linux-kernel - [tip:x86/asm] x86/xen/64: Fix the reported SS and CS in SYSCALL

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Message-ID: <tip-fa2016a8e7d846b306e431646d250500e1da0c33@git.kernel.org>
Date:   Tue, 15 Aug 2017 01:46:29 -0700
From:   tip-bot for Andy Lutomirski <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     luto@...nel.org, bp@...en8.de, jgross@...e.com, mingo@...nel.org,
        boris.ostrovsky@...cle.com, andrew.cooper3@...rix.com,
        peterz@...radead.org, tglx@...utronix.de,
        torvalds@...ux-foundation.org, brgerst@...il.com,
        linux-kernel@...r.kernel.org, hpa@...or.com
Subject: [tip:x86/asm] x86/xen/64: Fix the reported SS and CS in SYSCALL

Commit-ID:  fa2016a8e7d846b306e431646d250500e1da0c33
Gitweb:     http://git.kernel.org/tip/fa2016a8e7d846b306e431646d250500e1da0c33
Author:     Andy Lutomirski <luto@...nel.org>
AuthorDate: Mon, 14 Aug 2017 22:36:19 -0700
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Tue, 15 Aug 2017 10:10:58 +0200

x86/xen/64: Fix the reported SS and CS in SYSCALL

When I cleaned up the Xen SYSCALL entries, I inadvertently changed
the reported segment registers.  Before my patch, regs->ss was
__USER(32)_DS and regs->cs was __USER(32)_CS.  After the patch, they
are FLAT_USER_CS/DS(32).

This had a couple unfortunate effects.  It confused the
opportunistic fast return logic.  It also significantly increased
the risk of triggering a nasty glibc bug:

  https://sourceware.org/bugzilla/show_bug.cgi?id=21269

Update the Xen entry code to change it back.

Reported-by: Brian Gerst <brgerst@...il.com>
Signed-off-by: Andy Lutomirski <luto@...nel.org>
Cc: Andrew Cooper <andrew.cooper3@...rix.com>
Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Juergen Gross <jgross@...e.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: xen-devel@...ts.xenproject.org
Fixes: 8a9949bc71a7 ("x86/xen/64: Rearrange the SYSCALL entries")
Link: http://lkml.kernel.org/r/daba8351ea2764bb30272296ab9ce08a81bd8264.1502775273.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 arch/x86/xen/xen-asm_64.S | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/arch/x86/xen/xen-asm_64.S b/arch/x86/xen/xen-asm_64.S
index a8a4f4c..c5fee26 100644
--- a/arch/x86/xen/xen-asm_64.S
+++ b/arch/x86/xen/xen-asm_64.S
@@ -88,6 +88,15 @@ RELOC(xen_sysret64, 1b+1)
 ENTRY(xen_syscall_target)
 	popq %rcx
 	popq %r11
+
+	/*
+	 * Neither Xen nor the kernel really knows what the old SS and
+	 * CS were.  The kernel expects __USER_DS and __USER_CS, so
+	 * report those values even though Xen will guess its own values.
+	 */
+	movq $__USER_DS, 4*8(%rsp)
+	movq $__USER_CS, 1*8(%rsp)
+
 	jmp entry_SYSCALL_64_after_hwframe
 ENDPROC(xen_syscall_target)
 
@@ -97,6 +106,15 @@ ENDPROC(xen_syscall_target)
 ENTRY(xen_syscall32_target)
 	popq %rcx
 	popq %r11
+
+	/*
+	 * Neither Xen nor the kernel really knows what the old SS and
+	 * CS were.  The kernel expects __USER32_DS and __USER32_CS, so
+	 * report those values even though Xen will guess its own values.
+	 */
+	movq $__USER32_DS, 4*8(%rsp)
+	movq $__USER32_CS, 1*8(%rsp)
+
 	jmp entry_SYSCALL_compat_after_hwframe
 ENDPROC(xen_syscall32_target)