From: Andy Lutomirski <luto@kernel.org>
Date: Tue, 1 Aug 2017 14:11:34 +0000 (-0700)
Subject: x86/fsgsbase/64: Fully initialize FS and GS state in start_thread_common
X-Git-Tag: C0P2-H0.0--20200415~7529
X-Git-Url: https://git.somdevices.com/?a=commitdiff_plain;h=c7d1ddec251d39415cd488c29e9d60b22d4b61b7;p=linux.git

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

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>
---

diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index b3760b3c1ca0..02fa4701cc2e 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -216,10 +216,19 @@ start_thread_common(struct pt_regs *regs, unsigned long new_ip,
 		    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;