// SPDX-License-Identifier: GPL-2.0
#include <linux/kernel.h>
#include <linux/errno.h>
#include <linux/sched.h>
#include <linux/user.h>
#include <linux/regset.h>
#include <linux/syscalls.h>
#include <linux/nospec.h>

#include <linux/uaccess.h>
#include <asm/desc.h>
#include <asm/ldt.h>
#include <asm/processor.h>
#include <asm/proto.h>
#include <asm/gsseg.h>

#include "tls.h"

/*
 * sys_alloc_thread_area: get a yet unused TLS descriptor index.
 */
static int get_free_idx(void)
{
        struct thread_struct *t = &current->thread;
        int idx;

        for (idx = 0; idx < GDT_ENTRY_TLS_ENTRIES; idx++)
                if (desc_empty(&t->tls_array[idx]))
                        return idx + GDT_ENTRY_TLS_MIN;
        return -ESRCH;
}

static bool tls_desc_okay(const struct user_desc *info)
{
        /*
         * For historical reasons (i.e. no one ever documented how any
         * of the segmentation APIs work), user programs can and do
         * assume that a struct user_desc that's all zeros except for
         * entry_number means "no segment at all".  This never actually
         * worked.  In fact, up to Linux 3.19, a struct user_desc like
         * this would create a 16-bit read-write segment with base and
         * limit both equal to zero.
         *
         * That was close enough to "no segment at all" until we
         * hardened this function to disallow 16-bit TLS segments.  Fix
         * it up by interpreting these zeroed segments the way that they
         * were almost certainly intended to be interpreted.
         *
         * The correct way to ask for "no segment at all" is to specify
         * a user_desc that satisfies LDT_empty.  To keep everything
         * working, we accept both.
         *
         * Note that there's a similar kludge in modify_ldt -- look at
         * the distinction between modes 1 and 0x11.
         */
        if (LDT_empty(info) || LDT_zero(info))
                return true;

        /*
         * espfix is required for 16-bit data segments, but espfix
         * only works for LDT segments.
         */
        if (!info->seg_32bit)
                return false;

        /* Only allow data segments in the TLS array. */
        if (info->contents > 1)
                return false;

        /*
         * Non-present segments with DPL 3 present an interesting attack
         * surface.  The kernel should handle such segments correctly,
         * but TLS is very difficult to protect in a sandbox, so prevent
         * such segments from being created.
         *
         * If userspace needs to remove a TLS entry, it can still delete
         * it outright.
         */
        if (info->seg_not_present)
                return false;

        return true;
}

static void set_tls_desc(struct task_struct *p, int idx,
                         const struct user_desc *info, int n)
{
        struct thread_struct *t = &p->thread;
        struct desc_struct *desc = &t->tls_array[idx - GDT_ENTRY_TLS_MIN];
        int cpu;

        /*
         * We must not get preempted while modifying the TLS.
         */
        cpu = get_cpu();

        while (n-- > 0) {
                if (LDT_empty(info) || LDT_zero(info))
                        memset(desc, 0, sizeof(*desc));
                else
                        fill_ldt(desc, info);
                ++info;
                ++desc;
        }

        if (t == &current->thread)
                load_TLS(t, cpu);

        put_cpu();
}

/*
 * Set a given TLS descriptor:
 */
int do_set_thread_area(struct task_struct *p, int idx,
                       struct user_desc __user *u_info,
                       int can_allocate)
{
        struct user_desc info;
        unsigned short __maybe_unused sel, modified_sel;

        if (copy_from_user(&info, u_info, sizeof(info)))
                return -EFAULT;

        if (!tls_desc_okay(&info))
                return -EINVAL;

        if (idx == -1)
                idx = info.entry_number;

        /*
         * index -1 means the kernel should try to find and
         * allocate an empty descriptor:
         */
        if (idx == -1 && can_allocate) {
                idx = get_free_idx();
                if (idx < 0)
                        return idx;
                if (put_user(idx, &u_info->entry_number))
                        return -EFAULT;
        }

        if (idx < GDT_ENTRY_TLS_MIN || idx > GDT_ENTRY_TLS_MAX)
                return -EINVAL;

        set_tls_desc(p, idx, &info, 1);

        /*
         * If DS, ES, FS, or GS points to the modified segment, forcibly
         * refresh it.  Only needed on x86_64 because x86_32 reloads them
         * on return to user mode.
         */
        modified_sel = (idx << 3) | 3;

        if (p == current) {
#ifdef CONFIG_X86_64
                savesegment(ds, sel);
                if (sel == modified_sel)
                        loadsegment(ds, sel);

                savesegment(es, sel);
                if (sel == modified_sel)
                        loadsegment(es, sel);

                savesegment(fs, sel);
                if (sel == modified_sel)
                        loadsegment(fs, sel);
#endif

                savesegment(gs, sel);
                if (sel == modified_sel)
                        load_gs_index(sel);
        } else {
#ifdef CONFIG_X86_64
                if (p->thread.fsindex == modified_sel)
                        p->thread.fsbase = info.base_addr;

                if (p->thread.gsindex == modified_sel)
                        p->thread.gsbase = info.base_addr;
#endif
        }

        return 0;
}

SYSCALL_DEFINE1(set_thread_area, struct user_desc __user *, u_info)
{
        return do_set_thread_area(current, -1, u_info, 1);
}


/*
 * Get the current Thread-Local Storage area:
 */

static void fill_user_desc(struct user_desc *info, int idx,
                           const struct desc_struct *desc)

{
        memset(info, 0, sizeof(*info));
        info->entry_number = idx;
        info->base_addr = get_desc_base(desc);
        info->limit = get_desc_limit(desc);
        info->seg_32bit = desc->d;
        info->contents = desc->type >> 2;
        info->read_exec_only = !(desc->type & 2);
        info->limit_in_pages = desc->g;
        info->seg_not_present = !desc->p;
        info->useable = desc->avl;
#ifdef CONFIG_X86_64
        info->lm = desc->l;
#endif
}

int do_get_thread_area(struct task_struct *p, int idx,
                       struct user_desc __user *u_info)
{
        struct user_desc info;
        int index;

        if (idx == -1 && get_user(idx, &u_info->entry_number))
                return -EFAULT;

        if (idx < GDT_ENTRY_TLS_MIN || idx > GDT_ENTRY_TLS_MAX)
                return -EINVAL;

        index = idx - GDT_ENTRY_TLS_MIN;
        index = array_index_nospec(index,
                        GDT_ENTRY_TLS_MAX - GDT_ENTRY_TLS_MIN + 1);

        fill_user_desc(&info, idx, &p->thread.tls_array[index]);

        if (copy_to_user(u_info, &info, sizeof(info)))
                return -EFAULT;
        return 0;
}

SYSCALL_DEFINE1(get_thread_area, struct user_desc __user *, u_info)
{
        return do_get_thread_area(current, -1, u_info);
}

int regset_tls_active(struct task_struct *target,
                      const struct user_regset *regset)
{
        struct thread_struct *t = &target->thread;
        int n = GDT_ENTRY_TLS_ENTRIES;
        while (n > 0 && desc_empty(&t->tls_array[n - 1]))
                --n;
        return n;
}

int regset_tls_get(struct task_struct *target, const struct user_regset *regset,
                   struct membuf to)
{
        const struct desc_struct *tls;
        struct user_desc v;
        int pos;

        for (pos = 0, tls = target->thread.tls_array; to.left; pos++, tls++) {
                fill_user_desc(&v, GDT_ENTRY_TLS_MIN + pos, tls);
                membuf_write(&to, &v, sizeof(v));
        }
        return 0;
}

int regset_tls_set(struct task_struct *target, const struct user_regset *regset,
                   unsigned int pos, unsigned int count,
                   const void *kbuf, const void __user *ubuf)
{
        struct user_desc infobuf[GDT_ENTRY_TLS_ENTRIES];
        const struct user_desc *info;
        int i;

        if (pos >= GDT_ENTRY_TLS_ENTRIES * sizeof(struct user_desc) ||
            (pos % sizeof(struct user_desc)) != 0 ||
            (count % sizeof(struct user_desc)) != 0)
                return -EINVAL;

        if (kbuf)
                info = kbuf;
        else if (__copy_from_user(infobuf, ubuf, count))
                return -EFAULT;
        else
                info = infobuf;

        for (i = 0; i < count / sizeof(struct user_desc); i++)
                if (!tls_desc_okay(info + i))
                        return -EINVAL;

        set_tls_desc(target,
                     GDT_ENTRY_TLS_MIN + (pos / sizeof(struct user_desc)),
                     info, count / sizeof(struct user_desc));

        return 0;
}