linux-kernel - [tip:x86/mm] x86/fault: Improve the condition for signalling vs OOPSing

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Date:   Tue, 20 Nov 2018 00:16:51 -0800
From:   tip-bot for Andy Lutomirski <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     yu-cheng.yu@...el.com, dave.hansen@...ux.intel.com,
        brgerst@...il.com, torvalds@...ux-foundation.org, mingo@...nel.org,
        peterz@...radead.org, bp@...en8.de, tglx@...utronix.de,
        luto@...capital.net, dvlasenk@...hat.com, luto@...nel.org,
        riel@...riel.com, linux-kernel@...r.kernel.org, hpa@...or.com
Subject: [tip:x86/mm] x86/fault: Improve the condition for signalling vs
 OOPSing

Commit-ID:  6ea59b074f15e7ef4b042a108950861b383e7b02
Gitweb:     https://git.kernel.org/tip/6ea59b074f15e7ef4b042a108950861b383e7b02
Author:     Andy Lutomirski <luto@...nel.org>
AuthorDate: Mon, 19 Nov 2018 14:45:30 -0800
Committer:  Ingo Molnar <mingo@...nel.org>
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@...nel.org>
Cc: Andy Lutomirski <luto@...capital.net>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Brian Gerst <brgerst@...il.com>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: Denys Vlasenko <dvlasenk@...hat.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Rik van Riel <riel@...riel.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Yu-cheng Yu <yu-cheng.yu@...el.com>
Link: http://lkml.kernel.org/r/10e509c43893170e262e82027ea399130ae81159.1542667307.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@...nel.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 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:
 		 */