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?
But either way, if we prevent uprobe to be placed on end64 that will
essentially break any code that does compile with endbr64
(-fcf-protection=branch), which is very not great (I suspect most
people that care would just disable that option in such a case).
>
> Also, we tried very hard to not have a literal encode ENDBR (I really
> should teach objtool about this one :/). If it somehow makes sense to
> keep this clause, please use: gen_endbr()
I'll just use is_endbr(), no problem.
