[PATCH 06/13] x86/fault: Improve the condition for signalling vs OOPSing

[Andy Lutomirski <luto@kernel.org>]

__bad_area_nosemaphore() currently checks the X86_PF_USER bit in the
error code to decide whether to send a signal or to treat the fault
as a kernel error.  This can cause somewhat erratic behavior.  The
straightforward cases where the CPL agrees with the hardware USER
bit are all correct, but the other cases are confusing.

 - A user instruction accessing a kernel address with supervisor
   privilege (e.g. a descriptor table access failed).  The USER bit
   will be clear, and we OOPS.  This is correct, because it indicates
   a kernel bug, not a user error.

 - A user instruction accessing a user address with supervisor
   privilege (e.g. a descriptor table was incorrectly pointing at
   user memory).  __bad_area_nosemaphore() will be passed a modified
   error code with the user bit set, and we will send a signal.
   Sending the signal will work (because the regs and the entry
   frame genuinely come from user mode), but we really ought to
   OOPS, as this event indicates a severe kernel bug.

 - A kernel instruction with user privilege (i.e. WRUSS).  This
   should OOPS or get fixed up.  The current code would instead try
   send a signal and malfunction.

Change the logic: a signal should be sent if the faulting context is
user mode *and* the access has user privilege.  Otherwise it's
either a kernel mode fault or a failed implicit access, either of
which should end up in no_context().

Note to -stable maintainers: don't backport this unless you backport
CET.  The bug it fixes is unobservable in current kernels unless
something is extremely wrong.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
 arch/x86/mm/fault.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 95d94d48a10d..bd89b6f83aa2 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -794,7 +794,7 @@ __bad_area_nosemaphore(struct pt_regs *regs, unsigned long error_code,
 	struct task_struct *tsk = current;
 
 	/* User mode accesses just cause a SIGSEGV */
-	if (error_code & X86_PF_USER) {
+	if (user_mode(regs) && (error_code & X86_PF_USER)) {
 		/*
 		 * It's possible to have interrupts off here:
 		 */
-- 
2.17.2