x86/tls: Validate TLS entries to protect espfix

Installing a 16-bit RW data segment into the GDT defeats espfix.
AFAICT this will not affect glibc, Wine, or dosemu at all.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Acked-by: H. Peter Anvin <hpa@zytor.com>
Cc: stable@vger.kernel.org
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: security@kernel.org <security@kernel.org>
Cc: Willy Tarreau <w@1wt.eu>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 23 insertions, 0 deletions

diff --git a/arch/x86/kernel/tls.c b/arch/x86/kernel/tls.c
index f7fec09e3e3a..e7650bd71109 100644
--- a/arch/x86/kernel/tls.c
+++ b/arch/x86/kernel/tls.c
@@ -27,6 +27,21 @@ static int get_free_idx(void)
         return -ESRCH;
 }
 
+static bool tls_desc_okay(const struct user_desc *info)
+{
+        if (LDT_empty(info))
+                return true;
+
+        /*
+         * espfix is required for 16-bit data segments, but espfix
+         * only works for LDT segments.
+         */
+        if (!info->seg_32bit)
+                return false;
+
+        return true;
+}
+
 static void set_tls_desc(struct task_struct *p, int idx,
                          const struct user_desc *info, int n)
 {
@@ -66,6 +81,9 @@ int do_set_thread_area(struct task_struct *p, int idx,
         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;
 
@@ -192,6 +210,7 @@ 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 +224,10 @@ 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));