https://github.com/torvalds/linux
Tip revision: 30a7acd573899fd8b8ac39236eff6468b195ac7d authored by Linus Torvalds on 31 December 2017, 22:47:43 UTC
Linux 4.15-rc6
Linux 4.15-rc6
Tip revision: 30a7acd
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_sem
* 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/syscalls.h>
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;
}
/*
* If PTI is enabled, this maps the LDT into the kernelmode and
* usermode tables for the given mm.
*
* There is no corresponding unmap function. Even if the LDT is freed, we
* leave the PTEs around until the slot is reused or the mm is destroyed.
* This is harmless: the LDT is always in ordinary memory, and no one will
* access the freed slot.
*
* If we wanted to unmap freed LDTs, we'd also need to do a flush to make
* it useful, and the flush would slow down modify_ldt().
*/
static int
map_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt, int slot)
{
#ifdef CONFIG_PAGE_TABLE_ISOLATION
bool is_vmalloc, had_top_level_entry;
unsigned long va;
spinlock_t *ptl;
pgd_t *pgd;
int i;
if (!static_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);
/*
* Did we already have the top level entry allocated? We can't
* use pgd_none() for this because it doens't do anything on
* 4-level page table kernels.
*/
pgd = pgd_offset(mm, LDT_BASE_ADDR);
had_top_level_entry = (pgd->pgd != 0);
is_vmalloc = is_vmalloc_addr(ldt->entries);
for (i = 0; i * PAGE_SIZE < ldt->nr_entries * LDT_ENTRY_SIZE; i++) {
unsigned long offset = i << PAGE_SHIFT;
const void *src = (char *)ldt->entries + offset;
unsigned long pfn;
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 = pfn_pte(pfn, __pgprot(__PAGE_KERNEL_RO & ~_PAGE_GLOBAL));
set_pte_at(mm, va, ptep, pte);
pte_unmap_unlock(ptep, ptl);
}
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_top_level_entry);
if (static_cpu_has(X86_FEATURE_PTI))
WARN_ON(!kernel_to_user_pgdp(pgd)->pgd);
} else {
/*
* This is the first time we're mapping an LDT for this process.
* Sync the pgd to the usermode tables.
*/
WARN_ON(had_top_level_entry);
if (static_cpu_has(X86_FEATURE_PTI)) {
WARN_ON(kernel_to_user_pgdp(pgd)->pgd);
set_pgd(kernel_to_user_pgdp(pgd), *pgd);
}
}
va = (unsigned long)ldt_slot_va(slot);
flush_tlb_mm_range(mm, va, va + LDT_SLOT_STRIDE, 0);
ldt->slot = slot;
#endif
return 0;
}
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 = start + (1UL << PGDIR_SHIFT);
if (!static_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);
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;
}