From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932098AbaGTF4r (ORCPT ); Sun, 20 Jul 2014 01:56:47 -0400 Received: from mail-pd0-f178.google.com ([209.85.192.178]:63865 "EHLO mail-pd0-f178.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751525AbaGTF4q (ORCPT ); Sun, 20 Jul 2014 01:56:46 -0400 Date: Sat, 19 Jul 2014 22:56:43 -0700 From: Omar Sandoval To: linux-arm-kernel@lists.infradead.org, linux@arm.linux.org.uk Cc: linux-kernel@vger.kernel.org Subject: Re: [PATCH] ARM: Don't oops when userspace executes kgdb break instructions. Message-ID: <20140720055643.GA8413@mew.web-pass.com> References: <20140718225049.GA11937@mew.guest.dropbox.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20140718225049.GA11937@mew.guest.dropbox.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jul 18, 2014 at 03:51:31PM -0700, Omar Sandoval wrote: > Don't break into kgdb when userspace executes the kernel break instructions > (KGDB_BREAKINST and KGDB_COMPILED_BREAK). The kernel will oops in > kgdb_handle_exception. > > Signed-off-by: Omar Sandoval > --- > The following program will immediately cause a kernel oops: > .globl _start > _start: > udf #65006 @ KGDB_BREAKINST > > arch/arm/kernel/kgdb.c | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/arch/arm/kernel/kgdb.c b/arch/arm/kernel/kgdb.c > index 778c2f7..a74b53c 100644 > --- a/arch/arm/kernel/kgdb.c > +++ b/arch/arm/kernel/kgdb.c > @@ -160,12 +160,16 @@ static int kgdb_compiled_brk_fn(struct pt_regs *regs, unsigned int instr) > static struct undef_hook kgdb_brkpt_hook = { > .instr_mask = 0xffffffff, > .instr_val = KGDB_BREAKINST, > + .cpsr_mask = MODE_MASK, > + .cpsr_val = SVC_MODE, > .fn = kgdb_brk_fn > }; > > static struct undef_hook kgdb_compiled_brkpt_hook = { > .instr_mask = 0xffffffff, > .instr_val = KGDB_COMPILED_BREAK, > + .cpsr_mask = MODE_MASK, > + .cpsr_val = SVC_MODE, > .fn = kgdb_compiled_brk_fn > }; > > -- > 2.0.1 -- Following up/clarifying this. This only happens when the kernel is compiled with CONFIG_KGDB. When a userspace program executes KGDB_BREAKINST or KGDB_COMPILED_BREAK, the undef_hook for kgdb catches it. The reason in kdb_stub defaults to KDB_REASON_OOPS, so the bug manifests itself as an oops caused by userspace (a better description for the patch would be "Don't enter KGDB when userspace executes kgdb break instructions"). This means that a buggy/malicious program can take down the system just by executing an instruction. ARM64 might have the same issue, but I don't have a board to test that on. I verified that breaking normally (e.g., with kgdbwait or through /proc/sysrq-trigger) still works. — Omar