[PATCH 4.13 17/52] x86/fsgsbase/64: Fully initialize FS and GS state in start_thread_common

From: Greg Kroah-Hartman
Date: Mon Sep 18 2017 - 06:00:44 EST

4.13-stable review patch. If anyone has any objections, please let me know.


From: Andy Lutomirski <luto@xxxxxxxxxx>

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@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Borislav Petkov <bpetkov@xxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Chang Seok <chang.seok.bae@xxxxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

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
@@ -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);
regs->ip = new_ip;
regs->sp = new_sp;
regs->cs = _cs;