On Mon, Oct 23, 2023 at 02:04:10PM -0700, Pawan Gupta wrote: > On Mon, Oct 23, 2023 at 11:35:21AM -0700, Josh Poimboeuf wrote: > > On Fri, Oct 20, 2023 at 01:45:03PM -0700, Pawan Gupta wrote: > > > @@ -663,6 +665,10 @@ SYM_INNER_LABEL(swapgs_restore_regs_and_return_to_usermode, SYM_L_GLOBAL) > > > /* Restore RDI. */ > > > popq %rdi > > > swapgs > > > + > > > + /* Mitigate CPU data sampling attacks .e.g. MDS */ > > > + USER_CLEAR_CPU_BUFFERS > > > + > > > jmp .Lnative_iret > > > > > > > > > @@ -774,6 +780,9 @@ native_irq_return_ldt: > > > */ > > > popq %rax /* Restore user RAX */ > > > > > > + /* Mitigate CPU data sampling attacks .e.g. MDS */ > > > + USER_CLEAR_CPU_BUFFERS > > > + > > > > Can the above two USER_CLEAR_CPU_BUFFERS be replaced with a single one > > just above native_irq_return_iret? Otherwise the native_irq_return_ldt > > case ends up getting two VERWs. > > Wouldn't that make interrupts returning to kernel also execute VERWs? > > idtentry_body > error_return > restore_regs_and_return_to_kernel > verw > > native_irq_return_ldt doesn't look to be a common case. Anyways, I will > see how to remove the extra VERW. Ah, right. > > > /* > > > * RSP now points to an ordinary IRET frame, except that the page > > > * is read-only and RSP[31:16] are preloaded with the userspace > > > @@ -1502,6 +1511,9 @@ nmi_restore: > > > std > > > movq $0, 5*8(%rsp) /* clear "NMI executing" */ > > > > > > + /* Mitigate CPU data sampling attacks .e.g. MDS */ > > > + USER_CLEAR_CPU_BUFFERS > > > + > > > /* > > > * iretq reads the "iret" frame and exits the NMI stack in a > > > * single instruction. We are returning to kernel mode, so this > > > > This isn't needed here. This is the NMI return-to-kernel path. > > Yes, the VERW here can be omitted. But probably need to check if an NMI > occuring between VERW and ring transition will still execute VERW after > the NMI. That window does exist, though I'm not sure it's worth worrying about. -- Josh
