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