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-rw-r--r--arch/x86/include/asm/desc.h13
-rw-r--r--arch/x86/kernel/tls.c25
2 files changed, 36 insertions, 2 deletions
diff --git a/arch/x86/include/asm/desc.h b/arch/x86/include/asm/desc.h
index fc237fd0259a..a94b82e8f156 100644
--- a/arch/x86/include/asm/desc.h
+++ b/arch/x86/include/asm/desc.h
@@ -262,6 +262,19 @@ static inline void native_load_tls(struct thread_struct *t, unsigned int cpu)
(info)->seg_not_present == 1 && \
(info)->useable == 0)
+/* Lots of programs expect an all-zero user_desc to mean "no segment at all". */
+static inline bool LDT_zero(const struct user_desc *info)
+{
+ return (info->base_addr == 0 &&
+ info->limit == 0 &&
+ info->contents == 0 &&
+ info->read_exec_only == 0 &&
+ info->seg_32bit == 0 &&
+ info->limit_in_pages == 0 &&
+ info->seg_not_present == 0 &&
+ info->useable == 0);
+}
+
static inline void clear_LDT(void)
{
set_ldt(NULL, 0);
diff --git a/arch/x86/kernel/tls.c b/arch/x86/kernel/tls.c
index 4e942f31b1a7..7fc5e843f247 100644
--- a/arch/x86/kernel/tls.c
+++ b/arch/x86/kernel/tls.c
@@ -29,7 +29,28 @@ static int get_free_idx(void)
static bool tls_desc_okay(const struct user_desc *info)
{
- if (LDT_empty(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;
/*
@@ -71,7 +92,7 @@ static void set_tls_desc(struct task_struct *p, int idx,
cpu = get_cpu();
while (n-- > 0) {
- if (LDT_empty(info))
+ if (LDT_empty(info) || LDT_zero(info))
desc->a = desc->b = 0;
else
fill_ldt(desc, info);
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