On Tue, Jul 09, 2024 at 10:50:00AM -0700, Andrii Nakryiko wrote:
> On Tue, Jul 9, 2024 at 3:11 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > On Mon, Jul 08, 2024 at 04:11:27PM -0700, Andrii Nakryiko wrote:
> > > +#ifdef CONFIG_UPROBES
> > > +/*
> > > + * Heuristic-based check if uprobe is installed at the function entry.
> > > + *
> > > + * Under assumption of user code being compiled with frame pointers,
> > > + * `push %rbp/%ebp` is a good indicator that we indeed are.
> > > + *
> > > + * Similarly, `endbr64` (assuming 64-bit mode) is also a common pattern.
> > > + * If we get this wrong, captured stack trace might have one extra bogus
> > > + * entry, but the rest of stack trace will still be meaningful.
> > > + */
> > > +static bool is_uprobe_at_func_entry(struct pt_regs *regs)
> > > +{
> > > + struct arch_uprobe *auprobe;
> > > +
> > > + if (!current->utask)
> > > + return false;
> > > +
> > > + auprobe = current->utask->auprobe;
> > > + if (!auprobe)
> > > + return false;
> > > +
> > > + /* push %rbp/%ebp */
> > > + if (auprobe->insn[0] == 0x55)
> > > + return true;
> > > +
> > > + /* endbr64 (64-bit only) */
> > > + if (user_64bit_mode(regs) && *(u32 *)auprobe->insn == 0xfa1e0ff3)
> > > + return true;
> >
> > I meant to reply to Josh suggesting this, but... how can this be? If you
> > scribble the ENDBR with an INT3 things will #CP and we'll never get to
> > the #BP.
>
> Well, it seems like it works in practice, I just tried. Here's the
> disassembly of the function:
>
> 00000000000019d0 <urandlib_api_v1>:
> 19d0: f3 0f 1e fa endbr64
> 19d4: 55 pushq %rbp
> 19d5: 48 89 e5 movq %rsp, %rbp
> 19d8: 48 83 ec 10 subq $0x10, %rsp
> 19dc: 48 8d 3d fe ed ff ff leaq -0x1202(%rip), %rdi
> # 0x7e1 <__isoc99_scanf+0x7e1>
> 19e3: 48 8d 75 fc leaq -0x4(%rbp), %rsi
> 19e7: b0 00 movb $0x0, %al
> 19e9: e8 f2 00 00 00 callq 0x1ae0 <__isoc99_scanf+0x1ae0>
> 19ee: b8 01 00 00 00 movl $0x1, %eax
> 19f3: 48 83 c4 10 addq $0x10, %rsp
> 19f7: 5d popq %rbp
> 19f8: c3 retq
> 19f9: 0f 1f 80 00 00 00 00 nopl (%rax)
>
> And here's the state when uprobe is attached:
>
> (gdb) disass/r urandlib_api_v1
> Dump of assembler code for function urandlib_api_v1:
> 0x00007ffb734e39d0 <+0>: cc int3
> 0x00007ffb734e39d1 <+1>: 0f 1e fa nop %edx
> 0x00007ffb734e39d4 <+4>: 55 push %rbp
> 0x00007ffb734e39d5 <+5>: 48 89 e5 mov %rsp,%rbp
> 0x00007ffb734e39d8 <+8>: 48 83 ec 10 sub $0x10,%rsp
> 0x00007ffb734e39dc <+12>: 48 8d 3d fe ed ff ff lea
> -0x1202(%rip),%rdi # 0x7ffb734e27e1
> 0x00007ffb734e39e3 <+19>: 48 8d 75 fc lea -0x4(%rbp),%rsi
> => 0x00007ffb734e39e7 <+23>: b0 00 mov $0x0,%al
> 0x00007ffb734e39e9 <+25>: e8 f2 00 00 00 call
> 0x7ffb734e3ae0 <__isoc99_scanf@plt>
> 0x00007ffb734e39ee <+30>: b8 01 00 00 00 mov $0x1,%eax
> 0x00007ffb734e39f3 <+35>: 48 83 c4 10 add $0x10,%rsp
> 0x00007ffb734e39f7 <+39>: 5d pop %rbp
> 0x00007ffb734e39f8 <+40>: c3 ret
>
>
> You can see it replaced the first byte, the following 3 bytes are
> remnants of endb64 (gdb says it's a nop? :)), and then we proceeded,
> you can see I stepped through a few more instructions.
>
> Works by accident?
Yeah, we don't actually have Userspace IBT enabled yet, even on hardware
that supports it.
