Re: [PATCH v4 05/10] riscv: Add support for the tagged address ABI

[Samuel Holland]

Hi Charlie,

On 2024-09-12 9:45 PM, Charlie Jenkins wrote:
> On Wed, Aug 28, 2024 at 06:01:27PM -0700, Samuel Holland wrote:
>> When pointer masking is enabled for userspace, the kernel can accept
>> tagged pointers as arguments to some system calls. Allow this by
>> untagging the pointers in access_ok() and the uaccess routines. The
>> uaccess routines must peform untagging in software because U-mode and
>> S-mode have entirely separate pointer masking configurations. In fact,
>> hardware may not even implement pointer masking for S-mode.
>>
>> Since the number of tag bits is variable, untagged_addr_remote() needs
>> to know what PMLEN to use for the remote mm. Therefore, the pointer
>> masking mode must be the same for all threads sharing an mm. Enforce
>> this with a lock flag in the mm context, as x86 does for LAM. The flag
>> gets reset in init_new_context() during fork(), as the new mm is no
>> longer multithreaded.
>>
>> Signed-off-by: Samuel Holland <samuel.holland@xxxxxxxxxx>
>> ---
> 
> Not necessary, but what do you think about adding riscv to include/uapi/linux/prctl.h:
> 
> /* Tagged user address controls for arm64 */
> #define PR_SET_TAGGED_ADDR_CTRL		55
> #define PR_GET_TAGGED_ADDR_CTRL		56
> # define PR_TAGGED_ADDR_ENABLE		(1UL << 0)

Yes, I'll add this in v5.

> Also looks like this last line should probably be under SET rather than
> GET.

The same bit fields are used for both prctl() functions, so I think the current
grouping is okay (compare PR_RISCV_V_GET_CONTROL).

Regards,
Samuel

> Reviewed-by: Charlie Jenkins <charlie@xxxxxxxxxxxx>
> Tested-by: Charlie Jenkins <charlie@xxxxxxxxxxxx>
> 
>>
>> Changes in v4:
>>  - Combine __untagged_addr() and __untagged_addr_remote()
>>
>> Changes in v3:
>>  - Use IS_ENABLED instead of #ifdef when possible
>>  - Implement mm_untag_mask()
>>  - Remove pmlen from struct thread_info (now only in mm_context_t)
>>
>> Changes in v2:
>>  - Implement untagged_addr_remote()
>>  - Restrict PMLEN changes once a process is multithreaded
>>
>>  arch/riscv/include/asm/mmu.h         |  7 +++
>>  arch/riscv/include/asm/mmu_context.h | 13 +++++
>>  arch/riscv/include/asm/uaccess.h     | 43 ++++++++++++++--
>>  arch/riscv/kernel/process.c          | 73 ++++++++++++++++++++++++++--
>>  4 files changed, 126 insertions(+), 10 deletions(-)
>>
>> diff --git a/arch/riscv/include/asm/mmu.h b/arch/riscv/include/asm/mmu.h
>> index c9e03e9da3dc..1cc90465d75b 100644
>> --- a/arch/riscv/include/asm/mmu.h
>> +++ b/arch/riscv/include/asm/mmu.h
>> @@ -25,9 +25,16 @@ typedef struct {
>>  #ifdef CONFIG_BINFMT_ELF_FDPIC
>>  	unsigned long exec_fdpic_loadmap;
>>  	unsigned long interp_fdpic_loadmap;
>> +#endif
>> +	unsigned long flags;
>> +#ifdef CONFIG_RISCV_ISA_SUPM
>> +	u8 pmlen;
>>  #endif
>>  } mm_context_t;
>>  
>> +/* Lock the pointer masking mode because this mm is multithreaded */
>> +#define MM_CONTEXT_LOCK_PMLEN	0
>> +
>>  #define cntx2asid(cntx)		((cntx) & SATP_ASID_MASK)
>>  #define cntx2version(cntx)	((cntx) & ~SATP_ASID_MASK)
>>  
>> diff --git a/arch/riscv/include/asm/mmu_context.h b/arch/riscv/include/asm/mmu_context.h
>> index 7030837adc1a..8c4bc49a3a0f 100644
>> --- a/arch/riscv/include/asm/mmu_context.h
>> +++ b/arch/riscv/include/asm/mmu_context.h
>> @@ -20,6 +20,9 @@ void switch_mm(struct mm_struct *prev, struct mm_struct *next,
>>  static inline void activate_mm(struct mm_struct *prev,
>>  			       struct mm_struct *next)
>>  {
>> +#ifdef CONFIG_RISCV_ISA_SUPM
>> +	next->context.pmlen = 0;
>> +#endif
>>  	switch_mm(prev, next, NULL);
>>  }
>>  
>> @@ -30,11 +33,21 @@ static inline int init_new_context(struct task_struct *tsk,
>>  #ifdef CONFIG_MMU
>>  	atomic_long_set(&mm->context.id, 0);
>>  #endif
>> +	if (IS_ENABLED(CONFIG_RISCV_ISA_SUPM))
>> +		clear_bit(MM_CONTEXT_LOCK_PMLEN, &mm->context.flags);
>>  	return 0;
>>  }
>>  
>>  DECLARE_STATIC_KEY_FALSE(use_asid_allocator);
>>  
>> +#ifdef CONFIG_RISCV_ISA_SUPM
>> +#define mm_untag_mask mm_untag_mask
>> +static inline unsigned long mm_untag_mask(struct mm_struct *mm)
>> +{
>> +	return -1UL >> mm->context.pmlen;
>> +}
>> +#endif
>> +
>>  #include <asm-generic/mmu_context.h>
>>  
>>  #endif /* _ASM_RISCV_MMU_CONTEXT_H */
>> diff --git a/arch/riscv/include/asm/uaccess.h b/arch/riscv/include/asm/uaccess.h
>> index 72ec1d9bd3f3..fee56b0c8058 100644
>> --- a/arch/riscv/include/asm/uaccess.h
>> +++ b/arch/riscv/include/asm/uaccess.h
>> @@ -9,8 +9,41 @@
>>  #define _ASM_RISCV_UACCESS_H
>>  
>>  #include <asm/asm-extable.h>
>> +#include <asm/cpufeature.h>
>>  #include <asm/pgtable.h>		/* for TASK_SIZE */
>>  
>> +#ifdef CONFIG_RISCV_ISA_SUPM
>> +static inline unsigned long __untagged_addr_remote(struct mm_struct *mm, unsigned long addr)
>> +{
>> +	if (riscv_has_extension_unlikely(RISCV_ISA_EXT_SUPM)) {
>> +		u8 pmlen = mm->context.pmlen;
>> +
>> +		/* Virtual addresses are sign-extended; physical addresses are zero-extended. */
>> +		if (IS_ENABLED(CONFIG_MMU))
>> +			return (long)(addr << pmlen) >> pmlen;
>> +		else
>> +			return (addr << pmlen) >> pmlen;
>> +	}
>> +
>> +	return addr;
>> +}
>> +
>> +#define untagged_addr(addr) ({							\
>> +	unsigned long __addr = (__force unsigned long)(addr);			\
>> +	(__force __typeof__(addr))__untagged_addr_remote(current->mm, __addr);	\
>> +})
>> +
>> +#define untagged_addr_remote(mm, addr) ({					\
>> +	unsigned long __addr = (__force unsigned long)(addr);			\
>> +	mmap_assert_locked(mm);							\
>> +	(__force __typeof__(addr))__untagged_addr_remote(mm, __addr);		\
>> +})
>> +
>> +#define access_ok(addr, size) likely(__access_ok(untagged_addr(addr), size))
>> +#else
>> +#define untagged_addr(addr) (addr)
>> +#endif
>> +
>>  /*
>>   * User space memory access functions
>>   */
>> @@ -130,7 +163,7 @@ do {								\
>>   */
>>  #define __get_user(x, ptr)					\
>>  ({								\
>> -	const __typeof__(*(ptr)) __user *__gu_ptr = (ptr);	\
>> +	const __typeof__(*(ptr)) __user *__gu_ptr = untagged_addr(ptr); \
>>  	long __gu_err = 0;					\
>>  								\
>>  	__chk_user_ptr(__gu_ptr);				\
>> @@ -246,7 +279,7 @@ do {								\
>>   */
>>  #define __put_user(x, ptr)					\
>>  ({								\
>> -	__typeof__(*(ptr)) __user *__gu_ptr = (ptr);		\
>> +	__typeof__(*(ptr)) __user *__gu_ptr = untagged_addr(ptr); \
>>  	__typeof__(*__gu_ptr) __val = (x);			\
>>  	long __pu_err = 0;					\
>>  								\
>> @@ -293,13 +326,13 @@ unsigned long __must_check __asm_copy_from_user(void *to,
>>  static inline unsigned long
>>  raw_copy_from_user(void *to, const void __user *from, unsigned long n)
>>  {
>> -	return __asm_copy_from_user(to, from, n);
>> +	return __asm_copy_from_user(to, untagged_addr(from), n);
>>  }
>>  
>>  static inline unsigned long
>>  raw_copy_to_user(void __user *to, const void *from, unsigned long n)
>>  {
>> -	return __asm_copy_to_user(to, from, n);
>> +	return __asm_copy_to_user(untagged_addr(to), from, n);
>>  }
>>  
>>  extern long strncpy_from_user(char *dest, const char __user *src, long count);
>> @@ -314,7 +347,7 @@ unsigned long __must_check clear_user(void __user *to, unsigned long n)
>>  {
>>  	might_fault();
>>  	return access_ok(to, n) ?
>> -		__clear_user(to, n) : n;
>> +		__clear_user(untagged_addr(to), n) : n;
>>  }
>>  
>>  #define __get_kernel_nofault(dst, src, type, err_label)			\
>> diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
>> index f39221ab5ddd..6e9c84a41c29 100644
>> --- a/arch/riscv/kernel/process.c
>> +++ b/arch/riscv/kernel/process.c
>> @@ -204,6 +204,10 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
>>  	unsigned long tls = args->tls;
>>  	struct pt_regs *childregs = task_pt_regs(p);
>>  
>> +	/* Ensure all threads in this mm have the same pointer masking mode. */
>> +	if (IS_ENABLED(CONFIG_RISCV_ISA_SUPM) && p->mm && (clone_flags & CLONE_VM))
>> +		set_bit(MM_CONTEXT_LOCK_PMLEN, &p->mm->context.flags);
>> +
>>  	memset(&p->thread.s, 0, sizeof(p->thread.s));
>>  
>>  	/* p->thread holds context to be restored by __switch_to() */
>> @@ -249,10 +253,16 @@ enum {
>>  static bool have_user_pmlen_7;
>>  static bool have_user_pmlen_16;
>>  
>> +/*
>> + * Control the relaxed ABI allowing tagged user addresses into the kernel.
>> + */
>> +static unsigned int tagged_addr_disabled;
>> +
>>  long set_tagged_addr_ctrl(struct task_struct *task, unsigned long arg)
>>  {
>> -	unsigned long valid_mask = PR_PMLEN_MASK;
>> +	unsigned long valid_mask = PR_PMLEN_MASK | PR_TAGGED_ADDR_ENABLE;
>>  	struct thread_info *ti = task_thread_info(task);
>> +	struct mm_struct *mm = task->mm;
>>  	unsigned long pmm;
>>  	u8 pmlen;
>>  
>> @@ -267,16 +277,41 @@ long set_tagged_addr_ctrl(struct task_struct *task, unsigned long arg)
>>  	 * in case choosing a larger PMLEN has a performance impact.
>>  	 */
>>  	pmlen = FIELD_GET(PR_PMLEN_MASK, arg);
>> -	if (pmlen == PMLEN_0)
>> +	if (pmlen == PMLEN_0) {
>>  		pmm = ENVCFG_PMM_PMLEN_0;
>> -	else if (pmlen <= PMLEN_7 && have_user_pmlen_7)
>> +	} else if (pmlen <= PMLEN_7 && have_user_pmlen_7) {
>> +		pmlen = PMLEN_7;
>>  		pmm = ENVCFG_PMM_PMLEN_7;
>> -	else if (pmlen <= PMLEN_16 && have_user_pmlen_16)
>> +	} else if (pmlen <= PMLEN_16 && have_user_pmlen_16) {
>> +		pmlen = PMLEN_16;
>>  		pmm = ENVCFG_PMM_PMLEN_16;
>> -	else
>> +	} else {
>>  		return -EINVAL;
>> +	}
>> +
>> +	/*
>> +	 * Do not allow the enabling of the tagged address ABI if globally
>> +	 * disabled via sysctl abi.tagged_addr_disabled, if pointer masking
>> +	 * is disabled for userspace.
>> +	 */
>> +	if (arg & PR_TAGGED_ADDR_ENABLE && (tagged_addr_disabled || !pmlen))
>> +		return -EINVAL;
>> +
>> +	if (!(arg & PR_TAGGED_ADDR_ENABLE))
>> +		pmlen = PMLEN_0;
>> +
>> +	if (mmap_write_lock_killable(mm))
>> +		return -EINTR;
>> +
>> +	if (test_bit(MM_CONTEXT_LOCK_PMLEN, &mm->context.flags) && mm->context.pmlen != pmlen) {
>> +		mmap_write_unlock(mm);
>> +		return -EBUSY;
>> +	}
>>  
>>  	envcfg_update_bits(task, ENVCFG_PMM, pmm);
>> +	mm->context.pmlen = pmlen;
>> +
>> +	mmap_write_unlock(mm);
>>  
>>  	return 0;
>>  }
>> @@ -289,6 +324,10 @@ long get_tagged_addr_ctrl(struct task_struct *task)
>>  	if (is_compat_thread(ti))
>>  		return -EINVAL;
>>  
>> +	/*
>> +	 * The mm context's pmlen is set only when the tagged address ABI is
>> +	 * enabled, so the effective PMLEN must be extracted from envcfg.PMM.
>> +	 */
>>  	switch (task->thread.envcfg & ENVCFG_PMM) {
>>  	case ENVCFG_PMM_PMLEN_7:
>>  		ret = FIELD_PREP(PR_PMLEN_MASK, PMLEN_7);
>> @@ -298,6 +337,9 @@ long get_tagged_addr_ctrl(struct task_struct *task)
>>  		break;
>>  	}
>>  
>> +	if (task->mm->context.pmlen)
>> +		ret |= PR_TAGGED_ADDR_ENABLE;
>> +
>>  	return ret;
>>  }
>>  
>> @@ -307,6 +349,24 @@ static bool try_to_set_pmm(unsigned long value)
>>  	return (csr_read_clear(CSR_ENVCFG, ENVCFG_PMM) & ENVCFG_PMM) == value;
>>  }
>>  
>> +/*
>> + * Global sysctl to disable the tagged user addresses support. This control
>> + * only prevents the tagged address ABI enabling via prctl() and does not
>> + * disable it for tasks that already opted in to the relaxed ABI.
>> + */
>> +
>> +static struct ctl_table tagged_addr_sysctl_table[] = {
>> +	{
>> +		.procname	= "tagged_addr_disabled",
>> +		.mode		= 0644,
>> +		.data		= &tagged_addr_disabled,
>> +		.maxlen		= sizeof(int),
>> +		.proc_handler	= proc_dointvec_minmax,
>> +		.extra1		= SYSCTL_ZERO,
>> +		.extra2		= SYSCTL_ONE,
>> +	},
>> +};
>> +
>>  static int __init tagged_addr_init(void)
>>  {
>>  	if (!riscv_has_extension_unlikely(RISCV_ISA_EXT_SUPM))
>> @@ -320,6 +380,9 @@ static int __init tagged_addr_init(void)
>>  	have_user_pmlen_7 = try_to_set_pmm(ENVCFG_PMM_PMLEN_7);
>>  	have_user_pmlen_16 = try_to_set_pmm(ENVCFG_PMM_PMLEN_16);
>>  
>> +	if (!register_sysctl("abi", tagged_addr_sysctl_table))
>> +		return -EINVAL;
>> +
>>  	return 0;
>>  }
>>  core_initcall(tagged_addr_init);
>> -- 
>> 2.45.1
>>
>>
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