Re: [PATCH] accel/tcg: fix race in cpu_exec_step_atomic (bug 1863025)

[Paolo Bonzini]

On 14/02/20 15:49, Alex Bennée wrote:
> The bug describes a race whereby cpu_exec_step_atomic can acquire a TB
> which is invalidated by a tb_flush before we execute it. This doesn't
> affect the other cpu_exec modes as a tb_flush by it's nature can only
> occur on a quiescent system. The race was described as:
> 
>   B2. tcg_cpu_exec => cpu_exec => tb_find => tb_gen_code
>   B3. tcg_tb_alloc obtains a new TB
> 
>       C3. TB obtained with tb_lookup__cpu_state or tb_gen_code
>           (same TB as B2)
> 
>           A3. start_exclusive critical section entered
>           A4. do_tb_flush is called, TB memory freed/re-allocated
>           A5. end_exclusive exits critical section
> 
>   B2. tcg_cpu_exec => cpu_exec => tb_find => tb_gen_code
>   B3. tcg_tb_alloc reallocates TB from B2
> 
>       C4. start_exclusive critical section entered
>       C5. cpu_tb_exec executes the TB code that was free in A4
> 
> The simplest fix is to widen the exclusive period to include the TB
> lookup. As a result we can drop the complication of checking we are in
> the exclusive region before we end it.

Reviewed-by: Paolo Bonzini <address@hidden>

> Signed-off-by: Alex Bennée <address@hidden>
> Cc: Yifan <address@hidden>
> Cc: Bug 1863025 <address@hidden>
> ---
>  accel/tcg/cpu-exec.c | 21 +++++++++++----------
>  1 file changed, 11 insertions(+), 10 deletions(-)
> 
> diff --git a/accel/tcg/cpu-exec.c b/accel/tcg/cpu-exec.c
> index 2560c90eec7..d95c4848a47 100644
> --- a/accel/tcg/cpu-exec.c
> +++ b/accel/tcg/cpu-exec.c
> @@ -240,6 +240,8 @@ void cpu_exec_step_atomic(CPUState *cpu)
>      uint32_t cf_mask = cflags & CF_HASH_MASK;
>  
>      if (sigsetjmp(cpu->jmp_env, 0) == 0) {
> +        start_exclusive();
> +
>          tb = tb_lookup__cpu_state(cpu, &pc, &cs_base, &flags, cf_mask);
>          if (tb == NULL) {
>              mmap_lock();
> @@ -247,8 +249,6 @@ void cpu_exec_step_atomic(CPUState *cpu)
>              mmap_unlock();
>          }
>  
> -        start_exclusive();
> -
>          /* Since we got here, we know that parallel_cpus must be true.  */
>          parallel_cpus = false;
>          cc->cpu_exec_enter(cpu);
> @@ -271,14 +271,15 @@ void cpu_exec_step_atomic(CPUState *cpu)
>          qemu_plugin_disable_mem_helpers(cpu);
>      }
>  
> -    if (cpu_in_exclusive_context(cpu)) {
> -        /* We might longjump out of either the codegen or the
> -         * execution, so must make sure we only end the exclusive
> -         * region if we started it.
> -         */
> -        parallel_cpus = true;
> -        end_exclusive();
> -    }
> +
> +    /*
> +     * As we start the exclusive region before codegen we must still
> +     * be in the region if we longjump out of either the codegen or
> +     * the execution.
> +     */
> +    g_assert(cpu_in_exclusive_context(cpu));
> +    parallel_cpus = true;
> +    end_exclusive();
>  }
>  
>  struct tb_desc {
>