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Message-Id: <20190609164147.815178298@linuxfoundation.org>
Date: Sun, 9 Jun 2019 18:39:03 +0200
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-kernel@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
stable@...r.kernel.org, Andy Lutomirski <luto@...nel.org>,
Borislav Petkov <bp@...e.de>,
Frederic Weisbecker <frederic@...nel.org>,
Jon Masters <jcm@...hat.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Peter Zijlstra <peterz@...radead.org>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...nel.org>
Subject: [PATCH 4.4 001/241] x86/speculation/mds: Revert CPU buffer clear on double fault exit
From: Andy Lutomirski <luto@...nel.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@...nel.org>
Cc: Borislav Petkov <bp@...e.de>
Cc: Frederic Weisbecker <frederic@...nel.org>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Jon Masters <jcm@...hat.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: stable@...r.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@...nel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.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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