1 files changed, 295 insertions, 0 deletions
diff --git a/arch/x86/kernel/tls.c b/arch/x86/kernel/tls.c
new file mode 100644
index 000000000..3c883e064
--- /dev/null
+++ b/arch/x86/kernel/tls.c
@@ -0,0 +1,295 @@
+// 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 "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;
+}