@@ -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);
@@ -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);
This is a note to let you know that I have just added a patch titled x86, tls: Interpret an all-zero struct user_desc as "no segment" to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree which can be found at: http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.16.y-queue This patch is scheduled to be released in version 3.16.7-ckt6. If you, or anyone else, feels it should not be added to this tree, please reply to this email. For more information about the 3.16.y-ckt tree, see https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable Thanks. -Luis ------ From aa844bf25428923089e8203ff53ce72824a3aacc Mon Sep 17 00:00:00 2001 From: Andy Lutomirski <luto@amacapital.net> Date: Thu, 22 Jan 2015 11:27:59 -0800 Subject: x86, tls: Interpret an all-zero struct user_desc as "no segment" commit 3669ef9fa7d35f573ec9c0e0341b29251c2734a7 upstream. The Witcher 2 did something like this to allocate a TLS segment index: struct user_desc u_info; bzero(&u_info, sizeof(u_info)); u_info.entry_number = (uint32_t)-1; syscall(SYS_set_thread_area, &u_info); Strictly speaking, this code was never correct. It should have set read_exec_only and seg_not_present to 1 to indicate that it wanted to find a free slot without putting anything there, or it should have put something sensible in the TLS slot if it wanted to allocate a TLS entry for real. The actual effect of this code was to allocate a bogus segment that could be used to exploit espfix. The set_thread_area hardening patches changed the behavior, causing set_thread_area to return -EINVAL and crashing the game. This changes set_thread_area to interpret this as a request to find a free slot and to leave it empty, which isn't *quite* what the game expects but should be close enough to keep it working. In particular, using the code above to allocate two segments will allocate the same segment both times. According to FrostbittenKing on Github, this fixes The Witcher 2. If this somehow still causes problems, we could instead allocate a limit==0 32-bit data segment, but that seems rather ugly to me. Fixes: 41bdc78544b8 x86/tls: Validate TLS entries to protect espfix Signed-off-by: Andy Lutomirski <luto@amacapital.net> Cc: torvalds@linux-foundation.org Link: http://lkml.kernel.org/r/0cb251abe1ff0958b8e468a9a9a905b80ae3a746.1421954363.git.luto@amacapital.net Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Luis Henriques <luis.henriques@canonical.com> --- arch/x86/include/asm/desc.h | 13 +++++++++++++ arch/x86/kernel/tls.c | 25 +++++++++++++++++++++++-- 2 files changed, 36 insertions(+), 2 deletions(-) -- 2.1.4