Re: [PATCH 1/2] kprobes: Fix __get_insn_slot() after __counted_by annotation

[Nathan Chancellor <nathan@kernel.org>]

On Thu, Oct 31, 2024 at 10:58:27AM +0900, Masami Hiramatsu wrote:
> On Wed, 30 Oct 2024 09:14:48 -0700
> Nathan Chancellor <nathan@kernel.org> wrote:
> 
> > Commit 0888460c9050 ("kprobes: Annotate structs with __counted_by()")
> > added a __counted_by annotation without adjusting the code for the
> > __counted_by requirements, resulting in a panic when UBSAN_BOUNDS and
> > FORTIFY_SOURCE are enabled:
> > 
> >   | memset: detected buffer overflow: 512 byte write of buffer size 0
> >   | WARNING: CPU: 0 PID: 1 at lib/string_helpers.c:1032 __fortify_report+0x64/0x80
> >   | Call Trace:
> >   |  __fortify_report+0x60/0x80 (unreliable)
> >   |  __fortify_panic+0x18/0x1c
> >   |  __get_insn_slot+0x33c/0x340
> > 
> > __counted_by requires that the counter be set before accessing the
> > flexible array but ->nused is not set until after ->slot_used is
> > accessed via memset(). Even if the current ->nused assignment were moved
> > up before memset(), the value of 1 would be incorrect because the entire
> > array is being accessed, not just one element.
> 
> Ah, I think I misunderstood the __counted_by(). If so, ->nused can be
> smaller than the accessing element of slot_used[]. I should revert it.
> The accessing index and ->nused should have no relationship.
> 
> for example, slots_per_page(c) is 10, and 10 kprobes are registered
> and then, the 1st and 2nd kprobes are unregistered. At this moment,
> ->nused is 8 but slot_used[9] is still used. To unregister this 10th
> kprobe, we have to access slot_used[9].

Ah, I totally missed that bit of the code, sorry about that. Thanks for
the explanation!

> So let's just revert the commit 0888460c9050.

Reverting that change sounds totally reasonable to me based on the
above. Will you take care of that?

For what it's worth, I think patch #2 should still be applicable, if you
are okay with that one.

Cheers,
Nathan