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.

Cc: stable@vger.kernel.org Cc: Greg Kroah-Hartman gregkh@linuxfoundation.org Cc: Borislav Petkov bp@suse.de Cc: Frederic Weisbecker frederic@kernel.org Cc: Jon Masters jcm@redhat.com Fixes: 04dcbdb80578 ("x86/speculation/mds: Clear CPU buffers on exit to user") Signed-off-by: Andy Lutomirski luto@kernel.org --- 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 534e9baa4e1d..0dc812bb9249 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 7de466eb960b..8b6d03e55d2f 100644 --- a/arch/x86/kernel/traps.c +++ b/arch/x86/kernel/traps.c @@ -58,7 +58,6 @@ #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> #include <asm/umip.h> @@ -368,13 +367,6 @@ 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