Re: [PATCH v4 04/10] riscv: Add support for userspace pointer masking

[Charlie Jenkins <charlie@rivosinc.com>]

On Wed, Aug 28, 2024 at 06:01:26PM -0700, Samuel Holland wrote:
> 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().
> 
> Signed-off-by: Samuel Holland <samuel.holland@sifive.com>

Reviewed-by: Charlie Jenkins <charlie@rivosinc.com>
Tested-by: Charlie Jenkins <charlie@rivosinc.com>

> ---
> 
> 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()
> 
>  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         |  3 +
>  5 files changed, 124 insertions(+)
> 
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 0f3cd7c3a436..817437157138 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -512,6 +512,17 @@ 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 586e4ab701c4..5c4d4fb97314 100644
> --- a/arch/riscv/include/asm/processor.h
> +++ b/arch/riscv/include/asm/processor.h
> @@ -200,6 +200,14 @@ 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 e4bc61c4e58a..f39221ab5ddd 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>
> @@ -171,6 +172,10 @@ 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)
> @@ -233,3 +238,89 @@ 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..6e84c827869b 100644
> --- a/include/uapi/linux/prctl.h
> +++ b/include/uapi/linux/prctl.h
> @@ -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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