mmap locking API: convert mmap_sem comments

[mirror_ubuntu-jammy-kernel.git] / arch / x86 / kernel / ldt.c

// SPDX-License-Identifier: GPL-2.0
/*
 * Copyright (C) 1992 Krishna Balasubramanian and Linus Torvalds
 * Copyright (C) 1999 Ingo Molnar <mingo@redhat.com>
 * Copyright (C) 2002 Andi Kleen
 *
 * This handles calls from both 32bit and 64bit mode.
 *
 * Lock order:
 *      contex.ldt_usr_sem
 *        mmap_lock
 *          context.lock
 */

#include <linux/errno.h>
#include <linux/gfp.h>
#include <linux/sched.h>
#include <linux/string.h>
#include <linux/mm.h>
#include <linux/smp.h>
#include <linux/syscalls.h>
#include <linux/slab.h>
#include <linux/vmalloc.h>
#include <linux/uaccess.h>

#include <asm/ldt.h>
#include <asm/tlb.h>
#include <asm/desc.h>
#include <asm/mmu_context.h>
#include <asm/pgtable_areas.h>

/* This is a multiple of PAGE_SIZE. */
#define LDT_SLOT_STRIDE (LDT_ENTRIES * LDT_ENTRY_SIZE)

static inline void *ldt_slot_va(int slot)
{
        return (void *)(LDT_BASE_ADDR + LDT_SLOT_STRIDE * slot);
}

void load_mm_ldt(struct mm_struct *mm)
{
        struct ldt_struct *ldt;

        /* READ_ONCE synchronizes with smp_store_release */
        ldt = READ_ONCE(mm->context.ldt);

        /*
         * Any change to mm->context.ldt is followed by an IPI to all
         * CPUs with the mm active.  The LDT will not be freed until
         * after the IPI is handled by all such CPUs.  This means that,
         * if the ldt_struct changes before we return, the values we see
         * will be safe, and the new values will be loaded before we run
         * any user code.
         *
         * NB: don't try to convert this to use RCU without extreme care.
         * We would still need IRQs off, because we don't want to change
         * the local LDT after an IPI loaded a newer value than the one
         * that we can see.
         */

        if (unlikely(ldt)) {
                if (static_cpu_has(X86_FEATURE_PTI)) {
                        if (WARN_ON_ONCE((unsigned long)ldt->slot > 1)) {
                                /*
                                 * Whoops -- either the new LDT isn't mapped
                                 * (if slot == -1) or is mapped into a bogus
                                 * slot (if slot > 1).
                                 */
                                clear_LDT();
                                return;
                        }

                        /*
                         * If page table isolation is enabled, ldt->entries
                         * will not be mapped in the userspace pagetables.
                         * Tell the CPU to access the LDT through the alias
                         * at ldt_slot_va(ldt->slot).
                         */
                        set_ldt(ldt_slot_va(ldt->slot), ldt->nr_entries);
                } else {
                        set_ldt(ldt->entries, ldt->nr_entries);
                }
        } else {
                clear_LDT();
        }
}

void switch_ldt(struct mm_struct *prev, struct mm_struct *next)
{
        /*
         * Load the LDT if either the old or new mm had an LDT.
         *
         * An mm will never go from having an LDT to not having an LDT.  Two
         * mms never share an LDT, so we don't gain anything by checking to
         * see whether the LDT changed.  There's also no guarantee that
         * prev->context.ldt actually matches LDTR, but, if LDTR is non-NULL,
         * then prev->context.ldt will also be non-NULL.
         *
         * If we really cared, we could optimize the case where prev == next
         * and we're exiting lazy mode.  Most of the time, if this happens,
         * we don't actually need to reload LDTR, but modify_ldt() is mostly
         * used by legacy code and emulators where we don't need this level of
         * performance.
         *
         * This uses | instead of || because it generates better code.
         */
        if (unlikely((unsigned long)prev->context.ldt |
                     (unsigned long)next->context.ldt))
                load_mm_ldt(next);

        DEBUG_LOCKS_WARN_ON(preemptible());
}

static void refresh_ldt_segments(void)
{
#ifdef CONFIG_X86_64
        unsigned short sel;

        /*
         * Make sure that the cached DS and ES descriptors match the updated
         * LDT.
         */
        savesegment(ds, sel);
        if ((sel & SEGMENT_TI_MASK) == SEGMENT_LDT)
                loadsegment(ds, sel);

        savesegment(es, sel);
        if ((sel & SEGMENT_TI_MASK) == SEGMENT_LDT)
                loadsegment(es, sel);
#endif
}

/* context.lock is held by the task which issued the smp function call */
static void flush_ldt(void *__mm)
{
        struct mm_struct *mm = __mm;

        if (this_cpu_read(cpu_tlbstate.loaded_mm) != mm)
                return;

        load_mm_ldt(mm);

        refresh_ldt_segments();
}

/* The caller must call finalize_ldt_struct on the result. LDT starts zeroed. */
static struct ldt_struct *alloc_ldt_struct(unsigned int num_entries)
{
        struct ldt_struct *new_ldt;
        unsigned int alloc_size;

        if (num_entries > LDT_ENTRIES)
                return NULL;

        new_ldt = kmalloc(sizeof(struct ldt_struct), GFP_KERNEL);
        if (!new_ldt)
                return NULL;

        BUILD_BUG_ON(LDT_ENTRY_SIZE != sizeof(struct desc_struct));
        alloc_size = num_entries * LDT_ENTRY_SIZE;

        /*
         * Xen is very picky: it requires a page-aligned LDT that has no
         * trailing nonzero bytes in any page that contains LDT descriptors.
         * Keep it simple: zero the whole allocation and never allocate less
         * than PAGE_SIZE.
         */
        if (alloc_size > PAGE_SIZE)
                new_ldt->entries = vzalloc(alloc_size);
        else
                new_ldt->entries = (void *)get_zeroed_page(GFP_KERNEL);

        if (!new_ldt->entries) {
                kfree(new_ldt);
                return NULL;
        }

        /* The new LDT isn't aliased for PTI yet. */
        new_ldt->slot = -1;

        new_ldt->nr_entries = num_entries;
        return new_ldt;
}

#ifdef CONFIG_PAGE_TABLE_ISOLATION

static void do_sanity_check(struct mm_struct *mm,
                            bool had_kernel_mapping,
                            bool had_user_mapping)
{
        if (mm->context.ldt) {
                /*
                 * We already had an LDT.  The top-level entry should already
                 * have been allocated and synchronized with the usermode
                 * tables.
                 */
                WARN_ON(!had_kernel_mapping);
                if (boot_cpu_has(X86_FEATURE_PTI))
                        WARN_ON(!had_user_mapping);
        } else {
                /*
                 * This is the first time we're mapping an LDT for this process.
                 * Sync the pgd to the usermode tables.
                 */
                WARN_ON(had_kernel_mapping);
                if (boot_cpu_has(X86_FEATURE_PTI))
                        WARN_ON(had_user_mapping);
        }
}

#ifdef CONFIG_X86_PAE

static pmd_t *pgd_to_pmd_walk(pgd_t *pgd, unsigned long va)
{
        p4d_t *p4d;
        pud_t *pud;

        if (pgd->pgd == 0)
                return NULL;

        p4d = p4d_offset(pgd, va);
        if (p4d_none(*p4d))
                return NULL;

        pud = pud_offset(p4d, va);
        if (pud_none(*pud))
                return NULL;

        return pmd_offset(pud, va);
}

static void map_ldt_struct_to_user(struct mm_struct *mm)
{
        pgd_t *k_pgd = pgd_offset(mm, LDT_BASE_ADDR);
        pgd_t *u_pgd = kernel_to_user_pgdp(k_pgd);
        pmd_t *k_pmd, *u_pmd;

        k_pmd = pgd_to_pmd_walk(k_pgd, LDT_BASE_ADDR);
        u_pmd = pgd_to_pmd_walk(u_pgd, LDT_BASE_ADDR);

        if (boot_cpu_has(X86_FEATURE_PTI) && !mm->context.ldt)
                set_pmd(u_pmd, *k_pmd);
}

static void sanity_check_ldt_mapping(struct mm_struct *mm)
{
        pgd_t *k_pgd = pgd_offset(mm, LDT_BASE_ADDR);
        pgd_t *u_pgd = kernel_to_user_pgdp(k_pgd);
        bool had_kernel, had_user;
        pmd_t *k_pmd, *u_pmd;

        k_pmd      = pgd_to_pmd_walk(k_pgd, LDT_BASE_ADDR);
        u_pmd      = pgd_to_pmd_walk(u_pgd, LDT_BASE_ADDR);
        had_kernel = (k_pmd->pmd != 0);
        had_user   = (u_pmd->pmd != 0);

        do_sanity_check(mm, had_kernel, had_user);
}

#else /* !CONFIG_X86_PAE */

static void map_ldt_struct_to_user(struct mm_struct *mm)
{
        pgd_t *pgd = pgd_offset(mm, LDT_BASE_ADDR);

        if (boot_cpu_has(X86_FEATURE_PTI) && !mm->context.ldt)
                set_pgd(kernel_to_user_pgdp(pgd), *pgd);
}

static void sanity_check_ldt_mapping(struct mm_struct *mm)
{
        pgd_t *pgd = pgd_offset(mm, LDT_BASE_ADDR);
        bool had_kernel = (pgd->pgd != 0);
        bool had_user   = (kernel_to_user_pgdp(pgd)->pgd != 0);

        do_sanity_check(mm, had_kernel, had_user);
}

#endif /* CONFIG_X86_PAE */

/*
 * If PTI is enabled, this maps the LDT into the kernelmode and
 * usermode tables for the given mm.
 */
static int
map_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt, int slot)
{
        unsigned long va;
        bool is_vmalloc;
        spinlock_t *ptl;
        int i, nr_pages;

        if (!boot_cpu_has(X86_FEATURE_PTI))
                return 0;

        /*
         * Any given ldt_struct should have map_ldt_struct() called at most
         * once.
         */
        WARN_ON(ldt->slot != -1);

        /* Check if the current mappings are sane */
        sanity_check_ldt_mapping(mm);

        is_vmalloc = is_vmalloc_addr(ldt->entries);

        nr_pages = DIV_ROUND_UP(ldt->nr_entries * LDT_ENTRY_SIZE, PAGE_SIZE);

        for (i = 0; i < nr_pages; i++) {
                unsigned long offset = i << PAGE_SHIFT;
                const void *src = (char *)ldt->entries + offset;
                unsigned long pfn;
                pgprot_t pte_prot;
                pte_t pte, *ptep;

                va = (unsigned long)ldt_slot_va(slot) + offset;
                pfn = is_vmalloc ? vmalloc_to_pfn(src) :
                        page_to_pfn(virt_to_page(src));
                /*
                 * Treat the PTI LDT range as a *userspace* range.
                 * get_locked_pte() will allocate all needed pagetables
                 * and account for them in this mm.
                 */
                ptep = get_locked_pte(mm, va, &ptl);
                if (!ptep)
                        return -ENOMEM;
                /*
                 * Map it RO so the easy to find address is not a primary
                 * target via some kernel interface which misses a
                 * permission check.
                 */
                pte_prot = __pgprot(__PAGE_KERNEL_RO & ~_PAGE_GLOBAL);
                /* Filter out unsuppored __PAGE_KERNEL* bits: */
                pgprot_val(pte_prot) &= __supported_pte_mask;
                pte = pfn_pte(pfn, pte_prot);
                set_pte_at(mm, va, ptep, pte);
                pte_unmap_unlock(ptep, ptl);
        }

        /* Propagate LDT mapping to the user page-table */
        map_ldt_struct_to_user(mm);

        ldt->slot = slot;
        return 0;
}

static void unmap_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt)
{
        unsigned long va;
        int i, nr_pages;

        if (!ldt)
                return;

        /* LDT map/unmap is only required for PTI */
        if (!boot_cpu_has(X86_FEATURE_PTI))
                return;

        nr_pages = DIV_ROUND_UP(ldt->nr_entries * LDT_ENTRY_SIZE, PAGE_SIZE);

        for (i = 0; i < nr_pages; i++) {
                unsigned long offset = i << PAGE_SHIFT;
                spinlock_t *ptl;
                pte_t *ptep;

                va = (unsigned long)ldt_slot_va(ldt->slot) + offset;
                ptep = get_locked_pte(mm, va, &ptl);
                pte_clear(mm, va, ptep);
                pte_unmap_unlock(ptep, ptl);
        }

        va = (unsigned long)ldt_slot_va(ldt->slot);
        flush_tlb_mm_range(mm, va, va + nr_pages * PAGE_SIZE, PAGE_SHIFT, false);
}

#else /* !CONFIG_PAGE_TABLE_ISOLATION */

static int
map_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt, int slot)
{
        return 0;
}

static void unmap_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt)
{
}
#endif /* CONFIG_PAGE_TABLE_ISOLATION */

static void free_ldt_pgtables(struct mm_struct *mm)
{
#ifdef CONFIG_PAGE_TABLE_ISOLATION
        struct mmu_gather tlb;
        unsigned long start = LDT_BASE_ADDR;
        unsigned long end = LDT_END_ADDR;

        if (!boot_cpu_has(X86_FEATURE_PTI))
                return;

        tlb_gather_mmu(&tlb, mm, start, end);
        free_pgd_range(&tlb, start, end, start, end);
        tlb_finish_mmu(&tlb, start, end);
#endif
}

/* After calling this, the LDT is immutable. */
static void finalize_ldt_struct(struct ldt_struct *ldt)
{
        paravirt_alloc_ldt(ldt->entries, ldt->nr_entries);
}

static void install_ldt(struct mm_struct *mm, struct ldt_struct *ldt)
{
        mutex_lock(&mm->context.lock);

        /* Synchronizes with READ_ONCE in load_mm_ldt. */
        smp_store_release(&mm->context.ldt, ldt);

        /* Activate the LDT for all CPUs using currents mm. */
        on_each_cpu_mask(mm_cpumask(mm), flush_ldt, mm, true);

        mutex_unlock(&mm->context.lock);
}

static void free_ldt_struct(struct ldt_struct *ldt)
{
        if (likely(!ldt))
                return;

        paravirt_free_ldt(ldt->entries, ldt->nr_entries);
        if (ldt->nr_entries * LDT_ENTRY_SIZE > PAGE_SIZE)
                vfree_atomic(ldt->entries);
        else
                free_page((unsigned long)ldt->entries);
        kfree(ldt);
}

/*
 * Called on fork from arch_dup_mmap(). Just copy the current LDT state,
 * the new task is not running, so nothing can be installed.
 */
int ldt_dup_context(struct mm_struct *old_mm, struct mm_struct *mm)
{
        struct ldt_struct *new_ldt;
        int retval = 0;

        if (!old_mm)
                return 0;

        mutex_lock(&old_mm->context.lock);
        if (!old_mm->context.ldt)
                goto out_unlock;

        new_ldt = alloc_ldt_struct(old_mm->context.ldt->nr_entries);
        if (!new_ldt) {
                retval = -ENOMEM;
                goto out_unlock;
        }

        memcpy(new_ldt->entries, old_mm->context.ldt->entries,
               new_ldt->nr_entries * LDT_ENTRY_SIZE);
        finalize_ldt_struct(new_ldt);

        retval = map_ldt_struct(mm, new_ldt, 0);
        if (retval) {
                free_ldt_pgtables(mm);
                free_ldt_struct(new_ldt);
                goto out_unlock;
        }
        mm->context.ldt = new_ldt;

out_unlock:
        mutex_unlock(&old_mm->context.lock);
        return retval;
}

/*
 * No need to lock the MM as we are the last user
 *
 * 64bit: Don't touch the LDT register - we're already in the next thread.
 */
void destroy_context_ldt(struct mm_struct *mm)
{
        free_ldt_struct(mm->context.ldt);
        mm->context.ldt = NULL;
}

void ldt_arch_exit_mmap(struct mm_struct *mm)
{
        free_ldt_pgtables(mm);
}

static int read_ldt(void __user *ptr, unsigned long bytecount)
{
        struct mm_struct *mm = current->mm;
        unsigned long entries_size;
        int retval;

        down_read(&mm->context.ldt_usr_sem);

        if (!mm->context.ldt) {
                retval = 0;
                goto out_unlock;
        }

        if (bytecount > LDT_ENTRY_SIZE * LDT_ENTRIES)
                bytecount = LDT_ENTRY_SIZE * LDT_ENTRIES;

        entries_size = mm->context.ldt->nr_entries * LDT_ENTRY_SIZE;
        if (entries_size > bytecount)
                entries_size = bytecount;

        if (copy_to_user(ptr, mm->context.ldt->entries, entries_size)) {
                retval = -EFAULT;
                goto out_unlock;
        }

        if (entries_size != bytecount) {
                /* Zero-fill the rest and pretend we read bytecount bytes. */
                if (clear_user(ptr + entries_size, bytecount - entries_size)) {
                        retval = -EFAULT;
                        goto out_unlock;
                }
        }
        retval = bytecount;

out_unlock:
        up_read(&mm->context.ldt_usr_sem);
        return retval;
}

static int read_default_ldt(void __user *ptr, unsigned long bytecount)
{
        /* CHECKME: Can we use _one_ random number ? */
#ifdef CONFIG_X86_32
        unsigned long size = 5 * sizeof(struct desc_struct);
#else
        unsigned long size = 128;
#endif
        if (bytecount > size)
                bytecount = size;
        if (clear_user(ptr, bytecount))
                return -EFAULT;
        return bytecount;
}

static int write_ldt(void __user *ptr, unsigned long bytecount, int oldmode)
{
        struct mm_struct *mm = current->mm;
        struct ldt_struct *new_ldt, *old_ldt;
        unsigned int old_nr_entries, new_nr_entries;
        struct user_desc ldt_info;
        struct desc_struct ldt;
        int error;

        error = -EINVAL;
        if (bytecount != sizeof(ldt_info))
                goto out;
        error = -EFAULT;
        if (copy_from_user(&ldt_info, ptr, sizeof(ldt_info)))
                goto out;

        error = -EINVAL;
        if (ldt_info.entry_number >= LDT_ENTRIES)
                goto out;
        if (ldt_info.contents == 3) {
                if (oldmode)
                        goto out;
                if (ldt_info.seg_not_present == 0)
                        goto out;
        }

        if ((oldmode && !ldt_info.base_addr && !ldt_info.limit) ||
            LDT_empty(&ldt_info)) {
                /* The user wants to clear the entry. */
                memset(&ldt, 0, sizeof(ldt));
        } else {
                if (!IS_ENABLED(CONFIG_X86_16BIT) && !ldt_info.seg_32bit) {
                        error = -EINVAL;
                        goto out;
                }

                fill_ldt(&ldt, &ldt_info);
                if (oldmode)
                        ldt.avl = 0;
        }

        if (down_write_killable(&mm->context.ldt_usr_sem))
                return -EINTR;

        old_ldt       = mm->context.ldt;
        old_nr_entries = old_ldt ? old_ldt->nr_entries : 0;
        new_nr_entries = max(ldt_info.entry_number + 1, old_nr_entries);

        error = -ENOMEM;
        new_ldt = alloc_ldt_struct(new_nr_entries);
        if (!new_ldt)
                goto out_unlock;

        if (old_ldt)
                memcpy(new_ldt->entries, old_ldt->entries, old_nr_entries * LDT_ENTRY_SIZE);

        new_ldt->entries[ldt_info.entry_number] = ldt;
        finalize_ldt_struct(new_ldt);

        /*
         * If we are using PTI, map the new LDT into the userspace pagetables.
         * If there is already an LDT, use the other slot so that other CPUs
         * will continue to use the old LDT until install_ldt() switches
         * them over to the new LDT.
         */
        error = map_ldt_struct(mm, new_ldt, old_ldt ? !old_ldt->slot : 0);
        if (error) {
                /*
                 * This only can fail for the first LDT setup. If an LDT is
                 * already installed then the PTE page is already
                 * populated. Mop up a half populated page table.
                 */
                if (!WARN_ON_ONCE(old_ldt))
                        free_ldt_pgtables(mm);
                free_ldt_struct(new_ldt);
                goto out_unlock;
        }

        install_ldt(mm, new_ldt);
        unmap_ldt_struct(mm, old_ldt);
        free_ldt_struct(old_ldt);
        error = 0;

out_unlock:
        up_write(&mm->context.ldt_usr_sem);
out:
        return error;
}

SYSCALL_DEFINE3(modify_ldt, int , func , void __user * , ptr ,
                unsigned long , bytecount)
{
        int ret = -ENOSYS;

        switch (func) {
        case 0:
                ret = read_ldt(ptr, bytecount);
                break;
        case 1:
                ret = write_ldt(ptr, bytecount, 1);
                break;
        case 2:
                ret = read_default_ldt(ptr, bytecount);
                break;
        case 0x11:
                ret = write_ldt(ptr, bytecount, 0);
                break;
        }
        /*
         * The SYSCALL_DEFINE() macros give us an 'unsigned long'
         * return type, but tht ABI for sys_modify_ldt() expects
         * 'int'.  This cast gives us an int-sized value in %rax
         * for the return code.  The 'unsigned' is necessary so
         * the compiler does not try to sign-extend the negative
         * return codes into the high half of the register when
         * taking the value from int->long.
         */
        return (unsigned int)ret;
}