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Message-Id: <1473788797-10879-5-git-send-email-catalin.marinas@arm.com>
Date: Tue, 13 Sep 2016 18:46:34 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: linux-arm-kernel@...ts.infradead.org
Cc: Will Deacon <will.deacon@....com>,
James Morse <james.morse@....com>,
Kees Cook <keescook@...omium.org>,
Mark Rutland <mark.rutland@....com>,
Ard Biesheuvel <ard.biesheuvel@...aro.org>,
AKASHI Takahiro <takahiro.akashi@...aro.org>,
kernel-hardening@...ts.openwall.com
Subject: [PATCH v3 4/7] arm64: Disable TTBR0_EL1 during normal kernel execution
When the TTBR0 PAN feature is enabled, the kernel entry points need to
disable access to TTBR0_EL1. The PAN status of the interrupted context
is stored as part of the saved pstate, reusing the PSR_PAN_BIT (22).
Restoring access to TTBR0_PAN is done on exception return if returning
to user or returning to a context where PAN was disabled.
Context switching via switch_mm() must defer the update of TTBR0_EL1
until a return to user or an explicit uaccess_enable() call.
Special care needs to be taken for two cases where TTBR0_EL1 is set
outside the normal kernel context switch operation: EFI run-time
services (via efi_set_pgd) and CPU suspend (via cpu_(un)install_idmap).
Code has been added to avoid deferred TTBR0_EL1 switching as in
switch_mm() and restore the reserved TTBR0_EL1 when uninstalling the
special TTBR0_EL1.
This patch also removes a stale comment on the switch_mm() function.
Cc: Will Deacon <will.deacon@....com>
Cc: James Morse <james.morse@....com>
Cc: Kees Cook <keescook@...omium.org>
Cc: Mark Rutland <mark.rutland@....com>
Signed-off-by: Catalin Marinas <catalin.marinas@....com>
---
arch/arm64/include/asm/efi.h | 26 +++++++++++++-
arch/arm64/include/asm/mmu_context.h | 51 +++++++++++++++++++--------
arch/arm64/include/asm/ptrace.h | 2 ++
arch/arm64/kernel/entry.S | 67 ++++++++++++++++++++++++++++++++++++
arch/arm64/kernel/setup.c | 9 +++++
arch/arm64/mm/context.c | 7 +++-
6 files changed, 146 insertions(+), 16 deletions(-)
diff --git a/arch/arm64/include/asm/efi.h b/arch/arm64/include/asm/efi.h
index a9e54aad15ef..3a405dccb6cf 100644
--- a/arch/arm64/include/asm/efi.h
+++ b/arch/arm64/include/asm/efi.h
@@ -1,6 +1,7 @@
#ifndef _ASM_EFI_H
#define _ASM_EFI_H
+#include <asm/cpufeature.h>
#include <asm/io.h>
#include <asm/mmu_context.h>
#include <asm/neon.h>
@@ -75,7 +76,30 @@ static inline void efifb_setup_from_dmi(struct screen_info *si, const char *opt)
static inline void efi_set_pgd(struct mm_struct *mm)
{
- switch_mm(NULL, mm, NULL);
+ __switch_mm(mm);
+
+ if (system_uses_ttbr0_pan()) {
+ if (mm != current->active_mm) {
+ /*
+ * Update the current thread's saved ttbr0 since it is
+ * restored as part of a return from exception. Set
+ * the hardware TTBR0_EL1 using cpu_switch_mm()
+ * directly to enable potential errata workarounds.
+ */
+ update_saved_ttbr0(current, mm);
+ cpu_switch_mm(mm->pgd, mm);
+ } else {
+ /*
+ * 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);
+ }
+ }
}
void efi_virtmap_load(void);
diff --git a/arch/arm64/include/asm/mmu_context.h b/arch/arm64/include/asm/mmu_context.h
index b1892a0dbcb0..da0ecca8353c 100644
--- a/arch/arm64/include/asm/mmu_context.h
+++ b/arch/arm64/include/asm/mmu_context.h
@@ -23,6 +23,7 @@
#include <linux/sched.h>
#include <asm/cacheflush.h>
+#include <asm/cpufeature.h>
#include <asm/proc-fns.h>
#include <asm-generic/mm_hooks.h>
#include <asm/cputype.h>
@@ -113,7 +114,7 @@ static inline void cpu_uninstall_idmap(void)
local_flush_tlb_all();
cpu_set_default_tcr_t0sz();
- if (mm != &init_mm)
+ if (mm != &init_mm && !system_uses_ttbr0_pan())
cpu_switch_mm(mm->pgd, mm);
}
@@ -173,20 +174,26 @@ enter_lazy_tlb(struct mm_struct *mm, struct task_struct *tsk)
{
}
-/*
- * This is the actual mm switch as far as the scheduler
- * is concerned. No registers are touched. We avoid
- * calling the CPU specific function when the mm hasn't
- * actually changed.
- */
-static inline void
-switch_mm(struct mm_struct *prev, struct mm_struct *next,
- struct task_struct *tsk)
+#ifdef CONFIG_ARM64_SW_TTBR0_PAN
+static inline void update_saved_ttbr0(struct task_struct *tsk,
+ struct mm_struct *mm)
{
- unsigned int cpu = smp_processor_id();
+ 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;
+ }
+}
+#else
+static inline void update_saved_ttbr0(struct task_struct *tsk,
+ struct mm_struct *mm)
+{
+}
+#endif
- if (prev == next)
- return;
+static inline void __switch_mm(struct mm_struct *next)
+{
+ unsigned int cpu = smp_processor_id();
/*
* init_mm.pgd does not contain any user mappings and it is always
@@ -200,8 +207,24 @@ switch_mm(struct mm_struct *prev, struct mm_struct *next,
check_and_switch_context(next, cpu);
}
+static inline void
+switch_mm(struct mm_struct *prev, struct mm_struct *next,
+ struct task_struct *tsk)
+{
+ if (prev != next)
+ __switch_mm(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).
+ */
+ update_saved_ttbr0(tsk, next);
+}
+
#define deactivate_mm(tsk,mm) do { } while (0)
-#define activate_mm(prev,next) switch_mm(prev, next, NULL)
+#define activate_mm(prev,next) switch_mm(prev, next, current)
void verify_cpu_asid_bits(void);
diff --git a/arch/arm64/include/asm/ptrace.h b/arch/arm64/include/asm/ptrace.h
index ada08b5b036d..458773ac5ec9 100644
--- a/arch/arm64/include/asm/ptrace.h
+++ b/arch/arm64/include/asm/ptrace.h
@@ -21,6 +21,8 @@
#include <uapi/asm/ptrace.h>
+#define _PSR_PAN_BIT 22
+
/* Current Exception Level values, as contained in CurrentEL */
#define CurrentEL_EL1 (1 << 2)
#define CurrentEL_EL2 (2 << 2)
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index be1e3987c07a..2bbb5756f621 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -29,7 +29,9 @@
#include <asm/esr.h>
#include <asm/irq.h>
#include <asm/memory.h>
+#include <asm/ptrace.h>
#include <asm/thread_info.h>
+#include <asm/uaccess.h>
#include <asm/unistd.h>
/*
@@ -109,6 +111,34 @@
mrs x22, elr_el1
mrs x23, spsr_el1
stp lr, x21, [sp, #S_LR]
+
+#ifdef CONFIG_ARM64_SW_TTBR0_PAN
+ /*
+ * Set the TTBR0 PAN bit in SPSR. When the exception is taken from
+ * EL0, there is no need to check the state of TTBR0_EL1 since
+ * accesses are always enabled.
+ * Note that the meaning of this bit differs from the ARMv8.1 PAN
+ * feature as all TTBR0_EL1 accesses are disabled, not just those to
+ * user mappings.
+ */
+alternative_if_not ARM64_HAS_PAN
+ nop
+alternative_else
+ b 1f // skip TTBR0 PAN
+alternative_endif
+
+ .if \el != 0
+ mrs x21, ttbr0_el1
+ tst x21, #0xffff << 48 // Check for the reserved ASID
+ orr x23, x23, #PSR_PAN_BIT // Set the emulated PAN in the saved SPSR
+ b.eq 1f // TTBR0 access already disabled
+ and x23, x23, #~PSR_PAN_BIT // Clear the emulated PAN in the saved SPSR
+ .endif
+
+ uaccess_ttbr0_disable x21
+1:
+#endif
+
stp x22, x23, [sp, #S_PC]
/*
@@ -147,6 +177,42 @@
ldp x21, x22, [sp, #S_PC] // load ELR, SPSR
.if \el == 0
ct_user_enter
+ .endif
+
+#ifdef CONFIG_ARM64_SW_TTBR0_PAN
+ /*
+ * Restore access to TTBR0_EL1. If returning to EL0, no need for SPSR
+ * PAN bit checking.
+ */
+alternative_if_not ARM64_HAS_PAN
+ nop
+alternative_else
+ b 2f // skip TTBR0 PAN
+alternative_endif
+
+ .if \el != 0
+ tbnz x22, #_PSR_PAN_BIT, 1f // Skip re-enabling TTBR0 access if previously disabled
+ .endif
+
+ uaccess_ttbr0_enable x0
+
+ .if \el == 0
+ /*
+ * Enable errata workarounds only if returning to user. The only
+ * workaround currently required for TTBR0_EL1 changes are for the
+ * Cavium erratum 27456 (broadcast TLBI instructions may cause I-cache
+ * corruption).
+ */
+ post_ttbr0_update_workaround
+ .endif
+1:
+ .if \el != 0
+ and x22, x22, #~PSR_PAN_BIT // ARMv8.0 CPUs do not understand this bit
+ .endif
+2:
+#endif
+
+ .if \el == 0
ldr x23, [sp, #S_SP] // load return stack pointer
msr sp_el0, x23
#ifdef CONFIG_ARM64_ERRATUM_845719
@@ -168,6 +234,7 @@ alternative_else
alternative_endif
#endif
.endif
+
msr elr_el1, x21 // set up the return data
msr spsr_el1, x22
ldp x0, x1, [sp, #16 * 0]
diff --git a/arch/arm64/kernel/setup.c b/arch/arm64/kernel/setup.c
index 536dce22fe76..59b900d42cec 100644
--- a/arch/arm64/kernel/setup.c
+++ b/arch/arm64/kernel/setup.c
@@ -286,6 +286,15 @@ void __init setup_arch(char **cmdline_p)
smp_init_cpus();
smp_build_mpidr_hash();
+#ifdef CONFIG_ARM64_SW_TTBR0_PAN
+ /*
+ * Make sure init_thread_info.ttbr0 always generates translation
+ * faults in case uaccess_enable() is inadvertently called by the init
+ * thread.
+ */
+ init_thread_info.ttbr0 = virt_to_phys(empty_zero_page);
+#endif
+
#ifdef CONFIG_VT
#if defined(CONFIG_VGA_CONSOLE)
conswitchp = &vga_con;
diff --git a/arch/arm64/mm/context.c b/arch/arm64/mm/context.c
index efcf1f7ef1e4..4c63cb154859 100644
--- a/arch/arm64/mm/context.c
+++ b/arch/arm64/mm/context.c
@@ -221,7 +221,12 @@ void check_and_switch_context(struct mm_struct *mm, unsigned int cpu)
raw_spin_unlock_irqrestore(&cpu_asid_lock, flags);
switch_mm_fastpath:
- cpu_switch_mm(mm->pgd, mm);
+ /*
+ * Defer TTBR0_EL1 setting for user threads to uaccess_enable() when
+ * emulating PAN.
+ */
+ if (!system_uses_ttbr0_pan())
+ cpu_switch_mm(mm->pgd, mm);
}
static int asids_init(void)
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