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Message-ID: <1faba7e8-903d-40f5-8285-1b309d7b9410@ghiti.fr>
Date: Tue, 13 Aug 2024 13:35:50 +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 v2 05/10] riscv: Add support for the tagged address ABI
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?
>
> 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@...ive.com>
> ---
>
> 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.
> +
> + /* 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);
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