Patch "x86/fsgsbase/64: Fully initialize FS and GS state in start_thread_common" has been added to the 4.13-stable tree

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 4.13-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-4.13 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Andy Lutomirski <[email protected]>
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 <[email protected]>

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.

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

@@ -229,10 +229,19 @@ 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 [email protected] are

queue-4.13/x86-switch_to-64-rewrite-fs-gs-switching-yet-again-to-fix-amd-cpus.patch queue-4.13/x86-fsgsbase-64-fully-initialize-fs-and-gs-state-in-start_thread_common.patch queue-4.13/x86-fsgsbase-64-report-fsbase-and-gsbase-correctly-in-core-dumps.patch queue-4.13/x86-mm-mm-hwpoison-clear-present-bit-for-kernel-1-1-mappings-of-poison-pages.patch