linux-kernel - [ 04/25] x86-64, modify_ldt: Ban 16-bit segments on 64-bit kernels

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Message-Id: <20141206174148.596017245@1wt.eu>
Date:	Sat, 06 Dec 2014 18:41:52 +0100
From:	Willy Tarreau <w@....eu>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Ben Hutchings <ben@...adent.org.uk>, Willy Tarreau <w@....eu>
Subject: [ 04/25] x86-64, modify_ldt: Ban 16-bit segments on 64-bit
 kernels

2.6.32-longterm review patch.  If anyone has any objections, please let me know.

------------------

From: "H. Peter Anvin" <hpa@...ux.intel.com>

commit b3b42ac2cbae1f3cecbb6229964a4d48af31d382 upstream.

The IRET instruction, when returning to a 16-bit segment, only
restores the bottom 16 bits of the user space stack pointer.  We have
a software workaround for that ("espfix") for the 32-bit kernel, but
it relies on a nonzero stack segment base which is not available in
32-bit mode.

Since 16-bit support is somewhat crippled anyway on a 64-bit kernel
(no V86 mode), and most (if not quite all) 64-bit processors support
virtualization for the users who really need it, simply reject
attempts at creating a 16-bit segment when running on top of a 64-bit
kernel.

Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: H. Peter Anvin <hpa@...ux.intel.com>
Link: http://lkml.kernel.org/n/tip-kicdm89kzw9lldryb1br9od0@git.kernel.org
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
(cherry picked from 3.2 commit a862b5c4076b1ba4dd6c87aebac478853dc6db47)
Signed-off-by: Willy Tarreau <w@....eu>
---
 arch/x86/kernel/ldt.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/arch/x86/kernel/ldt.c b/arch/x86/kernel/ldt.c
index ec6ef60..75e356c 100644
--- a/arch/x86/kernel/ldt.c
+++ b/arch/x86/kernel/ldt.c
@@ -229,6 +229,17 @@ static int write_ldt(void __user *ptr, unsigned long bytecount, int oldmode)
 		}
 	}
 
+	/*
+	 * On x86-64 we do not support 16-bit segments due to
+	 * IRET leaking the high bits of the kernel stack address.
+	 */
+#ifdef CONFIG_X86_64
+	if (!ldt_info.seg_32bit) {
+		error = -EINVAL;
+		goto out_unlock;
+	}
+#endif
+
 	fill_ldt(&ldt, &ldt_info);
 	if (oldmode)
 		ldt.avl = 0;
-- 
1.7.12.2.21.g234cd45.dirty



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