[PATCH 7/7] x86/uaccess: OOPS or warn on a fault with KERNEL_DS and !pagefault_disabled()

[Andy Lutomirski]

If someone calls set_fs(KERNEL_DS), then they are responsible for
making sure that whatever addresses are accessed are safe.  If they
get it wrong on a kernel address, OOPS.  If they get it wrong on a user
address, warn.

This will make it harder to exploit bugs in which user code controls
a pointer accessed with KERNEL_DS: an attacker will OOPS if they
access an unmapped page, and they'll therefore need luck or a kASLR
bypass in addition.

To keep probe_kernel_read(), probe_kernel_write(), and
probe_kernel_address() working, skip this check if
pagefault_disabled().

Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
---
 arch/x86/mm/extable.c | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c
index 818cc7ffef79..4bf3ab2b8be1 100644
--- a/arch/x86/mm/extable.c
+++ b/arch/x86/mm/extable.c
@@ -60,6 +60,37 @@ static bool uaccess_fault_okay(int trapnr, unsigned long error_code,
 		return false;
 	}
 
+	/*
+	 * If fs == KERNEL_DS, then all uaccess should be directed to
+	 * known-good kernel addresses.
+	 *
+	 * We still need to support probe_kernel_read and
+	 * probe_kernel_address, which disable page faults. This could be
+	 * tightened up a bit if we explicitly annotated probe_kernel_read(),
+	 * probe_kernel_write() and probe_kernel_address(), perhaps by
+	 * introducing PROBE_KERNEL_DS.
+	 */
+	if (unlikely(!is_user_ds && !pagefault_disabled())) {
+		if (extra < TASK_SIZE_MAX) {
+			/*
+			 * Accessing user address under KERNEL_DS.  This is a
+			 * bug and should be fixed, but OOPSing is not helpful
+			 * for exploit mitigation.
+			 */
+			WARN_ONCE(1, "BUG: uaccess fault at 0x%lx with KERNEL_DS\n",
+				  extra);
+		} else {
+			/*
+			 * If a bug that allows user-controlled KERNEL_DS
+			 * access exists, this will prevent it from being used
+			 * to trivially bypass kASLR.
+			 */
+			pr_crit("BUG: uaccess fault at 0x%lx with KERNEL_DS\n",
+				extra);
+			return false;
+		}
+	}
+
 	return true;
 }
 
-- 
2.5.5