Re: [PATCH 2/6] x86/entry_64: Add VERW just before userspace transition
From: Josh Poimboeuf <jpoimboe@kernel.org>
Date: 2023-10-23 18:35:29
Also in:
kvm, lkml
On Fri, Oct 20, 2023 at 01:45:03PM -0700, Pawan Gupta wrote:
quoted hunk ↗ jump to hunk
@@ -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.
quoted hunk ↗ jump to hunk
/* * 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