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Date: Thu, 16 Jun 2016 10:57:30 -0700
From: Andy Lutomirski <luto@...capital.net>
To: Josh Poimboeuf <jpoimboe@...hat.com>
Cc: Andy Lutomirski <luto@...nel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
Borislav Petkov <bp@...en8.de>, Nadav Amit <nadav.amit@...il.com>, Kees Cook <keescook@...omium.org>,
Brian Gerst <brgerst@...il.com>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>,
Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 08/13] x86/dumpstack: When OOPSing, rewind the stack
before do_exit
On Thu, Jun 16, 2016 at 10:50 AM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> On Wed, Jun 15, 2016 at 05:28:30PM -0700, Andy Lutomirski wrote:
>> If we call do_exit with a clean stack, we greatly reduce the risk of
>> recursive oopses due to stack overflow in do_exit, and we allow
>> do_exit to work even if we OOPS from an IST stack. The latter gives
>> us a much better chance of surviving long enough after we detect a
>> stack overflow to write out our logs.
>>
>> I intentionally separated this from the preceding patch that
>> disables do_exit-on-OOPS on IST stacks. This way, if we need to
>> revert this patch, we still end up in an acceptable state wrt stack
>> overflow handling.
>>
>> Signed-off-by: Andy Lutomirski <luto@...nel.org>
>> ---
>> arch/x86/entry/entry_32.S | 11 +++++++++++
>> arch/x86/entry/entry_64.S | 11 +++++++++++
>> arch/x86/kernel/dumpstack.c | 13 +++++++++----
>> 3 files changed, 31 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
>> index 983e5d3a0d27..1499db695a88 100644
>> --- a/arch/x86/entry/entry_32.S
>> +++ b/arch/x86/entry/entry_32.S
>> @@ -1153,3 +1153,14 @@ ENTRY(async_page_fault)
>> jmp error_code
>> END(async_page_fault)
>> #endif
>> +
>> +ENTRY(rewind_stack_do_exit)
>> + /* Prevent any naive code from trying to unwind to our caller. */
>> + xorl %ebp, %ebp
>> +
>> + movl PER_CPU_VAR(cpu_current_top_of_stack), %esi
>> + leal -TOP_OF_KERNEL_STACK_PADDING-PT_OLDSS(%esi), %esp
>> +
>> + call do_exit
>> +1: jmp 1b
>> +END(rewind_stack_do_exit)
>> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
>> index 9ee0da1807ed..394cad73e890 100644
>> --- a/arch/x86/entry/entry_64.S
>> +++ b/arch/x86/entry/entry_64.S
>> @@ -1423,3 +1423,14 @@ ENTRY(ignore_sysret)
>> mov $-ENOSYS, %eax
>> sysret
>> END(ignore_sysret)
>> +
>> +ENTRY(rewind_stack_do_exit)
>> + /* Prevent any naive code from trying to unwind to our caller. */
>> + xorl %ebp, %ebp
>> +
>> + movq PER_CPU_VAR(cpu_current_top_of_stack), %rax
>> + leaq -TOP_OF_KERNEL_STACK_PADDING-SS(%rax), %rsp
>
> I think this should be:
>
> leaq -TOP_OF_KERNEL_STACK_PADDING-SIZEOF_PTREGS, %rsp
>
> That way when it calls do_exit(), the stack frame will be placed at the
> conventional spot where a smart unwinder would expect to find it.
Whoops!
--Andy
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