Re: [PATCH 1/2] x86/insn: Fix some potential undefined behavior.

Ingo Molnar
2020-10-15T08:01:37Z

* Ian Rogers <irogers@google.com> wrote:

> From: Numfor Mbiziwo-Tiapo <nums@google.com>
>
> If insn_init is given a NULL kaddr and 0 buflen then validate_next will
> perform arithmetic on NULL, add a guard to avoid this.
>
> Don't perform unaligned loads in __get_next and __peek_nbyte_next as
> these are forms of undefined behavior.

So, 'insn' is a kernel structure, usually allocated on the kernel stack.
How could these fields ever be unaligned?

>
> These problems were identified using the undefined behavior sanitizer
> (ubsan) with the tools version of the code and perf test. Part of this
> patch was previously posted here:
> https://lore.kernel.org/lkml/20190724184512.162887-4-nums@google.com/
>
> Signed-off-by: Ian Rogers <irogers@google.com>
> Signed-off-by: Numfor Mbiziwo-Tiapo <nums@google.com>
> ---
> arch/x86/lib/insn.c | 6 +++---
> 1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/lib/insn.c b/arch/x86/lib/insn.c
> index 404279563891..57236940de46 100644
> --- a/arch/x86/lib/insn.c
> +++ b/arch/x86/lib/insn.c
> @@ -17,13 +17,13 @@
>
> /* Verify next sizeof(t) bytes can be on the same instruction */
> #define validate_next(t, insn, n) \
> - ((insn)->next_byte + sizeof(t) + n <= (insn)->end_kaddr)
> + ((insn)->end_kaddr != 0 && (insn)->next_byte + sizeof(t) + n <= (insn)->end_kaddr)
>
> #define __get_next(t, insn) \
> - ({ t r = *(t*)insn->next_byte; insn->next_byte += sizeof(t); r; })
> + ({ t r; memcpy(&r, insn->next_byte, sizeof(t)); insn->next_byte += sizeof(t); r; })
>
> #define __peek_nbyte_next(t, insn, n) \
> - ({ t r = *(t*)((insn)->next_byte + n); r; })
> + ({ t r; memcpy(&r, (insn)->next_byte + n, sizeof(t)); r; })
>
> #define get_next(t, insn) \
> ({ if (unlikely(!validate_next(t, insn, 0))) goto err_out; __get_next(t, insn); })

Is there any code generation side effect of this change to the resulting
code?

Thanks,

Ingo