arch/x86/kernel/tls.c - linux - Git at Google

 // 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;
 }
	// 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;
	}