Messages in this thread | | | Date | Mon, 23 Oct 2023 11:35:21 -0700 | From | Josh Poimboeuf <> | Subject | Re: [PATCH 2/6] x86/entry_64: Add VERW just before userspace transition |
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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.
> /* > * 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.
The NMI return-to-user path is already mitigated as it goes through swapgs_restore_regs_and_return_to_usermode.
-- Josh
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