> On May 3, 2019, at 6:22 AM, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote: > > On Fri, 3 May 2019 11:29:59 +0200 > Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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 :)
