linux-kernel - [PATCH 1/2] x86_64,entry: Filter RFLAGS.NT on entry from userspace

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Date:	Tue, 30 Sep 2014 12:40:35 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Thomas Gleixner <tglx@...utronix.de>, X86 ML <x86@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>
Cc:	Sebastian Lackner <sebastian@...-team.de>,
	Anish Bhatt <anish@...lsio.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Chuck Ebbert <cebbert.lkml@...il.com>,
	Andy Lutomirski <luto@...capital.net>, stable@...nel.org
Subject: [PATCH 1/2] x86_64,entry: Filter RFLAGS.NT on entry from userspace

The NT flag doesn't do anything in long mode other than causing IRET
to #GP.  Oddly, CPL3 code can still net NT using popf.

Entry via hardware or software interrupt clears NT automatically, so
the only relevant entries are fast syscalls.

This patch programs the CPU to clear NT on entry via SYSCALL (both
32-bit and 64-bit, by my reading of the AMD APM).  It also clears NT
(and some other flags) in software on SYSENTER.

I haven't touched anything on 32-bit kernels.

If user code causes kernel code to run with NT set, then there's at
least some (small) chance that it could cause trouble.  For example,
user code could cause a call to EFI code with NT set, and who knows
what would happen.  Apparently Wine sometimes does this (!), and, if
an IRET return happens, Wine will segfault.

I think that Wine should be fixed to stop setting NT when a syscall
happens, but handling NT more gracefully is still nice.

The syscall mask change comes from a variant of this patch by Anish
Bhatt.

Cc: stable@...nel.org
Reported-by: Anish Bhatt <anish@...lsio.com>
Signed-off-by: Andy Lutomirski <luto@...capital.net>
---
 arch/x86/ia32/ia32entry.S    | 10 +++++++++-
 arch/x86/kernel/cpu/common.c |  2 +-
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/arch/x86/ia32/ia32entry.S b/arch/x86/ia32/ia32entry.S
index 4299eb05023c..079f42a7ad58 100644
--- a/arch/x86/ia32/ia32entry.S
+++ b/arch/x86/ia32/ia32entry.S
@@ -143,7 +143,15 @@ ENTRY(ia32_sysenter_target)
 	pushq_cfi %r10
 	CFI_REL_OFFSET rip,0
 	pushq_cfi %rax
-	cld
+
+	/*
+	 * Sysenter doesn't filter flags, so we should filter them
+	 * ourselves.
+	 */
+	pushfq_cfi
+	andl $~(X86_EFLAGS_TF|X86_EFLAGS_DF|X86_EFLAGS_NT|X86_EFLAGS_IOPL),(%rsp)
+	popfq_cfi
+
 	SAVE_ARGS 0,1,0
  	/* no need to do an access_ok check here because rbp has been
  	   32bit zero extended */ 
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index e4ab2b42bd6f..31265580c38a 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1184,7 +1184,7 @@ void syscall_init(void)
 	/* Flags to clear on syscall */
 	wrmsrl(MSR_SYSCALL_MASK,
 	       X86_EFLAGS_TF|X86_EFLAGS_DF|X86_EFLAGS_IF|
-	       X86_EFLAGS_IOPL|X86_EFLAGS_AC);
+	       X86_EFLAGS_IOPL|X86_EFLAGS_AC|X86_EFLAGS_NT);
 }

 /*
-- 
1.9.3

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