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Message-ID: <CAGXu5jL4hZTBdmLpOmjaKOan6NfBtnpyBk6m1rpMFbTZsd3cFw@mail.gmail.com>
Date: Tue, 25 Oct 2016 15:18:44 -0700
From: Kees Cook <keescook@...omium.org>
To: "Reshetova, Elena" <elena.reshetova@...el.com>
Cc: AKASHI Takahiro <takahiro.akashi@...aro.org>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>,
Hans Liljestrand <ishkamiel@...il.com>, David Windsor <dwindsor@...il.com>
Subject: Re: [RFC v2 PATCH 01/13] Add architecture
independent hardened atomic base
On Tue, Oct 25, 2016 at 11:20 AM, Reshetova, Elena
<elena.reshetova@...el.com> wrote:
>> struct list_head {
>> diff --git a/kernel/panic.c b/kernel/panic.c index e6480e2..cb1d6db
>> 100644
>> --- a/kernel/panic.c
>> +++ b/kernel/panic.c
>> @@ -616,3 +616,14 @@ static int __init oops_setup(char *s)
>> return 0;
>> }
>> early_param("oops", oops_setup);
>> +
>> +#ifdef CONFIG_HARDENED_ATOMIC
>> +void hardened_atomic_overflow(struct pt_regs *regs) {
>> + pr_emerg(KERN_EMERG "HARDENED_ATOMIC: overflow detected in: %s:%d, uid/euid: %u/%u\n",
>> + current->comm, task_pid_nr(current),
>> + from_kuid_munged(&init_user_ns, current_uid()),
>> + from_kuid_munged(&init_user_ns, current_euid()));
>> + BUG();
>
> BUG() will print a message like "kernel BUG at kernel/panic.c:627!"
> and a stack trace dump with extra frames including hardened_atomic_overflow() and some exception handler routines (do_trap() on x86), which are totally useless. So I don't want to call BUG() here.
>
> Instead, we will fall back to a normal "BUG" handler, bug_handler() on arm64, which eventually calls die(), generating more *intuitive* messages:
> ===8<===
> [ 29.082336] lkdtm: attempting good atomic_add_return
> [ 29.082391] lkdtm: attempting bad atomic_add_return
> [ 29.082830] ------------[ cut here ]------------
> [ 29.082889] Kernel BUG at ffff0000008b07fc [verbose debug info unavailable]
> (Actually, this is lkdtm_ATOMIC_ADD_RETURN_OVERFLOW)
> [ 29.082968] HARDENED_ATOMIC: overflow detected in: insmod:1152, uid/euid: 0/0
> [ 29.083043] Internal error: Oops - BUG: 0 [#1] PREEMPT SMP
> [ 29.083098] Modules linked in: lkdtm(+)
> [ 29.083189] CPU: 1 PID: 1152 Comm: insmod Not tainted 4.9.0-rc1-00024-gb757839-dirty #12
> [ 29.083262] Hardware name: FVP Base (DT)
> [ 29.083324] task: ffff80087aa21900 task.stack: ffff80087a36c000
> [ 29.083557] PC is at lkdtm_ATOMIC_ADD_RETURN_OVERFLOW+0x6c/0xa0 [lkdtm]
> [ 29.083627] LR is at 0x7fffffff
> [ 29.083687] pc : [<ffff0000008b07fc>] lr : [<000000007fffffff>] pstate: 90400149
> [ 29.083757] sp : ffff80087a36fbe0
> [ 29.083810] x29: ffff80087a36fbe0 [ 29.083858] x28: ffff000008ec3000
> [ 29.083906]
>
> ...
>
> [ 29.090842] [<ffff0000008b07fc>] lkdtm_ATOMIC_ADD_RETURN_OVERFLOW+0x6c/0xa0 [lkdtm]
> [ 29.091090] [<ffff0000008b20a4>] lkdtm_do_action+0x1c/0x28 [lkdtm]
> [ 29.091334] [<ffff0000008bb118>] lkdtm_module_init+0x118/0x210 [lkdtm]
> [ 29.091422] [<ffff000008083150>] do_one_initcall+0x38/0x128
> [ 29.091503] [<ffff000008166ad4>] do_init_module+0x5c/0x1c8
> [ 29.091586] [<ffff00000812e1ec>] load_module+0x1b24/0x20b0
> [ 29.091670] [<ffff00000812e920>] SyS_init_module+0x1a8/0x1d8
> [ 29.091753] [<ffff000008082ef0>] el0_svc_naked+0x24/0x28
> [ 29.091843] Code: 910063a1 b8e0003e 2b1e0010 540000c7 (d4210020)
> ===>8===
>
> So, you propose to remove call to BUG() fully from there? Funny, I think on x86 the output was actually like you wanted with just calling BUG().
The x86 BUG isn't as nice:
- "kernel BUG at kernel/panic.c:627" is bogus, the bug is a frame above, etc
- the meaningful message "HARDENED_ATOMIC: overflow detected" happens
above the ==cut== line
-Kees
--
Kees Cook
Nexus Security
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