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Subject[PATCH 4.4 001/241] x86/speculation/mds: Revert CPU buffer clear on double fault exit
Date
From: Andy Lutomirski <luto@kernel.org>

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@kernel.org>
Cc: Borislav Petkov <bp@suse.de>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jon Masters <jcm@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Fixes: 04dcbdb80578 ("x86/speculation/mds: Clear CPU buffers on exit to user")
Link: http://lkml.kernel.org/r/ac97612445c0a44ee10374f6ea79c222fe22a5c4.1557865329.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
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
@@ -61,7 +61,6 @@
#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>

@@ -338,13 +337,6 @@ 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

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