linux/arch/x86/kernel/ldt.c
Linus Torvalds bb5a93aaf2 x86/ldt: use "pr_info_once()" instead of open-coding it badly
Using a mutex for "print this warning only once" is so overdesigned as
to be actively offensive to my sensitive stomach.

Just use "pr_info_once()" that already does this, although in a
(harmlessly) racy manner that can in theory cause the message to be
printed twice if more than one CPU races on that "is this the first
time" test.

[ If somebody really cares about that harmless data race (which sounds
  very unlikely indeed), that person can trivially fix printk_once() by
  using a simple atomic access, preferably with an optimistic non-atomic
  test first before even bothering to treat the pointless "make sure it
  is _really_ just once" case.

  A mutex is most definitely never the right primitive to use for
  something like this. ]

Yes, this is a small and meaningless detail in a code path that hardly
matters.  But let's keep some code quality standards here, and not
accept outrageously bad code.

Link: https://lore.kernel.org/lkml/CAHk-=wgV9toS7GU3KmNpj8hCS9SeF+A0voHS8F275_mgLhL4Lw@mail.gmail.com/
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2020-07-05 12:50:20 -07:00

689 lines
17 KiB
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>
#include <xen/xen.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 bool allow_16bit_segments(void)
{
if (!IS_ENABLED(CONFIG_X86_16BIT))
return false;
#ifdef CONFIG_XEN_PV
/*
* Xen PV does not implement ESPFIX64, which means that 16-bit
* segments will not work correctly. Until either Xen PV implements
* ESPFIX64 and can signal this fact to the guest or unless someone
* provides compelling evidence that allowing broken 16-bit segments
* is worthwhile, disallow 16-bit segments under Xen PV.
*/
if (xen_pv_domain()) {
pr_info_once("Warning: 16-bit segments do not work correctly in a Xen PV guest\n");
return false;
}
#endif
return true;
}
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 (!ldt_info.seg_32bit && !allow_16bit_segments()) {
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;
}