Re: [PATCH 08/13] x86/dumpstack: When OOPSing, rewind the stack before do_exit

[Andy Lutomirski]

On Thu, Jun 16, 2016 at 10:50 AM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> 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@xxxxxxxxxx>
>> ---
>>  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