Hi Alex,
On 2024-08-13 6:35 AM, Alexandre Ghiti wrote:
> Hi Samuel,
>
> On 25/06/2024 23:09, 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.
>
>
> Would it make sense to have a fast path when S-mode and U-mode PMLENs are equal?
I don't think so? Different userspace processes can have different PMLEN values,
including PMLEN==0, so it wouldn't be possible to patch out the untagging
operation based on PMLEN. (It's already skipped with a static branch if the
hardware doesn't support pointer masking). The untagging sequence is only 4
instructions (3 with pmlen in struct thread_info):
746: 41023603 ld a2,1040(tp) current->mm
74a: 46064603 lbu a2,1120(a2) current->mm->context.pmlen
74e: 00c51533 sll a0,a0,a2
752: 40c55533 sra a0,a0,a2
so I'm not sure how to make this faster.
>> 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.
>>
>> Unlike x86, untagged_addr() gets pmlen from struct thread_info instead
>> of a percpu variable, as this both avoids context switch overhead and
>> loads the value more efficiently.
>>
>> Signed-off-by: Samuel Holland <samuel.holland@xxxxxxxxxx>
>> ---
>>
>> 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 | 6 +++
>> arch/riscv/include/asm/thread_info.h | 3 ++
>> arch/riscv/include/asm/uaccess.h | 58 +++++++++++++++++++++--
>> arch/riscv/kernel/process.c | 69 +++++++++++++++++++++++++++-
>> 5 files changed, 136 insertions(+), 7 deletions(-)
>>
>> diff --git a/arch/riscv/include/asm/mmu.h b/arch/riscv/include/asm/mmu.h
>> index 947fd60f9051..361a9623f8c8 100644
>> --- a/arch/riscv/include/asm/mmu.h
>> +++ b/arch/riscv/include/asm/mmu.h
>> @@ -26,8 +26,15 @@ typedef struct {
>> unsigned long exec_fdpic_loadmap;
>> unsigned long interp_fdpic_loadmap;
>> #endif
>> +#ifdef CONFIG_RISCV_ISA_POINTER_MASKING
>> + unsigned long flags;
>> + 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..62a9f76cf257 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_POINTER_MASKING
>> + next->context.pmlen = 0;
>> +#endif
>> switch_mm(prev, next, NULL);
>> }
>> @@ -29,6 +32,9 @@ static inline int init_new_context(struct task_struct *tsk,
>> {
>> #ifdef CONFIG_MMU
>> atomic_long_set(&mm->context.id, 0);
>> +#endif
>> +#ifdef CONFIG_RISCV_ISA_POINTER_MASKING
>> + clear_bit(MM_CONTEXT_LOCK_PMLEN, &mm->context.flags);
>> #endif
>> return 0;
>> }
>> diff --git a/arch/riscv/include/asm/thread_info.h
>> b/arch/riscv/include/asm/thread_info.h
>> index 5d473343634b..cd355f8a550f 100644
>> --- a/arch/riscv/include/asm/thread_info.h
>> +++ b/arch/riscv/include/asm/thread_info.h
>> @@ -60,6 +60,9 @@ struct thread_info {
>> void *scs_base;
>> void *scs_sp;
>> #endif
>> +#ifdef CONFIG_RISCV_ISA_POINTER_MASKING
>> + u8 pmlen;
>> +#endif
>> };
>> #ifdef CONFIG_SHADOW_CALL_STACK
>> diff --git a/arch/riscv/include/asm/uaccess.h b/arch/riscv/include/asm/uaccess.h
>> index 72ec1d9bd3f3..153495997bc1 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_POINTER_MASKING
>> +static inline unsigned long __untagged_addr(unsigned long addr)
>> +{
>> + if (riscv_has_extension_unlikely(RISCV_ISA_EXT_SUPM)) {
>> + u8 pmlen = current->thread_info.pmlen;
>
>
> Why don't we use mm->pmlen? I don't see the need to introduce this variable that
> mirrors what is in mm already but I may be missing something.
Only that caching the value in struct thread_info saves an instruction/cache
line load from the pointer chasing. current->mm is likely to be hot anyway, so
it probably doesn't make too much difference. I will simplify this in v3.
Regards,
Samuel
>> +
>> + /* 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;
>> +}
>> +
>> +#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 dec5ccc44697..7bd445dade92 100644
>> --- a/arch/riscv/kernel/process.c
>> +++ b/arch/riscv/kernel/process.c
>> @@ -173,8 +173,10 @@ void flush_thread(void)
>> clear_tsk_thread_flag(current, TIF_RISCV_V_DEFER_RESTORE);
>> #endif
>> #ifdef CONFIG_RISCV_ISA_POINTER_MASKING
>> - if (riscv_has_extension_unlikely(RISCV_ISA_EXT_SUPM))
>> + if (riscv_has_extension_unlikely(RISCV_ISA_EXT_SUPM)) {
>> envcfg_update_bits(current, ENVCFG_PMM, ENVCFG_PMM_PMLEN_0);
>> + current->thread_info.pmlen = 0;
>> + }
>> #endif
>> }
>> @@ -204,6 +206,12 @@ 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);
>> +#ifdef CONFIG_RISCV_ISA_POINTER_MASKING
>> + /* Ensure all threads in this mm have the same pointer masking mode. */
>> + if (p->mm && (clone_flags & CLONE_VM))
>> + set_bit(MM_CONTEXT_LOCK_PMLEN, &p->mm->context.flags);
>> +#endif
>> +
>> memset(&p->thread.s, 0, sizeof(p->thread.s));
>> /* p->thread holds context to be restored by __switch_to() */
>> @@ -243,10 +251,16 @@ void __init arch_task_cache_init(void)
>> 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;
>> @@ -277,6 +291,14 @@ long set_tagged_addr_ctrl(struct task_struct *task,
>> unsigned long arg)
>> 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 (pmlen == 7)
>> pmm = ENVCFG_PMM_PMLEN_7;
>> else if (pmlen == 16)
>> @@ -284,7 +306,22 @@ long set_tagged_addr_ctrl(struct task_struct *task,
>> unsigned long arg)
>> else
>> pmm = ENVCFG_PMM_PMLEN_0;
>> + if (!(arg & PR_TAGGED_ADDR_ENABLE))
>> + 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);
>> + task->mm->context.pmlen = pmlen;
>> + task->thread_info.pmlen = pmlen;
>> +
>> + mmap_write_unlock(mm);
>> return 0;
>> }
>> @@ -297,6 +334,13 @@ long get_tagged_addr_ctrl(struct task_struct *task)
>> if (is_compat_thread(ti))
>> return -EINVAL;
>> + if (task->thread_info.pmlen)
>> + ret = PR_TAGGED_ADDR_ENABLE;
>> +
>> + /*
>> + * The task's pmlen is only set if 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, 7);
>> @@ -315,6 +359,24 @@ 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))
>> @@ -328,6 +390,9 @@ 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);
