1 files changed, 114 insertions, 31 deletions

diff --git a/arch/x86/kernel/tls.c b/arch/x86/kernel/tls.c
index f7fec09e3e3a..3ffbab0081f4 100644
--- a/arch/x86/kernel/tls.c
+++ b/arch/x86/kernel/tls.c
@@ -1,15 +1,18 @@
+// 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 <asm/uaccess.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"
 
@@ -27,6 +30,58 @@ static int get_free_idx(void)
 	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)
 {
@@ -40,8 +95,8 @@ static void set_tls_desc(struct task_struct *p, int idx,
 	cpu = get_cpu();
 
 	while (n-- > 0) {
-		if (LDT_empty(info))
-			desc->a = desc->b = 0;
+		if (LDT_empty(info) || LDT_zero(info))
+			memset(desc, 0, sizeof(*desc));
 		else
 			fill_ldt(desc, info);
 		++info;
@@ -62,10 +117,14 @@ int do_set_thread_area(struct task_struct *p, int idx,
 		       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;
 
@@ -86,6 +145,41 @@ int do_set_thread_area(struct task_struct *p, int idx,
 
 	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;
 }
 
@@ -122,6 +216,7 @@ 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;
@@ -129,8 +224,11 @@ int do_get_thread_area(struct task_struct *p, int idx,
 	if (idx < GDT_ENTRY_TLS_MIN || idx > GDT_ENTRY_TLS_MAX)
 		return -EINVAL;
 
-	fill_user_desc(&info, idx,
-		       &p->thread.tls_array[idx - GDT_ENTRY_TLS_MIN]);
+	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;
@@ -153,36 +251,16 @@ int regset_tls_active(struct task_struct *target,
 }
 
 int regset_tls_get(struct task_struct *target, const struct user_regset *regset,
-		   unsigned int pos, unsigned int count,
-		   void *kbuf, void __user *ubuf)
+		   struct membuf to)
 {
 	const struct desc_struct *tls;
+	struct user_desc v;
+	int pos;
 
-	if (pos >= GDT_ENTRY_TLS_ENTRIES * sizeof(struct user_desc) ||
-	    (pos % sizeof(struct user_desc)) != 0 ||
-	    (count % sizeof(struct user_desc)) != 0)
-		return -EINVAL;
-
-	pos /= sizeof(struct user_desc);
-	count /= sizeof(struct user_desc);
-
-	tls = &target->thread.tls_array[pos];
-
-	if (kbuf) {
-		struct user_desc *info = kbuf;
-		while (count-- > 0)
-			fill_user_desc(info++, GDT_ENTRY_TLS_MIN + pos++,
-				       tls++);
-	} else {
-		struct user_desc __user *u_info = ubuf;
-		while (count-- > 0) {
-			struct user_desc info;
-			fill_user_desc(&info, GDT_ENTRY_TLS_MIN + pos++, tls++);
-			if (__copy_to_user(u_info++, &info, sizeof(info)))
-				return -EFAULT;
-		}
+	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;
 }
 
@@ -192,6 +270,7 @@ int regset_tls_set(struct task_struct *target, const struct user_regset *regset,
 {
 	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 ||
@@ -205,6 +284,10 @@ int regset_tls_set(struct task_struct *target, const struct user_regset *regset,
 	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));