzr, [temp_pte, #8 * (\level + 2)]      // break before make
> > +     dsb     ishst
> > +     add     pte, flag_ptr, #PAGE_SIZE * (\level + 2)
> > +     lsr     pte, pte, #12
> > +     tlbi    vaae1is, pte
> > +     dsb     ish
> > +     isb
> > +
> > +     phys_to_pte pte, cur_\type\()p
> > +     add     cur_\type\()p, flag_ptr, #PAGE_SIZE * (\level + 2)
> > +     orr     pte, pte, pte_flags
> > +     str     pte, [temp_pte, #8 * (\level + 2)]
> > +     dsb     ishst
> > +     .endm
>
> If we only had the boot CPU map the fixmap TTBR, we could reduce the scope of
> barriers and TLB maintenance here to be nSH, and removing the broadcast
> maintenance could be a big win. We'd just need a final DSB ISHST prior to
> waking secondaries to ensure they see all the updates.
>
> Mybe that's miniscule compared to keeping the MMU on, though.
>

To be honest, the only observable cq. measurable difference on TX2 is
keeping the MMU enabled. Everything else was in the noise.


> > +
> >  /*
> > - * void __kpti_install_ng_mappings(int cpu, int num_cpus, phys_addr_t swapper)
> > + * void __kpti_install_ng_mappings(int cpu, int num_secondaries, phys_addr_t temp_pgd,
> > + *                              unsigned long kpti_ng_temp_va)
> >   *
> >   * Called exactly once from stop_machine context by each CPU found during boot.
> >   */
> > -__idmap_kpti_flag:
> > -     .long   1
> >  SYM_FUNC_START(idmap_kpti_install_ng_mappings)
> > -     cpu             .req    w0
> > +     cpu             .req    w0      // at entry
> > +     pte_flags       .req    x0
> >       num_cpus        .req    w1
> > -     swapper_pa      .req    x2
> > -     swapper_ttb     .req    x3
> > -     flag_ptr        .req    x4
> > +     temp_pgd_phys   .req    x2      // at entry
> > +     temp_pte        .req    x2
> > +     flag_ptr        .req    x3
> > +     swapper_ttb     .req    x4
> >       cur_pgdp        .req    x5
> >       end_pgdp        .req    x6
> >       pgd             .req    x7
> > @@ -245,10 +270,15 @@ SYM_FUNC_START(idmap_kpti_install_ng_mappings)
> >       cur_ptep        .req    x14
> >       end_ptep        .req    x15
> >       pte             .req    x16
> > +     valid           .req    x17
> >
> >       mrs     swapper_ttb, ttbr1_el1
> > -     restore_ttbr1   swapper_ttb
> > -     adr     flag_ptr, __idmap_kpti_flag
> > +
> > +     /* Uninstall swapper before surgery begins */
> > +     __idmap_cpu_set_reserved_ttbr1 x8, x9
> > +     offset_ttbr1 temp_pgd_phys, x8
> > +     msr     ttbr1_el1, temp_pgd_phys
> > +     isb
> >
> >       cbnz    cpu, __idmap_kpti_secondary
> >
> > @@ -259,31 +289,24 @@ SYM_FUNC_START(idmap_kpti_install_ng_mappings)
> >       eor     w17, w17, num_cpus
> >       cbnz    w17, 1b
> >
> > -     /* We need to walk swapper, so turn off the MMU. */
> > -     pre_disable_mmu_workaround
> > -     mrs     x17, sctlr_el1
> > -     bic     x17, x17, #SCTLR_ELx_M
> > -     msr     sctlr_el1, x17
> > -     isb
> > +     mov     pte_flags, #KPTI_NG_PTE_FLAGS
> > +
> > +     /* Advance temp_pte to the fixmap page */
> > +     add     temp_pte, flag_ptr, #PAGE_SIZE
> >
> >       /* Everybody is enjoying the idmap, so we can rewrite swapper. */
> >       /* PGD */
> > -     mov             cur_pgdp, swapper_pa
> > +     adrp            cur_pgdp, swapper_pg_dir
> > +     kpti_map_pgtbl  pgd, 0
> >       kpti_mk_tbl_ng  pgd, PTRS_PER_PGD
> >
> > -     /* Publish the updated tables and nuke all the TLBs */
> > -     dsb     sy
> > -     tlbi    vmalle1is
> > -     dsb     ish
> > -     isb
> > -
> > -     /* We're done: fire up the MMU again */
> > -     mrs     x17, sctlr_el1
> > -     orr     x17, x17, #SCTLR_ELx_M
> > -     set_sctlr_el1   x17
> > -
> >       /* Set the flag to zero to indicate that we're all done */
> >       str     wzr, [flag_ptr]
> > +
> > +     /* We're done: fire up swapper again */
> > +     __idmap_cpu_set_reserved_ttbr1 x8, x9
> > +     msr     ttbr1_el1, swapper_ttb
> > +     isb
> >       ret
> >
> >  .Lderef_pgd:
> > @@ -291,6 +314,7 @@ SYM_FUNC_START(idmap_kpti_install_ng_mappings)
> >       .if             CONFIG_PGTABLE_LEVELS > 3
> >       pud             .req    x10
> >       pte_to_phys     cur_pudp, pgd
> > +     kpti_map_pgtbl  pud, 1
> >       kpti_mk_tbl_ng  pud, PTRS_PER_PUD
> >       b               .Lnext_pgd
> >       .else           /* CONFIG_PGTABLE_LEVELS <= 3 */
> > @@ -303,6 +327,7 @@ SYM_FUNC_START(idmap_kpti_install_ng_mappings)
> >       .if             CONFIG_PGTABLE_LEVELS > 2
> >       pmd             .req    x13
> >       pte_to_phys     cur_pmdp, pud
> > +     kpti_map_pgtbl  pmd, 2
> >       kpti_mk_tbl_ng  pmd, PTRS_PER_PMD
> >       b               .Lnext_pud
> >       .else           /* CONFIG_PGTABLE_LEVELS <= 2 */
> > @@ -313,12 +338,14 @@ SYM_FUNC_START(idmap_kpti_install_ng_mappings)
> >  .Lderef_pmd:
> >       /* PTE */
> >       pte_to_phys     cur_ptep, pmd
> > +     kpti_map_pgtbl  pte, 3
> >       kpti_mk_tbl_ng  pte, PTRS_PER_PTE
> >       b               .Lnext_pmd
> >
> >       .unreq  cpu
> > +     .unreq  pte_flags
> >       .unreq  num_cpus
> > -     .unreq  swapper_pa
> > +     .unreq  temp_pgd_phys
> >       .unreq  cur_pgdp
> >       .unreq  end_pgdp
> >       .unreq  pgd
> > @@ -331,12 +358,10 @@ SYM_FUNC_START(idmap_kpti_install_ng_mappings)
> >       .unreq  cur_ptep
> >       .unreq  end_ptep
> >       .unreq  pte
> > +     .unreq  valid
> >
> >       /* Secondary CPUs end up here */
> >  __idmap_kpti_secondary:
> > -     /* Uninstall swapper before surgery begins */
> > -     __idmap_cpu_set_reserved_ttbr1 x16, x17
> > -
> >       /* Increment the flag to let the boot CPU we're ready */
> >  1:   ldxr    w16, [flag_ptr]
> >       add     w16, w16, #1
> > @@ -350,7 +375,7 @@ __idmap_kpti_secondary:
> >       cbnz    w16, 1b
> >
> >       /* All done, act like nothing happened */
> > -     offset_ttbr1 swapper_ttb, x16
> > +     __idmap_cpu_set_reserved_ttbr1 x8, x9
> >       msr     ttbr1_el1, swapper_ttb
> >       isb
> >       ret
> > --
> > 2.30.2
> >

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