Re: [PATCH v3 05/10] riscv: Add support for the tagged address ABI

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Hi Alex,

On 2024-08-14 10:10 AM, Alexandre Ghiti wrote:
> On 14/08/2024 10:13, 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>
>> ---
>>
>> 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     | 58 ++++++++++++++++++++--
>>   arch/riscv/kernel/process.c          | 73 ++++++++++++++++++++++++++--
>>   4 files changed, 141 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..6416559232a2 100644
>> --- a/arch/riscv/include/asm/uaccess.h
>> +++ b/arch/riscv/include/asm/uaccess.h
>> @@ -9,8 +9,56 @@
>>   #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(unsigned long addr)
>> +{
>> +    if (riscv_has_extension_unlikely(RISCV_ISA_EXT_SUPM)) {
>> +        u8 pmlen = current->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(__addr);        \
>> +})
>> +
>> +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;
>> +}
> 
> 
> I should have mentioned that in v2: now that you removed the thread_info pmlen
> field, __untagged_addr_remote() and __untagged_addr() are almost the same, can
> you merge them?

I can merge them, but this places the load of current->mm outside the static
branch. If you think that is okay, then I'll merge them for v4.

Regards,
Samuel

>> +
>> +#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 +178,7 @@ 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 +294,7 @@ 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 +341,13 @@ 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 +362,7 @@ 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 1280a7c4a412..f4d8e5c3bb84 100644
>> --- a/arch/riscv/kernel/process.c
>> +++ b/arch/riscv/kernel/process.c
>> @@ -203,6 +203,10 @@ 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() */
>> @@ -248,10 +252,16 @@ 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;
>>   @@ -266,16 +276,41 @@ 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;
>>   }
>> @@ -288,6 +323,10 @@ 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);
>> @@ -297,6 +336,9 @@ long get_tagged_addr_ctrl(struct task_struct *task)
>>           break;
>>       }
>>   +    if (task->mm->context.pmlen)
>> +        ret |= PR_TAGGED_ADDR_ENABLE;
>> +
>>       return ret;
>>   }
>>   @@ -306,6 +348,24 @@ 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))
>> @@ -319,6 +379,9 @@ 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);





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