Re: [PATCH v2] target/s390x: Fix translation exception on illegal instruction

[David Hildenbrand <david@redhat.com>]

On 13.04.21 18:52, Ilya Leoshkevich wrote:
> Hitting an uretprobe in a s390x TCG guest causes a SIGSEGV. What
> happens is:
> 
> * uretprobe maps a userspace page containing an invalid instruction.
> * uretprobe replaces the target function's return address with the
>    address of that page.
> * When tb_gen_code() is called on that page, tb->size ends up being 0
>    (because the page starts with the invalid instruction), which causes
>    virt_page2 to point to the previous page.
> * The previous page is not mapped, so this causes a spurious
>    translation exception.
> 
> The bug is that tb->size must never be 0: even if there is an illegal
> instruction, the instruction bytes that have been looked at must count
> towards tb->size. So adjust s390x's translate_one() to act this way
> for both illegal instructions and instructions that are known to
> generate exceptions.
> 
> Also add an assertion to tb_gen_code() in order to detect such
> situations in future.
> 
> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
> ---
> 
> v1: https://lists.nongnu.org/archive/html/qemu-devel/2021-04/msg02037.html
> v1 -> v2: Fix target/s390x instead of trying to tolerate tb->size == 0
>            in tb_gen_code().
> 
>   accel/tcg/translate-all.c |  1 +
>   target/s390x/translate.c  | 16 +++++++++++-----
>   2 files changed, 12 insertions(+), 5 deletions(-)
> 
> diff --git a/accel/tcg/translate-all.c b/accel/tcg/translate-all.c
> index ba6ab09790..93b2dae112 100644
> --- a/accel/tcg/translate-all.c
> +++ b/accel/tcg/translate-all.c
> @@ -1913,6 +1913,7 @@ TranslationBlock *tb_gen_code(CPUState *cpu,
>   
>       tcg_ctx->cpu = env_cpu(env);
>       gen_intermediate_code(cpu, tb, max_insns);
> +    assert(tb->size != 0);
>       tcg_ctx->cpu = NULL;
>       max_insns = tb->icount;
>   
> diff --git a/target/s390x/translate.c b/target/s390x/translate.c
> index 4f953ddfba..e243624d2a 100644
> --- a/target/s390x/translate.c
> +++ b/target/s390x/translate.c
> @@ -6412,7 +6412,8 @@ static DisasJumpType translate_one(CPUS390XState *env, DisasContext *s)
>           qemu_log_mask(LOG_UNIMP, "unimplemented opcode 0x%02x%02x\n",
>                         s->fields.op, s->fields.op2);
>           gen_illegal_opcode(s);
> -        return DISAS_NORETURN;
> +        ret = DISAS_NORETURN;
> +        goto out;
>       }
>   
>   #ifndef CONFIG_USER_ONLY
> @@ -6428,7 +6429,8 @@ static DisasJumpType translate_one(CPUS390XState *env, DisasContext *s)
>           /* privileged instruction */
>           if ((s->base.tb->flags & FLAG_MASK_PSTATE) && (insn->flags & IF_PRIV)) {
>               gen_program_exception(s, PGM_PRIVILEGED);
> -            return DISAS_NORETURN;
> +            ret = DISAS_NORETURN;
> +            goto out;
>           }
>   
>           /* if AFP is not enabled, instructions and registers are forbidden */
> @@ -6455,7 +6457,8 @@ static DisasJumpType translate_one(CPUS390XState *env, DisasContext *s)
>               }
>               if (dxc) {
>                   gen_data_exception(dxc);
> -                return DISAS_NORETURN;
> +                ret = DISAS_NORETURN;
> +                goto out;
>               }
>           }
>   
> @@ -6463,7 +6466,8 @@ static DisasJumpType translate_one(CPUS390XState *env, DisasContext *s)
>           if (insn->flags & IF_VEC) {
>               if (!((s->base.tb->flags & FLAG_MASK_VECTOR))) {
>                   gen_data_exception(0xfe);
> -                return DISAS_NORETURN;
> +                ret = DISAS_NORETURN;
> +                goto out;
>               }
>           }
>   
> @@ -6484,7 +6488,8 @@ static DisasJumpType translate_one(CPUS390XState *env, DisasContext *s)
>               (insn->spec & SPEC_r1_f128 && !is_fp_pair(get_field(s, r1))) ||
>               (insn->spec & SPEC_r2_f128 && !is_fp_pair(get_field(s, r2)))) {
>               gen_program_exception(s, PGM_SPECIFICATION);
> -            return DISAS_NORETURN;
> +            ret = DISAS_NORETURN;
> +            goto out;
>           }
>       }
>   
> @@ -6544,6 +6549,7 @@ static DisasJumpType translate_one(CPUS390XState *env, DisasContext *s)
>       }
>   #endif
>   
> +out:
>       /* Advance to the next instruction.  */
>       s->base.pc_next = s->pc_tmp;
>       return ret;
> 

Reviewed-by: David Hildenbrand <david@redhat.com>

-- 
Thanks,

David / dhildenb