Patch "x86/tls: Don't validate lm in set_thread_area() after all" has been added to the 3.18-stable tree

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This is a note to let you know that I've just added the patch titled

    x86/tls: Don't validate lm in set_thread_area() after all

to the 3.18-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     x86-tls-don-t-validate-lm-in-set_thread_area-after-all.patch
and it can be found in the queue-3.18 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.


>From 3fb2f4237bb452eb4e98f6a5dbd5a445b4fed9d0 Mon Sep 17 00:00:00 2001
From: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Date: Wed, 17 Dec 2014 14:48:30 -0800
Subject: x86/tls: Don't validate lm in set_thread_area() after all

From: Andy Lutomirski <luto@xxxxxxxxxxxxxx>

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@xxxxxxxxxxxxxx>
Acked-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/d7875b60e28c512f6a6fc0baf5714d58e7eaadbb.1418856405.git.luto@xxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 arch/x86/include/uapi/asm/ldt.h |    7 +++++++
 arch/x86/kernel/tls.c           |    6 ------
 2 files changed, 7 insertions(+), 6 deletions(-)

--- 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
 };
--- a/arch/x86/kernel/tls.c
+++ b/arch/x86/kernel/tls.c
@@ -55,12 +55,6 @@ static bool tls_desc_okay(const struct u
 	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;
 }
 


Patches currently in stable-queue which might be from luto@xxxxxxxxxxxxxx are

queue-3.18/x86_64-switch_to-load-tls-descriptors-before-switching-ds-and-es.patch
queue-3.18/x86-asm-traps-disable-tracing-and-kprobes-in-fixup_bad_iret-and-sync_regs.patch
queue-3.18/x86-tls-disallow-unusual-tls-segments.patch
queue-3.18/x86-tls-validate-tls-entries-to-protect-espfix.patch
queue-3.18/x86-tls-don-t-validate-lm-in-set_thread_area-after-all.patch
queue-3.18/x86-kvm-clear-paravirt_enabled-on-kvm-guests-for-espfix32-s-benefit.patch
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