[PATCH v3 16/23] arm64: mte: Allow user control of the tag check mode via prctl()

Catalin Marinas
2020-04-21T14:26:41Z

By default, even if PROT_MTE is set on a memory range, there is no tag
check fault reporting (SIGSEGV). Introduce a set of option to the
exiting prctl(PR_SET_TAGGED_ADDR_CTRL) to allow user control of the tag
check fault mode:

PR_MTE_TCF_NONE - no reporting (default)
PR_MTE_TCF_SYNC - synchronous tag check fault reporting
PR_MTE_TCF_ASYNC - asynchronous tag check fault reporting

These options translate into the corresponding SCTLR_EL1.TCF0 bitfield,
context-switched by the kernel. Note that uaccess done by the kernel is
not checked and cannot be configured by the user.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
---

Notes:
v3:
- Use SCTLR_EL1_TCF0_NONE instead of 0 for consistency.
- Move mte_thread_switch() in this patch from an earlier one. In
addition, it is called after the dsb() in __switch_to() so that any
asynchronous tag check faults have been registered in the TFSR_EL1
registers (to be added with the in-kernel MTE support.

v2:
- Handle SCTLR_EL1_TCF0_NONE explicitly for consistency with PR_MTE_TCF_NONE.
- Fix SCTLR_EL1 register setting in flush_mte_state() (thanks to Peter
Collingbourne).
- Added ISB to update_sctlr_el1_tcf0() since, with the latest
architecture update/fix, the TCF0 field is used by the uaccess
routines.

arch/arm64/include/asm/mte.h | 14 ++++++
arch/arm64/include/asm/processor.h | 3 ++
arch/arm64/kernel/mte.c | 77 ++++++++++++++++++++++++++++++
arch/arm64/kernel/process.c | 26 ++++++++--
include/uapi/linux/prctl.h | 6 +++
5 files changed, 123 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/include/asm/mte.h b/arch/arm64/include/asm/mte.h
index e9711ea51eb5..3dc0a7977124 100644
--- a/arch/arm64/include/asm/mte.h
+++ b/arch/arm64/include/asm/mte.h
@@ -11,10 +11,24 @@ int mte_memcmp_pages(const void *page1_addr, const void *page2_addr);

#ifdef CONFIG_ARM64_MTE
void flush_mte_state(void);
+void mte_thread_switch(struct task_struct *next);
+long set_mte_ctrl(unsigned long arg);
+long get_mte_ctrl(void);
#else
static inline void flush_mte_state(void)
{
}
+static inline void mte_thread_switch(struct task_struct *next)
+{
+}
+static inline long set_mte_ctrl(unsigned long arg)
+{
+ return 0;
+}
+static inline long get_mte_ctrl(void)
+{
+ return 0;
+}
#endif

#endif /* __ASSEMBLY__ */
diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
index 240fe5e5b720..80e7f0573309 100644
--- a/arch/arm64/include/asm/processor.h
+++ b/arch/arm64/include/asm/processor.h
@@ -151,6 +151,9 @@ struct thread_struct {
struct ptrauth_keys_user keys_user;
struct ptrauth_keys_kernel keys_kernel;
#endif
+#ifdef CONFIG_ARM64_MTE
+ u64 sctlr_tcf0;
+#endif
};

static inline void arch_thread_struct_whitelist(unsigned long *offset,
diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index 032016823957..e62d02890d12 100644
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c
@@ -3,12 +3,34 @@
* Copyright (C) 2020 ARM Ltd.
*/

+#include <linux/prctl.h>
+#include <linux/sched.h>
#include <linux/thread_info.h>

#include <asm/cpufeature.h>
#include <asm/mte.h>
#include <asm/sysreg.h>

+static void update_sctlr_el1_tcf0(u64 tcf0)
+{
+ /* ISB required for the kernel uaccess routines */
+ sysreg_clear_set(sctlr_el1, SCTLR_EL1_TCF0_MASK, tcf0);
+ isb();
+}
+
+static void set_sctlr_el1_tcf0(u64 tcf0)
+{
+ /*
+ * mte_thread_switch() checks current->thread.sctlr_tcf0 as an
+ * optimisation. Disable preemption so that it does not see
+ * the variable update before the SCTLR_EL1.TCF0 one.
+ */
+ preempt_disable();
+ current->thread.sctlr_tcf0 = tcf0;
+ update_sctlr_el1_tcf0(tcf0);
+ preempt_enable();
+}
+
void flush_mte_state(void)
{
if (!system_supports_mte())
@@ -18,4 +40,59 @@ void flush_mte_state(void)
dsb(ish);
write_sysreg_s(0, SYS_TFSRE0_EL1);
clear_thread_flag(TIF_MTE_ASYNC_FAULT);
+ /* disable tag checking */
+ set_sctlr_el1_tcf0(SCTLR_EL1_TCF0_NONE);
+}
+
+void mte_thread_switch(struct task_struct *next)
+{
+ if (!system_supports_mte())
+ return;
+
+ /* avoid expensive SCTLR_EL1 accesses if no change */
+ if (current->thread.sctlr_tcf0 != next->thread.sctlr_tcf0)
+ update_sctlr_el1_tcf0(next->thread.sctlr_tcf0);
+}
+
+long set_mte_ctrl(unsigned long arg)
+{
+ u64 tcf0;
+
+ if (!system_supports_mte())
+ return 0;
+
+ switch (arg & PR_MTE_TCF_MASK) {
+ case PR_MTE_TCF_NONE:
+ tcf0 = SCTLR_EL1_TCF0_NONE;
+ break;
+ case PR_MTE_TCF_SYNC:
+ tcf0 = SCTLR_EL1_TCF0_SYNC;
+ break;
+ case PR_MTE_TCF_ASYNC:
+ tcf0 = SCTLR_EL1_TCF0_ASYNC;
+ break;
+ default:
+ return -EINVAL;
+ }
+
+ set_sctlr_el1_tcf0(tcf0);
+
+ return 0;
+}
+
+long get_mte_ctrl(void)
+{
+ if (!system_supports_mte())
+ return 0;
+
+ switch (current->thread.sctlr_tcf0) {
+ case SCTLR_EL1_TCF0_NONE:
+ return PR_MTE_TCF_NONE;
+ case SCTLR_EL1_TCF0_SYNC:
+ return PR_MTE_TCF_SYNC;
+ case SCTLR_EL1_TCF0_ASYNC:
+ return PR_MTE_TCF_ASYNC;
+ }
+
+ return 0;
}
diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index 740047c9cd13..ff6031a398d0 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -529,6 +529,13 @@ __notrace_funcgraph struct task_struct *__switch_to(struct task_struct *prev,
*/
dsb(ish);

+ /*
+ * MTE thread switching must happen after the DSB above to ensure that
+ * any asynchronous tag check faults have been logged in the TFSR*_EL1
+ * registers.
+ */
+ mte_thread_switch(next);
+
/* the actual thread switch */
last = cpu_switch_to(prev, next);

@@ -588,9 +595,15 @@ static unsigned int tagged_addr_disabled;

long set_tagged_addr_ctrl(unsigned long arg)
{
+ unsigned long valid_mask = PR_TAGGED_ADDR_ENABLE;
+
if (is_compat_task())
return -EINVAL;
- if (arg & ~PR_TAGGED_ADDR_ENABLE)
+
+ if (system_supports_mte())
+ valid_mask |= PR_MTE_TCF_MASK;
+
+ if (arg & ~valid_mask)
return -EINVAL;

/*
@@ -600,6 +613,9 @@ long set_tagged_addr_ctrl(unsigned long arg)
if (arg & PR_TAGGED_ADDR_ENABLE && tagged_addr_disabled)
return -EINVAL;

+ if (set_mte_ctrl(arg) != 0)
+ return -EINVAL;
+
update_thread_flag(TIF_TAGGED_ADDR, arg & PR_TAGGED_ADDR_ENABLE);

return 0;
@@ -607,13 +623,17 @@ long set_tagged_addr_ctrl(unsigned long arg)

long get_tagged_addr_ctrl(void)
{
+ long ret = 0;
+
if (is_compat_task())
return -EINVAL;

if (test_thread_flag(TIF_TAGGED_ADDR))
- return PR_TAGGED_ADDR_ENABLE;
+ ret = PR_TAGGED_ADDR_ENABLE;

- return 0;
+ ret |= get_mte_ctrl();
+
+ return ret;
}

/*
diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
index 07b4f8131e36..2390ab324afa 100644
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h
@@ -233,6 +233,12 @@ struct prctl_mm_map {
#define PR_SET_TAGGED_ADDR_CTRL 55
#define PR_GET_TAGGED_ADDR_CTRL 56
# define PR_TAGGED_ADDR_ENABLE (1UL << 0)
+/* MTE tag check fault modes */
+# define PR_MTE_TCF_SHIFT 1
+# define PR_MTE_TCF_NONE (0UL << PR_MTE_TCF_SHIFT)
+# define PR_MTE_TCF_SYNC (1UL << PR_MTE_TCF_SHIFT)
+# define PR_MTE_TCF_ASYNC (2UL << PR_MTE_TCF_SHIFT)
+# define PR_MTE_TCF_MASK (3UL << PR_MTE_TCF_SHIFT)

/* Control reclaim behavior when allocating memory */
#define PR_SET_IO_FLUSHER 57