[PATCH entry v2 2/6] x86/entry, selftests: Further improve user entry sanity checks

[Andy Lutomirski]

Chasing down a Xen bug caused me to realize that the new entry sanity
checks are still fairly weak.  Add some more checks.

Signed-off-by: Andy Lutomirski <l...@kernel.org>
---
 arch/x86/entry/common.c                  | 19 +++++++++++++++++++
 tools/testing/selftests/x86/syscall_nt.c | 11 +++++++++++
 2 files changed, 30 insertions(+)

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index f392a8bcd1c3..e83b3f14897c 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -49,6 +49,23 @@
 static void check_user_regs(struct pt_regs *regs)
 {
        if (IS_ENABLED(CONFIG_DEBUG_ENTRY)) {
+               /*
+                * Make sure that the entry code gave us a sensible EFLAGS
+                * register.  Native because we want to check the actual CPU
+                * state, not the interrupt state as imagined by Xen.
+                */
+               unsigned long flags = native_save_fl();
+               WARN_ON_ONCE(flags & (X86_EFLAGS_AC | X86_EFLAGS_DF |
+                                     X86_EFLAGS_NT));
+
+               /* We think we came from user mode. Make sure pt_regs agrees. */
+               WARN_ON_ONCE(!user_mode(regs));
+
+               /*
+                * All entries from user mode (except #DF) should be on the
+                * normal thread stack and should have user pt_regs in the
+                * correct location.
+                */
                WARN_ON_ONCE(!on_thread_stack());
                WARN_ON_ONCE(regs != task_pt_regs(current));
        }
@@ -577,6 +594,7 @@ SYSCALL_DEFINE0(ni_syscall)
 bool noinstr idtentry_enter_cond_rcu(struct pt_regs *regs)
 {
        if (user_mode(regs)) {
+               check_user_regs(regs);
                enter_from_user_mode();
                return false;
        }
@@ -710,6 +728,7 @@ void noinstr idtentry_exit_cond_rcu(struct pt_regs *regs, 
bool rcu_exit)
  */
 void noinstr idtentry_enter_user(struct pt_regs *regs)
 {
+       check_user_regs(regs);
        enter_from_user_mode();
 }
 
diff --git a/tools/testing/selftests/x86/syscall_nt.c 
b/tools/testing/selftests/x86/syscall_nt.c
index 970e5e14d96d..a108b80dd082 100644
--- a/tools/testing/selftests/x86/syscall_nt.c
+++ b/tools/testing/selftests/x86/syscall_nt.c
@@ -81,5 +81,16 @@ int main(void)
        printf("[RUN]\tSet NT|AC|TF and issue a syscall\n");
        do_it(X86_EFLAGS_NT | X86_EFLAGS_AC | X86_EFLAGS_TF);
 
+       /*
+        * Now try DF.  This is evil and it's plausible that we will crash
+        * glibc, but glibc would have to do something rather surprising
+        * for this to happen.
+        */
+       printf("[RUN]\tSet DF and issue a syscall\n");
+       do_it(X86_EFLAGS_DF);
+
+       printf("[RUN]\tSet TF|DF and issue a syscall\n");
+       do_it(X86_EFLAGS_TF | X86_EFLAGS_DF);
+
        return nerrs == 0 ? 0 : 1;
 }
-- 
2.25.4