From 393ca9ea37fb1c8cd74838282bf569825d18e3bf Mon Sep 17 00:00:00 2001 From: Andy Lutomirski Date: Tue, 14 May 2019 13:24:39 -0700 Subject: [PATCH] x86/speculation/mds: Revert CPU buffer clear on double fault exit 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 Cc: Borislav Petkov Cc: Frederic Weisbecker Cc: Greg Kroah-Hartman Cc: Jon Masters Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Gleixner 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 Signed-off-by: Greg Kroah-Hartman --- 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 534e9ba..0dc812b 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 0a5efd7..e6db475 100644 --- a/arch/x86/kernel/traps.c +++ b/arch/x86/kernel/traps.c @@ -58,7 +58,6 @@ #include #include #include -#include #include #include #include @@ -388,13 +387,6 @@ 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 -- 2.7.4