Commit-ID: 6ea59b074f15e7ef4b042a108950861b383e7b02
Gitweb: https://git.kernel.org/tip/6ea59b074f15e7ef4b042a108950861b383e7b02
Author: Andy Lutomirski <luto@xxxxxxxxxx>
AuthorDate: Mon, 19 Nov 2018 14:45:30 -0800
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Tue, 20 Nov 2018 08:44:29 +0100
x86/fault: Improve the condition for signalling vs OOPSing
__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@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
Link: http://lkml.kernel.org/r/10e509c43893170e262e82027ea399130ae81159.1542667307.git.luto@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
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 7a69b66cf071..3c9aed03d18e 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:
*/
|