Patch "x86/fsgsbase/64: Fully initialize FS and GS state in start_thread_common" has been added to the 4.4-stable tree

Patch "x86/fsgsbase/64: Fully initialize FS and GS state in start_thread_common" has been added to the 4.4-stable tree

[gregkh]

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

    x86/fsgsbase/64: Fully initialize FS and GS state in start_thread_common

to the 4.4-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-fsgsbase-64-fully-initialize-fs-and-gs-state-in-start_thread_common.patch
and it can be found in the queue-4.4 subdirectory.

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


>From 767d035d838f4fd6b5a5bbd7a3f6d293b7f65a49 Mon Sep 17 00:00:00 2001
From: Andy Lutomirski <luto@kernel.org>
Date: Tue, 1 Aug 2017 07:11:34 -0700
Subject: x86/fsgsbase/64: Fully initialize FS and GS state in start_thread_common

From: Andy Lutomirski <luto@kernel.org>

commit 767d035d838f4fd6b5a5bbd7a3f6d293b7f65a49 upstream.

execve used to leak FSBASE and GSBASE on AMD CPUs.  Fix it.

The security impact of this bug is small but not quite zero -- it
could weaken ASLR when a privileged task execs a less privileged
program, but only if program changed bitness across the exec, or the
child binary was highly unusual or actively malicious.  A child
program that was compromised after the exec would not have access to
the leaked base.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Borislav Petkov <bpetkov@suse.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Chang Seok <chang.seok.bae@intel.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 arch/x86/kernel/process_64.c |    9 +++++++++
 1 file changed, 9 insertions(+)

--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -232,10 +232,19 @@ start_thread_common(struct pt_regs *regs
 		    unsigned long new_sp,
 		    unsigned int _cs, unsigned int _ss, unsigned int _ds)
 {
+	WARN_ON_ONCE(regs != current_pt_regs());
+
+	if (static_cpu_has(X86_BUG_NULL_SEG)) {
+		/* Loading zero below won't clear the base. */
+		loadsegment(fs, __USER_DS);
+		load_gs_index(__USER_DS);
+	}
+
 	loadsegment(fs, 0);
 	loadsegment(es, _ds);
 	loadsegment(ds, _ds);
 	load_gs_index(0);
+
 	regs->ip		= new_ip;
 	regs->sp		= new_sp;
 	regs->cs		= _cs;


Patches currently in stable-queue which might be from luto@kernel.org are

queue-4.4/x86-fsgsbase-64-fully-initialize-fs-and-gs-state-in-start_thread_common.patch
queue-4.4/x86-fsgsbase-64-report-fsbase-and-gsbase-correctly-in-core-dumps.patch