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Subject[PATCH 26/29] arm64: kpti: Add ->enable callback to remap swapper using nG mappings
Date
From: Will Deacon <will.deacon@arm.com>

commit f992b4dfd58b upstream.

Defaulting to global mappings for kernel space is generally good for
performance and appears to be necessary for Cavium ThunderX. If we
subsequently decide that we need to enable kpti, then we need to rewrite
our existing page table entries to be non-global. This is fiddly, and
made worse by the possible use of contiguous mappings, which require
a strict break-before-make sequence.

Since the enable callback runs on each online CPU from stop_machine
context, we can have all CPUs enter the idmap, where secondaries can
wait for the primary CPU to rewrite swapper with its MMU off. It's all
fairly horrible, but at least it only runs once.

Nicolas Dechesne <nicolas.dechesne@linaro.org> found a bug on this commit
which cause boot failure on db410c etc board. Ard Biesheuvel found it
writting wrong contenct to ttbr1_el1 in __idmap_cpu_set_reserved_ttbr1
macro and fixed it by give it the right content.

Tested-by: Marc Zyngier <marc.zyngier@arm.com>
Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>

Signed-off-by: Alex Shi <alex.shi@linaro.org>

Conflicts:
no get_thread_info/post_ttbr_update_workaround/pre_disable_mmu_workaround
in arch/arm64/include/asm/assembler.h and arch/arm64/mm/proc.S
---
arch/arm64/include/asm/assembler.h | 3 +
arch/arm64/kernel/cpufeature.c | 25 +++++
arch/arm64/mm/proc.S | 201 +++++++++++++++++++++++++++++++++++--
3 files changed, 222 insertions(+), 7 deletions(-)

diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h
index 851290d..7193bf9 100644
--- a/arch/arm64/include/asm/assembler.h
+++ b/arch/arm64/include/asm/assembler.h
@@ -413,4 +413,7 @@ alternative_endif
movk \reg, :abs_g0_nc:\val
.endm

+ .macro pte_to_phys, phys, pte
+ and \phys, \pte, #(((1 << (48 - PAGE_SHIFT)) - 1) << PAGE_SHIFT)
+ .endm
#endif /* __ASM_ASSEMBLER_H */
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index e62583d..0fba610 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -778,6 +778,30 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
ID_AA64PFR0_CSV3_SHIFT);
}

+static int kpti_install_ng_mappings(void *__unused)
+{
+ typedef void (kpti_remap_fn)(int, int, phys_addr_t);
+ extern kpti_remap_fn idmap_kpti_install_ng_mappings;
+ kpti_remap_fn *remap_fn;
+
+ static bool kpti_applied = false;
+ int cpu = smp_processor_id();
+
+ if (kpti_applied)
+ return 0;
+
+ remap_fn = (void *)__pa_symbol(idmap_kpti_install_ng_mappings);
+
+ cpu_install_idmap();
+ remap_fn(cpu, num_online_cpus(), __pa_symbol(swapper_pg_dir));
+ cpu_uninstall_idmap();
+
+ if (!cpu)
+ kpti_applied = true;
+
+ return 0;
+}
+
static int __init parse_kpti(char *str)
{
bool enabled;
@@ -881,6 +905,7 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
.capability = ARM64_UNMAP_KERNEL_AT_EL0,
.def_scope = SCOPE_SYSTEM,
.matches = unmap_kernel_at_el0,
+ .enable = kpti_install_ng_mappings,
},
#endif
{},
diff --git a/arch/arm64/mm/proc.S b/arch/arm64/mm/proc.S
index 3378f3e..5c268f5 100644
--- a/arch/arm64/mm/proc.S
+++ b/arch/arm64/mm/proc.S
@@ -148,6 +148,16 @@ alternative_else_nop_endif
ENDPROC(cpu_do_switch_mm)

.pushsection ".idmap.text", "ax"
+
+.macro __idmap_cpu_set_reserved_ttbr1, tmp1, tmp2
+ adrp \tmp1, empty_zero_page
+ msr ttbr1_el1, \tmp1
+ isb
+ tlbi vmalle1
+ dsb nsh
+ isb
+.endm
+
/*
* void idmap_cpu_replace_ttbr1(phys_addr_t new_pgd)
*
@@ -158,13 +168,7 @@ ENTRY(idmap_cpu_replace_ttbr1)
mrs x2, daif
msr daifset, #0xf

- adrp x1, empty_zero_page
- msr ttbr1_el1, x1
- isb
-
- tlbi vmalle1
- dsb nsh
- isb
+ __idmap_cpu_set_reserved_ttbr1 x1, x3

msr ttbr1_el1, x0
isb
@@ -175,6 +179,189 @@ ENTRY(idmap_cpu_replace_ttbr1)
ENDPROC(idmap_cpu_replace_ttbr1)
.popsection

+#ifdef CONFIG_UNMAP_KERNEL_AT_EL0
+ .pushsection ".idmap.text", "ax"
+
+ .macro __idmap_kpti_get_pgtable_ent, 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
+ tbz \type, #0, next_\()\type // Skip invalid entries
+ .endm
+
+ .macro __idmap_kpti_put_pgtable_ent_ng, type
+ orr \type, \type, #PTE_NG // Same bit for blocks and pages
+ str \type, [cur_\()\type\()p] // Update the entry and ensure it
+ dc civac, cur_\()\type\()p // is visible to all CPUs.
+ .endm
+
+/*
+ * void __kpti_install_ng_mappings(int cpu, int num_cpus, phys_addr_t swapper)
+ *
+ * Called exactly once from stop_machine context by each CPU found during boot.
+ */
+__idmap_kpti_flag:
+ .long 1
+ENTRY(idmap_kpti_install_ng_mappings)
+ cpu .req w0
+ num_cpus .req w1
+ swapper_pa .req x2
+ swapper_ttb .req x3
+ flag_ptr .req x4
+ cur_pgdp .req x5
+ end_pgdp .req x6
+ pgd .req x7
+ cur_pudp .req x8
+ end_pudp .req x9
+ pud .req x10
+ cur_pmdp .req x11
+ end_pmdp .req x12
+ pmd .req x13
+ cur_ptep .req x14
+ end_ptep .req x15
+ pte .req x16
+
+ mrs swapper_ttb, ttbr1_el1
+ adr flag_ptr, __idmap_kpti_flag
+
+ cbnz cpu, __idmap_kpti_secondary
+
+ /* We're the boot CPU. Wait for the others to catch up */
+ sevl
+1: wfe
+ ldaxr w18, [flag_ptr]
+ eor w18, w18, num_cpus
+ cbnz w18, 1b
+
+ /* We need to walk swapper, so turn off the MMU. */
+ mrs x18, sctlr_el1
+ bic x18, x18, #SCTLR_ELx_M
+ msr sctlr_el1, x18
+ isb
+
+ /* Everybody is enjoying the idmap, so we can rewrite swapper. */
+ /* PGD */
+ mov cur_pgdp, swapper_pa
+ add end_pgdp, cur_pgdp, #(PTRS_PER_PGD * 8)
+do_pgd: __idmap_kpti_get_pgtable_ent pgd
+ tbnz pgd, #1, walk_puds
+ __idmap_kpti_put_pgtable_ent_ng pgd
+next_pgd:
+ add cur_pgdp, cur_pgdp, #8
+ cmp cur_pgdp, end_pgdp
+ b.ne do_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 x18, sctlr_el1
+ orr x18, x18, #SCTLR_ELx_M
+ msr sctlr_el1, x18
+ isb
+
+ /* Set the flag to zero to indicate that we're all done */
+ str wzr, [flag_ptr]
+ ret
+
+ /* PUD */
+walk_puds:
+ .if CONFIG_PGTABLE_LEVELS > 3
+ pte_to_phys cur_pudp, pgd
+ add end_pudp, cur_pudp, #(PTRS_PER_PUD * 8)
+do_pud: __idmap_kpti_get_pgtable_ent pud
+ tbnz pud, #1, walk_pmds
+ __idmap_kpti_put_pgtable_ent_ng pud
+next_pud:
+ add cur_pudp, cur_pudp, 8
+ cmp cur_pudp, end_pudp
+ b.ne do_pud
+ b next_pgd
+ .else /* CONFIG_PGTABLE_LEVELS <= 3 */
+ mov pud, pgd
+ b walk_pmds
+next_pud:
+ b next_pgd
+ .endif
+
+ /* PMD */
+walk_pmds:
+ .if CONFIG_PGTABLE_LEVELS > 2
+ pte_to_phys cur_pmdp, pud
+ add end_pmdp, cur_pmdp, #(PTRS_PER_PMD * 8)
+do_pmd: __idmap_kpti_get_pgtable_ent pmd
+ tbnz pmd, #1, walk_ptes
+ __idmap_kpti_put_pgtable_ent_ng pmd
+next_pmd:
+ add cur_pmdp, cur_pmdp, #8
+ cmp cur_pmdp, end_pmdp
+ b.ne do_pmd
+ b next_pud
+ .else /* CONFIG_PGTABLE_LEVELS <= 2 */
+ mov pmd, pud
+ b walk_ptes
+next_pmd:
+ b next_pud
+ .endif
+
+ /* PTE */
+walk_ptes:
+ pte_to_phys cur_ptep, pmd
+ add end_ptep, cur_ptep, #(PTRS_PER_PTE * 8)
+do_pte: __idmap_kpti_get_pgtable_ent pte
+ __idmap_kpti_put_pgtable_ent_ng pte
+next_pte:
+ add cur_ptep, cur_ptep, #8
+ cmp cur_ptep, end_ptep
+ b.ne do_pte
+ b next_pmd
+
+ /* Secondary CPUs end up here */
+__idmap_kpti_secondary:
+ /* Uninstall swapper before surgery begins */
+ __idmap_cpu_set_reserved_ttbr1 x18, x17
+
+ /* Increment the flag to let the boot CPU we're ready */
+1: ldxr w18, [flag_ptr]
+ add w18, w18, #1
+ stxr w17, w18, [flag_ptr]
+ cbnz w17, 1b
+
+ /* Wait for the boot CPU to finish messing around with swapper */
+ sevl
+1: wfe
+ ldxr w18, [flag_ptr]
+ cbnz w18, 1b
+
+ /* All done, act like nothing happened */
+ msr ttbr1_el1, swapper_ttb
+ isb
+ ret
+
+ .unreq cpu
+ .unreq num_cpus
+ .unreq swapper_pa
+ .unreq swapper_ttb
+ .unreq flag_ptr
+ .unreq cur_pgdp
+ .unreq end_pgdp
+ .unreq pgd
+ .unreq cur_pudp
+ .unreq end_pudp
+ .unreq pud
+ .unreq cur_pmdp
+ .unreq end_pmdp
+ .unreq pmd
+ .unreq cur_ptep
+ .unreq end_ptep
+ .unreq pte
+ENDPROC(idmap_kpti_install_ng_mappings)
+ .popsection
+#endif
+
/*
* __cpu_setup
*
--
2.7.4
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