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... 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 @@ -62,7 +62,6 @@ #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>
@@ -341,13 +340,6 @@ 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