From 3c73b81a9164d0c1b6379d6672d2772a9e95168e Mon Sep 17 00:00:00 2001
From: Andy Lutomirski <luto@kernel.org>
Date: Fri, 3 Jul 2020 10:02:54 -0700
Subject: [PATCH] x86/entry, selftests: Further improve user entry sanity
checks
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 <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/881de09e786ab93ce56ee4a2437ba2c308afe7a9.1593795633.git.luto@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 f392a8bcd1c3e..e83b3f14897cc 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 970e5e14d96d1..a108b80dd0823 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;
}
--
2.30.2