[PATCH v2 2/6] x86/entry_64: Add VERW just before userspace transition

Pawan Gupta
2023-10-24T08:08:37Z

Mitigation for MDS is to use VERW instruction to clear any secrets in
CPU Buffers. Any memory accesses after VERW execution can still remain
in CPU buffers. It is safer to execute VERW late in return to user path
to minimize the window in which kernel data can end up in CPU buffers.
There are not many kernel secrets to be had after SWITCH_TO_USER_CR3.

Add support for deploying VERW mitigation after user register state is
restored. This helps minimize the chances of kernel data ending up into
CPU buffers after executing VERW.

Note that the mitigation at the new location is not yet enabled.

Corner case not handled
=======================
Interrupts returning to kernel don't clear CPUs buffers since the
exit-to-user path is expected to do that anyways. But, there could be
a case when an NMI is generated in kernel after the exit-to-user path
has cleared the buffers. This case is not handled and NMI returning to
kernel don't clear CPU buffers because:

1. It is rare to get an NMI after VERW, but before returning to userspace.
2. For an unprivileged user, there is no known way to make that NMI
less rare or target it.
3. It would take a large number of these precisely-timed NMIs to mount
an actual attack. There's presumably not enough bandwidth.
4. The NMI in question occurs after a VERW, i.e. when user state is
restored and most interesting data is already scrubbed. Whats left
is only the data that NMI touches, and that may or may not be of
any interest.

Suggested-by: Dave Hansen <dave.hansen@intel.com>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
---
arch/x86/entry/entry_64.S | 11 +++++++++++
arch/x86/entry/entry_64_compat.S | 1 +
2 files changed, 12 insertions(+)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 43606de22511..9f97a8bd11e8 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -223,6 +223,7 @@ syscall_return_via_sysret:
SYM_INNER_LABEL(entry_SYSRETQ_unsafe_stack, SYM_L_GLOBAL)
ANNOTATE_NOENDBR
swapgs
+ CLEAR_CPU_BUFFERS
sysretq
SYM_INNER_LABEL(entry_SYSRETQ_end, SYM_L_GLOBAL)
ANNOTATE_NOENDBR
@@ -663,6 +664,7 @@ SYM_INNER_LABEL(swapgs_restore_regs_and_return_to_usermode, SYM_L_GLOBAL)
/* Restore RDI. */
popq %rdi
swapgs
+ CLEAR_CPU_BUFFERS
jmp .Lnative_iret

@@ -774,6 +776,8 @@ native_irq_return_ldt:
*/
popq %rax /* Restore user RAX */

+ CLEAR_CPU_BUFFERS
+
/*
* 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 +1506,12 @@ nmi_restore:
std
movq $0, 5*8(%rsp) /* clear "NMI executing" */

+ /*
+ * Skip CLEAR_CPU_BUFFERS here, since it only helps in rare cases like
+ * NMI in kernel after user state is restored. For an unprivileged user
+ * these conditions are hard to meet.
+ */
+
/*
* iretq reads the "iret" frame and exits the NMI stack in a
* single instruction. We are returning to kernel mode, so this
@@ -1520,6 +1530,7 @@ SYM_CODE_START(ignore_sysret)
UNWIND_HINT_END_OF_STACK
ENDBR
mov $-ENOSYS, %eax
+ CLEAR_CPU_BUFFERS
sysretl
SYM_CODE_END(ignore_sysret)
#endif
diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
index 70150298f8bd..245697eb8485 100644
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -271,6 +271,7 @@ SYM_INNER_LABEL(entry_SYSRETL_compat_unsafe_stack, SYM_L_GLOBAL)
xorl %r9d, %r9d
xorl %r10d, %r10d
swapgs
+ CLEAR_CPU_BUFFERS
sysretl
SYM_INNER_LABEL(entry_SYSRETL_compat_end, SYM_L_GLOBAL)
ANNOTATE_NOENDBR

--
2.34.1