Message-ID: <05eb614b-f4fc-4154-96b3-a30e5adc789f@ghiti.fr>
Date: Wed, 14 Aug 2024 21:29:05 +0200
From: Alexandre Ghiti <alex@...ti.fr>
To: Samuel Holland <samuel.holland@...ive.com>,
 Palmer Dabbelt <palmer@...belt.com>, linux-riscv@...ts.infradead.org
Cc: devicetree@...r.kernel.org, Catalin Marinas <catalin.marinas@....com>,
 linux-kernel@...r.kernel.org, Anup Patel <anup@...infault.org>,
 Conor Dooley <conor@...nel.org>, kasan-dev@...glegroups.com,
 Atish Patra <atishp@...shpatra.org>, Evgenii Stepanov <eugenis@...gle.com>,
 Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
 Rob Herring <robh+dt@...nel.org>,
 "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v3 05/10] riscv: Add support for the tagged address ABI


On 14/08/2024 18:14, Samuel Holland wrote:
> 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@...ive.com>
>>> ---
>>>
>>> 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.


I think it's ok, it's not a big overhead :)

Thanks,

Alex


> 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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