linux-kernel - [PATCH 1/2] x86/speculation/mds: Revert CPU buffer clear on double fault exit

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Date:   Tue, 14 May 2019 13:24:39 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     x86@...nel.org
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>, stable@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Borislav Petkov <bp@...e.de>,
        Frederic Weisbecker <frederic@...nel.org>,
        Jon Masters <jcm@...hat.com>
Subject: [PATCH 1/2] 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.

Cc: stable@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Borislav Petkov <bp@...e.de>
Cc: Frederic Weisbecker <frederic@...nel.org>
Cc: Jon Masters <jcm@...hat.com>
Fixes: 04dcbdb80578 ("x86/speculation/mds: Clear CPU buffers on exit to user")
Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
 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
      coverage.
 
-   - 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();
 		return;
 	}
 #endif
-- 
2.21.0