[tip:x86/urgent] x86/speculation/mds: Revert CPU buffer clear on double fault exit

From: tip-bot for Andy Lutomirski
Date: Thu May 16 2019 - 03:12:51 EST

Commit-ID: 88640e1dcd089879530a49a8d212d1814678dfe7
Gitweb: https://git.kernel.org/tip/88640e1dcd089879530a49a8d212d1814678dfe7
Author: Andy Lutomirski <luto@xxxxxxxxxx>
AuthorDate: Tue, 14 May 2019 13:24:39 -0700
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Thu, 16 May 2019 09:05:11 +0200

x86/speculation/mds: Revert CPU buffer clear on double fault exit

The double fault ESPFIX path doesn't return to user mode at all --
it returns back to the kernel by simulating a #GP fault.
prepare_exit_to_usermode() will run on the way out of
general_protection before running user code.

Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxx>
Cc: Frederic Weisbecker <frederic@xxxxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Jon Masters <jcm@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Fixes: 04dcbdb80578 ("x86/speculation/mds: Clear CPU buffers on exit to user")
Link: http://lkml.kernel.org/r/ac97612445c0a44ee10374f6ea79c222fe22a5c4.1557865329.git.luto@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Documentation/x86/mds.rst | 7 -------
arch/x86/kernel/traps.c | 8 --------
2 files changed, 15 deletions(-)

diff --git a/Documentation/x86/mds.rst b/Documentation/x86/mds.rst
index 534e9baa4e1d..0dc812bb9249 100644
--- a/Documentation/x86/mds.rst
+++ b/Documentation/x86/mds.rst
@@ -158,13 +158,6 @@ Mitigation points
mitigated on the return from do_nmi() to provide almost complete

- - Double fault (#DF):
- A double fault is usually fatal, but the ESPFIX workaround, which can
- be triggered from user space through modify_ldt(2) is a recoverable
- double fault. #DF uses the paranoid exit path, so explicit mitigation
- in the double fault handler is required.
- Machine Check Exception (#MC):

Another corner case is a #MC which hits between the CPU buffer clear
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 7de466eb960b..8b6d03e55d2f 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -58,7 +58,6 @@
#include <asm/alternative.h>
#include <asm/fpu/xstate.h>
#include <asm/trace/mpx.h>
-#include <asm/nospec-branch.h>
#include <asm/mpx.h>
#include <asm/vm86.h>
#include <asm/umip.h>
@@ -368,13 +367,6 @@ dotraplinkage void do_double_fault(struct pt_regs *regs, long error_code)
regs->ip = (unsigned long)general_protection;
regs->sp = (unsigned long)&gpregs->orig_ax;

- /*
- * This situation can be triggered by userspace via
- * modify_ldt(2) and the return does not take the regular
- * user space exit, so a CPU buffer clear is required when
- * MDS mitigation is enabled.
- */
- mds_user_clear_cpu_buffers();