> On May 3, 2019, at 6:22 AM, Steven Rostedt <rost...@goodmis.org> wrote:
>
> On Fri, 3 May 2019 11:29:59 +0200
> Peter Zijlstra <pet...@infradead.org> wrote:
>
>
>> OMG, WTF, ARGH... That code is fsck'ing horrible. I'd almost argue to
>> always do the INT3 thing, just to avoid games like that.
>
> Hehe, that's almost the exact same thoughts I had when seeing this
> code ;-)
>
>>
>> That said; for normal traps ®s->sp is indeed the previous context --
>> if it doesn't fall off the stack. Your hack detects the regular INT3
>> frame. Howver if regs->sp has been modified (int3_emulate_push, for
>> example) your detectoring comes unstuck.
>
> Yep. I realized the issue as well. But wanted to make sure this did
> work when sp wasn't changed.
>
>>
>> Now, it is rather unlikely these two code paths interact, but just to be
>> safe, something like so might be more reliable:
>>
>>
>> diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
>> index 4b8ee05dd6ad..aceaad0cc9a9 100644
>> --- a/arch/x86/kernel/ptrace.c
>> +++ b/arch/x86/kernel/ptrace.c
>> @@ -163,6 +163,9 @@ static inline bool invalid_selector(u16 value)
>> * stack pointer we fall back to regs as stack if no previous stack
>> * exists.
>> *
>> + * There is a special case for INT3, there we construct a full pt_regs
>> + * environment. We can detect this case by a high bit in regs->cs
>> + *
>> * This is valid only for kernel mode traps.
>> */
>> unsigned long kernel_stack_pointer(struct pt_regs *regs)
>> @@ -171,6 +174,9 @@ unsigned long kernel_stack_pointer(struct pt_regs *regs)
>> unsigned long sp = (unsigned long)®s->sp;
>> u32 *prev_esp;
>>
>> + if (regs->__csh & (1 << 13)) /* test CS_FROM_INT3 */
>> + return regs->sp;
>> +
>
> Thanks, I was looking into doing something like this (setting a flag in
> the int3 code), but didn't have the time to see the best way to do this.
>
> I'll add this version of the code and run it through my tests.
>
> -- Steve
>
>> if (context == (sp & ~(THREAD_SIZE - 1)))
>> return sp;
>>
>> --- a/arch/x86/entry/entry_32.S
>> +++ b/arch/x86/entry/entry_32.S
>> @@ -388,6 +388,7 @@
>>
>> #define CS_FROM_ENTRY_STACK (1 << 31)
>> #define CS_FROM_USER_CR3 (1 << 30)
>> +#define CS_FROM_INT3 (1 << 29)
>>
>> .macro SWITCH_TO_KERNEL_STACK
>>
>> @@ -1515,6 +1516,9 @@ ENTRY(int3)
>>
>> add $16, 12(%esp) # point sp back at the previous context
>>
>> + andl $0x0000ffff, 4(%esp)
>> + orl $CS_FROM_INT3, 4(%esp)
>> +
>> pushl $-1 # orig_eax; mark as interrupt
>>
>> SAVE_ALL
>
So here’s a somewhat nutty suggestion: how about we tweak the 32-bit entry code
to emulate the sane 64-bit frame, not just for int3 but always? Basically, the
entry asm for entries from kernel mode would do, roughly:
push $0 ;dummy for call emulation
push %ss
push $0 ;a dummy for ESP
push 3*4(%esp) ;EFLAGS
push 3*4(%esp) ;CS
push 3*4(%esp) ;EIP
push %rax
lea 7*4(%esp), %rax
mov %rax, 4*4(%esp) ;ESP
And the exit asm would do a little dance to write EFLAGS, CS, and EIP to the
right spot, then load ESP-3*4 into %esp and do IRET.
Now the annoying kernel_stack_pointer() hack can just go away, since regs->sp
is always correct!
I probably screwed up some arithmetic there, but it’s the idea that counts :)
