On Mon, Sep 21, 2020 at 04:48:25PM -0700, Andy Lutomirski wrote:

On Mon, Sep 21, 2020 at 3:37 PM Yu-cheng Yu [off-list ref] wrote:

quoted

diff --git a/arch/x86/entry/vsyscall/vsyscall_64.c

b/arch/x86/entry/vsyscall/vsyscall_64.c
index 44c33103a955..0131c9f7f9c5 100644

--- a/arch/x86/entry/vsyscall/vsyscall_64.c
+++ b/arch/x86/entry/vsyscall/vsyscall_64.c

@@ -38,6 +38,9 @@
 #include <asm/fixmap.h>
 #include <asm/traps.h>
 #include <asm/paravirt.h>
+#include <asm/fpu/xstate.h>
+#include <asm/fpu/types.h>
+#include <asm/fpu/internal.h>

 #define CREATE_TRACE_POINTS
 #include "vsyscall_trace.h"

@@ -286,6 +289,32 @@ bool emulate_vsyscall(unsigned long error_code,
        /* Emulate a ret instruction. */
        regs->ip = caller;
        regs->sp += 8;
+
+       if (current->thread.cet.shstk_size ||
+           current->thread.cet.ibt_enabled) {
+               u64 r;
+
+               fpregs_lock();
+               if (test_thread_flag(TIF_NEED_FPU_LOAD))
+                       __fpregs_load_activate();

Wouldn't this be nicer if you operated on the memory image, not the registers?

quoted

+
+#ifdef CONFIG_X86_INTEL_BRANCH_TRACKING_USER
+               /* Fixup branch tracking */
+               if (current->thread.cet.ibt_enabled) {
+                       rdmsrl(MSR_IA32_U_CET, r);
+                       wrmsrl(MSR_IA32_U_CET, r & ~CET_WAIT_ENDBR);
+               }
+#endif

Seems reasonable on first glance.

quoted

+
+#ifdef CONFIG_X86_INTEL_SHADOW_STACK_USER
+               /* Unwind shadow stack. */
+               if (current->thread.cet.shstk_size) {
+                       rdmsrl(MSR_IA32_PL3_SSP, r);
+                       wrmsrl(MSR_IA32_PL3_SSP, r + 8);
+               }
+#endif

What happens if the result is noncanonical?  A quick skim of the SDM
didn't find anything.  This latter issue goes away if you operate on
the memory image, though -- writing a bogus value is just fine, since
the FP restore will handle it.

#GP, the SSP MSRs do canonical checks.