__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 <l...@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
