[PATCH 3.16 100/132] x86/speculation/mds: Revert CPU buffer clear on double fault exit

[Ben Hutchings]

3.16.74-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Andy Lutomirski <luto@xxxxxxxxxx>

commit 88640e1dcd089879530a49a8d212d1814678dfe7 upstream.

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>
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>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 Documentation/x86/mds.rst | 7 -------
 arch/x86/kernel/traps.c   | 8 --------
 2 files changed, 15 deletions(-)

--- 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
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -55,7 +55,6 @@
 #include <asm/fixmap.h>
 #include <asm/mach_traps.h>
 #include <asm/alternative.h>
-#include <asm/nospec-branch.h>
 
 #ifdef CONFIG_X86_64
 #include <asm/x86_init.h>
@@ -260,13 +259,6 @@ dotraplinkage void do_double_fault(struc
 		regs->ip = (unsigned long)general_protection;
 		regs->sp = (unsigned long)&normal_regs->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