[PATCH 5/7] x86/uaccess: Warn on uaccess faults other than #PF

From: Andy Lutomirski
Date: Tue May 24 2016 - 18:49:09 EST

If a uaccess instruction fails due to an8 error other than #PF,
warn. If the fault is #GP, it most likely indicates access to a
non-canonical address, which means that an access_ok check is
missing, and that's bad. If the fault is something else (#UD?),
then something is very wrong and we should diagnose it rather
than ignoring it.

Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
arch/x86/mm/extable.c | 13 +++++++++++++
1 file changed, 13 insertions(+)

diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c
index 658292fdee5e..c1933471fce7 100644
--- a/arch/x86/mm/extable.c
+++ b/arch/x86/mm/extable.c
@@ -29,6 +29,19 @@ EXPORT_SYMBOL(ex_handler_default);
static bool uaccess_fault_okay(int trapnr, unsigned long error_code,
unsigned long extra)
+ /*
+ * For uaccess, only page faults should be fixed up. I can't see
+ * any exploit mitigation value in OOPSing on other types of faults,
+ * so just warn and continue if that happens. This means that
+ * uaccess faults to non-canonical addresses will warn. That's okay
+ * -- this will only happen if an access_ok is missing, and we want to
+ * detect that error if it happens.
+ */
+ if (WARN_ONCE(trapnr != X86_TRAP_PF,
+ "unexpected uaccess trap %d (may indicate a missing access_ok on a non-canonical address)\n",
+ trapnr))
+ return true; /* no good reason to OOPS. */
return true;