[PATCH v5 04/10] riscv: Add support for userspace pointer masking

From: Samuel Holland
Date: Wed Oct 16 2024 - 16:29:27 EST

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RISC-V supports pointer masking with a variable number of tag bits
(which is called "PMLEN" in the specification) and which is configured
at the next higher privilege level.

Wire up the PR_SET_TAGGED_ADDR_CTRL and PR_GET_TAGGED_ADDR_CTRL prctls
so userspace can request a lower bound on the number of tag bits and
determine the actual number of tag bits. As with arm64's
PR_TAGGED_ADDR_ENABLE, the pointer masking configuration is
thread-scoped, inherited on clone() and fork() and cleared on execve().

Reviewed-by: Charlie Jenkins <charlie@xxxxxxxxxxxx>
Tested-by: Charlie Jenkins <charlie@xxxxxxxxxxxx>
Signed-off-by: Samuel Holland <samuel.holland@xxxxxxxxxx>
---

Changes in v5:
- Document how PR_[SG]ET_TAGGED_ADDR_CTRL are used on RISC-V

Changes in v4:
- Switch IS_ENABLED back to #ifdef to fix riscv32 build

Changes in v3:
- Rename CONFIG_RISCV_ISA_POINTER_MASKING to CONFIG_RISCV_ISA_SUPM,
since it only controls the userspace part of pointer masking
- Use IS_ENABLED instead of #ifdef when possible
- Use an enum for the supported PMLEN values
- Simplify the logic in set_tagged_addr_ctrl()

Changes in v2:
- Rebase on riscv/linux.git for-next
- Add and use the envcfg_update_bits() helper function
- Inline flush_tagged_addr_state()

Documentation/arch/riscv/uabi.rst | 12 ++++
arch/riscv/Kconfig | 11 ++++
arch/riscv/include/asm/processor.h | 8 +++
arch/riscv/include/asm/switch_to.h | 11 ++++
arch/riscv/kernel/process.c | 91 ++++++++++++++++++++++++++++++
include/uapi/linux/prctl.h | 5 +-
6 files changed, 137 insertions(+), 1 deletion(-)

diff --git a/Documentation/arch/riscv/uabi.rst b/Documentation/arch/riscv/uabi.rst
index 2b420bab0527..ddb8359a46ed 100644
--- a/Documentation/arch/riscv/uabi.rst
+++ b/Documentation/arch/riscv/uabi.rst
@@ -68,3 +68,15 @@ Misaligned accesses
Misaligned scalar accesses are supported in userspace, but they may perform
poorly. Misaligned vector accesses are only supported if the Zicclsm extension
is supported.
+
+Pointer masking
+---------------
+
+Support for pointer masking in userspace (the Supm extension) is provided via
+the ``PR_SET_TAGGED_ADDR_CTRL`` and ``PR_GET_TAGGED_ADDR_CTRL`` ``prctl()``
+operations. Pointer masking is disabled by default. To enable it, userspace
+must call ``PR_SET_TAGGED_ADDR_CTRL`` with the ``PR_PMLEN`` field set to the
+number of mask/tag bits needed by the application. ``PR_PMLEN`` is interpreted
+as a lower bound; if the kernel is unable to satisfy the request, the
+``PR_SET_TAGGED_ADDR_CTRL`` operation will fail. The actual number of tag bits
+is returned in ``PR_PMLEN`` by the ``PR_GET_TAGGED_ADDR_CTRL`` operation.
diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 22dc5ea4196c..0ef449465378 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -531,6 +531,17 @@ config RISCV_ISA_C

If you don't know what to do here, say Y.

+config RISCV_ISA_SUPM
+ bool "Supm extension for userspace pointer masking"
+ depends on 64BIT
+ default y
+ help
+ Add support for pointer masking in userspace (Supm) when the
+ underlying hardware extension (Smnpm or Ssnpm) is detected at boot.
+
+ If this option is disabled, userspace will be unable to use
+ the prctl(PR_{SET,GET}_TAGGED_ADDR_CTRL) API.
+
config RISCV_ISA_SVNAPOT
bool "Svnapot extension support for supervisor mode NAPOT pages"
depends on 64BIT && MMU
diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h
index c1a492508835..5f56eb9d114a 100644
--- a/arch/riscv/include/asm/processor.h
+++ b/arch/riscv/include/asm/processor.h
@@ -178,6 +178,14 @@ extern int set_unalign_ctl(struct task_struct *tsk, unsigned int val);
#define RISCV_SET_ICACHE_FLUSH_CTX(arg1, arg2) riscv_set_icache_flush_ctx(arg1, arg2)
extern int riscv_set_icache_flush_ctx(unsigned long ctx, unsigned long per_thread);

+#ifdef CONFIG_RISCV_ISA_SUPM
+/* PR_{SET,GET}_TAGGED_ADDR_CTRL prctl */
+long set_tagged_addr_ctrl(struct task_struct *task, unsigned long arg);
+long get_tagged_addr_ctrl(struct task_struct *task);
+#define SET_TAGGED_ADDR_CTRL(arg) set_tagged_addr_ctrl(current, arg)
+#define GET_TAGGED_ADDR_CTRL() get_tagged_addr_ctrl(current)
+#endif
+
#endif /* __ASSEMBLY__ */

#endif /* _ASM_RISCV_PROCESSOR_H */
diff --git a/arch/riscv/include/asm/switch_to.h b/arch/riscv/include/asm/switch_to.h
index 9685cd85e57c..94e33216b2d9 100644
--- a/arch/riscv/include/asm/switch_to.h
+++ b/arch/riscv/include/asm/switch_to.h
@@ -70,6 +70,17 @@ static __always_inline bool has_fpu(void) { return false; }
#define __switch_to_fpu(__prev, __next) do { } while (0)
#endif

+static inline void envcfg_update_bits(struct task_struct *task,
+ unsigned long mask, unsigned long val)
+{
+ unsigned long envcfg;
+
+ envcfg = (task->thread.envcfg & ~mask) | val;
+ task->thread.envcfg = envcfg;
+ if (task == current)
+ csr_write(CSR_ENVCFG, envcfg);
+}
+
static inline void __switch_to_envcfg(struct task_struct *next)
{
asm volatile (ALTERNATIVE("nop", "csrw " __stringify(CSR_ENVCFG) ", %0",
diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
index e3142d8a6e28..200d2ed64dfe 100644
--- a/arch/riscv/kernel/process.c
+++ b/arch/riscv/kernel/process.c
@@ -7,6 +7,7 @@
* Copyright (C) 2017 SiFive
*/

+#include <linux/bitfield.h>
#include <linux/cpu.h>
#include <linux/kernel.h>
#include <linux/sched.h>
@@ -180,6 +181,10 @@ void flush_thread(void)
memset(&current->thread.vstate, 0, sizeof(struct __riscv_v_ext_state));
clear_tsk_thread_flag(current, TIF_RISCV_V_DEFER_RESTORE);
#endif
+#ifdef CONFIG_RISCV_ISA_SUPM
+ if (riscv_has_extension_unlikely(RISCV_ISA_EXT_SUPM))
+ envcfg_update_bits(current, ENVCFG_PMM, ENVCFG_PMM_PMLEN_0);
+#endif
}

void arch_release_task_struct(struct task_struct *tsk)
@@ -242,3 +247,89 @@ void __init arch_task_cache_init(void)
{
riscv_v_setup_ctx_cache();
}
+
+#ifdef CONFIG_RISCV_ISA_SUPM
+enum {
+ PMLEN_0 = 0,
+ PMLEN_7 = 7,
+ PMLEN_16 = 16,
+};
+
+static bool have_user_pmlen_7;
+static bool have_user_pmlen_16;
+
+long set_tagged_addr_ctrl(struct task_struct *task, unsigned long arg)
+{
+ unsigned long valid_mask = PR_PMLEN_MASK;
+ struct thread_info *ti = task_thread_info(task);
+ unsigned long pmm;
+ u8 pmlen;
+
+ if (is_compat_thread(ti))
+ return -EINVAL;
+
+ if (arg & ~valid_mask)
+ return -EINVAL;
+
+ /*
+ * Prefer the smallest PMLEN that satisfies the user's request,
+ * in case choosing a larger PMLEN has a performance impact.
+ */
+ pmlen = FIELD_GET(PR_PMLEN_MASK, arg);
+ if (pmlen == PMLEN_0)
+ pmm = ENVCFG_PMM_PMLEN_0;
+ else if (pmlen <= PMLEN_7 && have_user_pmlen_7)
+ pmm = ENVCFG_PMM_PMLEN_7;
+ else if (pmlen <= PMLEN_16 && have_user_pmlen_16)
+ pmm = ENVCFG_PMM_PMLEN_16;
+ else
+ return -EINVAL;
+
+ envcfg_update_bits(task, ENVCFG_PMM, pmm);
+
+ return 0;
+}
+
+long get_tagged_addr_ctrl(struct task_struct *task)
+{
+ struct thread_info *ti = task_thread_info(task);
+ long ret = 0;
+
+ if (is_compat_thread(ti))
+ return -EINVAL;
+
+ switch (task->thread.envcfg & ENVCFG_PMM) {
+ case ENVCFG_PMM_PMLEN_7:
+ ret = FIELD_PREP(PR_PMLEN_MASK, PMLEN_7);
+ break;
+ case ENVCFG_PMM_PMLEN_16:
+ ret = FIELD_PREP(PR_PMLEN_MASK, PMLEN_16);
+ break;
+ }
+
+ return ret;
+}
+
+static bool try_to_set_pmm(unsigned long value)
+{
+ csr_set(CSR_ENVCFG, value);
+ return (csr_read_clear(CSR_ENVCFG, ENVCFG_PMM) & ENVCFG_PMM) == value;
+}
+
+static int __init tagged_addr_init(void)
+{
+ if (!riscv_has_extension_unlikely(RISCV_ISA_EXT_SUPM))
+ return 0;
+
+ /*
+ * envcfg.PMM is a WARL field. Detect which values are supported.
+ * Assume the supported PMLEN values are the same on all harts.
+ */
+ csr_clear(CSR_ENVCFG, ENVCFG_PMM);
+ have_user_pmlen_7 = try_to_set_pmm(ENVCFG_PMM_PMLEN_7);
+ have_user_pmlen_16 = try_to_set_pmm(ENVCFG_PMM_PMLEN_16);
+
+ return 0;
+}
+core_initcall(tagged_addr_init);
+#endif /* CONFIG_RISCV_ISA_SUPM */
diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
index 35791791a879..cefd656ebf43 100644
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h
@@ -230,7 +230,7 @@ struct prctl_mm_map {
# define PR_PAC_APDBKEY (1UL << 3)
# define PR_PAC_APGAKEY (1UL << 4)

-/* Tagged user address controls for arm64 */
+/* Tagged user address controls for arm64 and RISC-V */
#define PR_SET_TAGGED_ADDR_CTRL 55
#define PR_GET_TAGGED_ADDR_CTRL 56
# define PR_TAGGED_ADDR_ENABLE (1UL << 0)
@@ -244,6 +244,9 @@ struct prctl_mm_map {
# define PR_MTE_TAG_MASK (0xffffUL << PR_MTE_TAG_SHIFT)
/* Unused; kept only for source compatibility */
# define PR_MTE_TCF_SHIFT 1
+/* RISC-V pointer masking tag length */
+# define PR_PMLEN_SHIFT 24
+# define PR_PMLEN_MASK (0x7fUL << PR_PMLEN_SHIFT)

/* Control reclaim behavior when allocating memory */
#define PR_SET_IO_FLUSHER 57
--
2.45.1

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