[PATCH v4 2/2] arm64: mm: install KPTI nG mappings with MMU enabled
Ard Biesheuvel
ardb at kernel.org
Thu Jun 9 07:50:10 PDT 2022
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 a minimal, ad-hoc
fixmap that we can use to map each page table in turn as we traverse the
hierarchy.
Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
---
arch/arm64/kernel/cpufeature.c | 54 ++++++++++++-
arch/arm64/mm/mmu.c | 7 ++
arch/arm64/mm/proc.S | 81 +++++++++++++-------
3 files changed, 113 insertions(+), 29 deletions(-)
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 42ea2bd856c6..cf7f6beabefe 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -1645,14 +1645,34 @@ 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))
+
+extern
+void create_kpti_ng_temp_pgd(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);
+
+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);
+ 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);
+ u64 kpti_ng_temp_pgd_pa = 0;
+ pgd_t *kpti_ng_temp_pgd;
+ u64 alloc = 0;
if (__this_cpu_read(this_cpu_vector) == vectors) {
const char *v = arm64_get_bp_hardening_vector(EL1_VECTOR_KPTI);
@@ -1670,12 +1690,40 @@ kpti_install_ng_mappings(const struct arm64_cpu_capabilities *__unused)
remap_fn = (void *)__pa_symbol(function_nocfi(idmap_kpti_install_ng_mappings));
+ if (!cpu) {
+ alloc = __get_free_pages(GFP_ATOMIC | __GFP_ZERO, order);
+ kpti_ng_temp_pgd = (pgd_t *)(alloc + levels * PAGE_SIZE);
+ kpti_ng_temp_alloc = kpti_ng_temp_pgd_pa = __pa(kpti_ng_temp_pgd);
+
+ //
+ // Create a minimal page table hierarchy that permits us to map
+ // the swapper page tables temporarily as we traverse them.
+ //
+ // The physical pages are laid out as follows:
+ //
+ // +--------+-/-------+-/------ +-\\--------+
+ // : PTE[] : | PMD[] : | PUD[] : || PGD[] :
+ // +--------+-\-------+-\------ +-//--------+
+ // ^
+ // The first page is mapped into this hierarchy at a PMD_SHIFT
+ // aligned virtual address, so that we can manipulate the PTE
+ // level entries while the mapping is active. The first entry
+ // covers the PTE[] page itself, the remaining entries are free
+ // to be used as a ad-hoc fixmap.
+ //
+ create_kpti_ng_temp_pgd(kpti_ng_temp_pgd, __pa(alloc),
+ KPTI_NG_TEMP_VA, PAGE_SIZE, PAGE_KERNEL,
+ kpti_ng_pgd_alloc, 0);
+ }
+
cpu_install_idmap();
- remap_fn(cpu, num_online_cpus(), __pa_symbol(swapper_pg_dir));
+ remap_fn(cpu, num_online_cpus(), kpti_ng_temp_pgd_pa, KPTI_NG_TEMP_VA);
cpu_uninstall_idmap();
- if (!cpu)
+ if (!cpu) {
+ 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 be4d6c3f5692..c5563ff990da 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -388,6 +388,13 @@ static void __create_pgd_mapping(pgd_t *pgdir, phys_addr_t phys,
} while (pgdp++, addr = next, addr != end);
}
+#ifdef CONFIG_UNMAP_KERNEL_AT_EL0
+extern __alias(__create_pgd_mapping)
+void create_kpti_ng_temp_pgd(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);
+#endif
+
static phys_addr_t __pgd_pgtable_alloc(int shift)
{
void *ptr = (void *)__get_free_page(GFP_PGTABLE_KERNEL);
diff --git a/arch/arm64/mm/proc.S b/arch/arm64/mm/proc.S
index 660887152dba..0cd052bd2fb1 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 +201,19 @@ 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] // loading the entry
+ ldr \type, [cur_\type\()p] // Load 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] // Update the entry and ensure
- dmb sy // that it is visible to all
- dc civac, cur_\()\type\()p // CPUs.
+ str \type, [cur_\type\()p] // Update the entry
.ifnc \type, pte
tbnz \type, #1, .Lderef_\type
.endif
@@ -223,8 +223,29 @@ 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.
+ */
+ .macro kpti_map_pgtbl, type, level
+ str xzr, [temp_pte, #8 * (\level + 1)] // break before make
+ dsb nshst
+ add pte, flag_ptr, #PAGE_SIZE * (\level + 1)
+ lsr pte, pte, #12
+ tlbi vaae1, pte
+ dsb nsh
+ isb
+
+ phys_to_pte pte, cur_\type\()p
+ add cur_\type\()p, flag_ptr, #PAGE_SIZE * (\level + 1)
+ orr pte, pte, pte_flags
+ str pte, [temp_pte, #8 * (\level + 1)]
+ dsb nshst
+ .endm
+
/*
- * 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 temp_pte_va)
*
* Called exactly once from stop_machine context by each CPU found during boot.
*/
@@ -232,8 +253,10 @@ __idmap_kpti_flag:
.long 1
SYM_FUNC_START(idmap_kpti_install_ng_mappings)
cpu .req w0
+ temp_pte .req x0
num_cpus .req w1
- swapper_pa .req x2
+ pte_flags .req x1
+ temp_pgd_phys .req x2
swapper_ttb .req x3
flag_ptr .req x4
cur_pgdp .req x5
@@ -246,9 +269,10 @@ SYM_FUNC_START(idmap_kpti_install_ng_mappings)
cur_ptep .req x14
end_ptep .req x15
pte .req x16
+ valid .req x17
+ mov x5, x3 // preserve temp_pte arg
mrs swapper_ttb, ttbr1_el1
- restore_ttbr1 swapper_ttb
adr flag_ptr, __idmap_kpti_flag
cbnz cpu, __idmap_kpti_secondary
@@ -260,28 +284,28 @@ 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
+ /* Switch to the temporary page tables on this CPU only */
+ __idmap_cpu_set_reserved_ttbr1 x8, x9
+ offset_ttbr1 temp_pgd_phys, x8
+ msr ttbr1_el1, temp_pgd_phys
isb
+ mov temp_pte, x5
+ mov pte_flags, #KPTI_NG_PTE_FLAGS
+
/* 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
+ /* Ensure all the updated entries are visible to secondary CPUs */
+ dsb ishst
- /* We're done: fire up the MMU again */
- mrs x17, sctlr_el1
- orr x17, x17, #SCTLR_ELx_M
- set_sctlr_el1 x17
+ /* We're done: fire up swapper_pg_dir again */
+ __idmap_cpu_set_reserved_ttbr1 x8, x9
+ msr ttbr1_el1, swapper_ttb
+ isb
/* Set the flag to zero to indicate that we're all done */
str wzr, [flag_ptr]
@@ -292,6 +316,7 @@ 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 */
@@ -304,6 +329,7 @@ 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 */
@@ -314,12 +340,15 @@ 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 temp_pte
.unreq num_cpus
- .unreq swapper_pa
+ .unreq pte_flags
+ .unreq temp_pgd_phys
.unreq cur_pgdp
.unreq end_pgdp
.unreq pgd
@@ -332,6 +361,7 @@ 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:
@@ -351,7 +381,6 @@ __idmap_kpti_secondary:
cbnz w16, 1b
/* All done, act like nothing happened */
- offset_ttbr1 swapper_ttb, x16
msr ttbr1_el1, swapper_ttb
isb
ret
--
2.30.2
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