[PATCH 3.16.y-ckt 187/216] x86/tls: Don't validate lm in set_thread_area() after all

Luis Henriques
2015-01-12T18:40:59Z

From: Andy Lutomirski <luto@amacapital.net>

commit 3fb2f4237bb452eb4e98f6a5dbd5a445b4fed9d0 upstream.

It turns out that there's a lurking ABI issue. GCC, when
compiling this in a 32-bit program:

struct user_desc desc = {
.entry_number = idx,
.base_addr = base,
.limit = 0xfffff,
.seg_32bit = 1,
.contents = 0, /* Data, grow-up */
.read_exec_only = 0,
.limit_in_pages = 1,
.seg_not_present = 0,
.useable = 0,
};

will leave .lm uninitialized. This means that anything in the
kernel that reads user_desc.lm for 32-bit tasks is unreliable.

Revert the .lm check in set_thread_area(). The value never did
anything in the first place.

Fixes: 0e58af4e1d21 ("x86/tls: Disallow unusual TLS segments")
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/d7875b60e28c512f6a6fc0baf5714d58e7eaadbb.1418856405.git.luto@amacapital.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
---
arch/x86/include/uapi/asm/ldt.h | 7 +++++++
arch/x86/kernel/tls.c | 6 ------
2 files changed, 7 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/uapi/asm/ldt.h b/arch/x86/include/uapi/asm/ldt.h
index 46727eb37bfe..6e1aaf73852a 100644
--- a/arch/x86/include/uapi/asm/ldt.h
+++ b/arch/x86/include/uapi/asm/ldt.h
@@ -28,6 +28,13 @@ struct user_desc {
unsigned int seg_not_present:1;
unsigned int useable:1;
#ifdef __x86_64__
+ /*
+ * Because this bit is not present in 32-bit user code, user
+ * programs can pass uninitialized values here. Therefore, in
+ * any context in which a user_desc comes from a 32-bit program,
+ * the kernel must act as though lm == 0, regardless of the
+ * actual value.
+ */
unsigned int lm:1;
#endif
};
diff --git a/arch/x86/kernel/tls.c b/arch/x86/kernel/tls.c
index 3e551eee87b9..4e942f31b1a7 100644
--- a/arch/x86/kernel/tls.c
+++ b/arch/x86/kernel/tls.c
@@ -55,12 +55,6 @@ static bool tls_desc_okay(const struct user_desc *info)
if (info->seg_not_present)
return false;

-#ifdef CONFIG_X86_64
- /* The L bit makes no sense for data. */
- if (info->lm)
- return false;
-#endif
-
return true;
}

--
2.1.4