linux-kernel - [tip:x86/urgent] x86/tls: Don't validate lm in set_thread

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Message-ID: <tip-3fb2f4237bb452eb4e98f6a5dbd5a445b4fed9d0@git.kernel.org>
Date:	Thu, 18 Dec 2014 03:16:10 -0800
From:	tip-bot for Andy Lutomirski <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	tglx@...utronix.de, luto@...capital.net,
	linux-kernel@...r.kernel.org, mingo@...nel.org, hpa@...or.com,
	torvalds@...ux-foundation.org
Subject: [tip:x86/urgent] x86/tls: Don't validate lm in set_thread_area()
  after all

Commit-ID:  3fb2f4237bb452eb4e98f6a5dbd5a445b4fed9d0
Gitweb:     http://git.kernel.org/tip/3fb2f4237bb452eb4e98f6a5dbd5a445b4fed9d0
Author:     Andy Lutomirski <luto@...capital.net>
AuthorDate: Wed, 17 Dec 2014 14:48:30 -0800
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Thu, 18 Dec 2014 12:12:26 +0100

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

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@...capital.net>
Acked-by: Thomas Gleixner <tglx@...utronix.de>
Cc: stable@...r.kernel.org # Only if 0e58af4e1d21 is backported
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Link: http://lkml.kernel.org/r/d7875b60e28c512f6a6fc0baf5714d58e7eaadbb.1418856405.git.luto@amacapital.net
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 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 46727eb..6e1aaf7 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 3e551ee..4e942f3 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;
 }
 
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