Re: [PATCH 1/1] sched: fix user_mask double free

[Peter Zijlstra]

So you failed:

 - to Cc the original author of this code (Will Deacon)
 - to report what version this is against (apparently Linus' tree)
 - to check if this still applies to the latest tree (it doesn't)
 - to Cc the author of the code it now conflicts with (Waiman)
 - write something coherent in the changelog.
 - to include a Fixes tag.

Still, let me try and make sense of things...

On Mon, Nov 21, 2022 at 06:04:20PM +0800, wangbiao3@xxxxxxxxxx wrote:
> From: wangbiao3 <wangbiao3@xxxxxxxxxx>
> 
> Clone/Fork a new task,call dup_task_struct->arch_dup_task_struct(tsk,orig)
> which copy the data of parent/sibling task inclding p->user_cpus_ptr,so
> the user_cpus_ptr of newtask is the same with orig task's.When
> dup_task_struct call dup_user_cpus_ptr(tsk, orig, node),it return 0
> dircetly if src->user_cpus_ptris free by other task,in this case ,
> the newtask's address of user_cpus_ptr is not changed.

(even just inserting some whitespace would've made it so much easier to
read)

But, the only way that would be possible is if
force_compatible_cpus_allowed_ptr() were to be called on !current, and
that just doesn't happen, the only callsite is:

arch/arm64/kernel/process.c:                    force_compatible_cpus_allowed_ptr(current);

And you can't be in fork() and exec() at the same time.

If it were possible to call restrict_cpus_allowed_ptr() on a non-current
task then yes, absolutely, which is why:

  8f9ea86fdf99 ("sched: Always preserve the user requested cpumask")

also wraps the thing in pi_lock, but looking at it now, perhaps it needs
to do the alloc/copy first and swap under pi_lock instead.

(leaving the rest for the newly Cc'ed)

> Finally,
> wakup newtask to execute, call task_cpu_possible_mask-->
> do_set_cpus_allowed to set new task's user_cpus_ptr(user_mask) which
> call kfree user_mask at the end. So cause a slub double free panic.
> 
> Use pi_lock to protect content of user_cpus_ptr in dup_user_cpus_ptr and
> clear dst->user_cpus_ptr when found src->user_cpus_ptr is null
> 
> kernel BUG at mm/slub.c:363!
> Call trace:
>  __slab_free+0x230/0x28c
>  kfree+0x220/0x2cc
>  do_set_cpus_allowed+0x74/0xa4
>  select_fallback_rq+0x12c/0x200
>  wake_up_new_task+0x26c/0x304
>  kernel_clone+0x2c0/0x470
>  __arm64_sys_clone+0x5c/0x8c
>  invoke_syscall+0x60/0x150
>  el0_svc_common.llvm.13030543509303927816+0x98/0x114
>  do_el0_svc_compat+0x20/0x30
>  el0_svc_compat+0x28/0x90
>  el0t_32_sync_handler+0x7c/0xbc
>  el0t_32_sync+0x1b8/0x1bc
> 
> Signed-off-by: wangbiao3 <wangbiao3@xxxxxxxxxx>
> ---
>  kernel/sched/core.c | 31 ++++++++++++++++++++-----------
>  1 file changed, 20 insertions(+), 11 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index daff72f00385..b013d8b777b4 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2584,29 +2584,38 @@ void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask)
>         __do_set_cpus_allowed(p, new_mask, 0);
>  }
> 
> +static inline struct cpumask *clear_user_cpus_ptr(struct task_struct *p)
> +{
> +       struct cpumask *user_mask = NULL;
> +
> +       swap(p->user_cpus_ptr, user_mask);
> +
> +       return user_mask;
> +}
> +
>  int dup_user_cpus_ptr(struct task_struct *dst, struct task_struct *src,
>                       int node)
>  {
> -       if (!src->user_cpus_ptr)
> -               return 0;
> +       unsigned long flags;
> +       struct cpumask *user_mask = NULL;
> 
>         dst->user_cpus_ptr = kmalloc_node(cpumask_size(), GFP_KERNEL, node);
>         if (!dst->user_cpus_ptr)
>                 return -ENOMEM;
> 
> +       /* Use pi_lock to protect content of user_cpus_ptr */
> +       raw_spin_lock_irqsave(&src->pi_lock, flags);
> +       if (!src->user_cpus_ptr) {
> +               user_mask = clear_user_cpus_ptr(dst);
> +               raw_spin_unlock_irqrestore(&src->pi_lock, flags);
> +               kfree(user_mask);
> +               return 0;
> +       }
>         cpumask_copy(dst->user_cpus_ptr, src->user_cpus_ptr);
> +       raw_spin_unlock_irqrestore(&src->pi_lock, flags);
>         return 0;
>  }
> 
> -static inline struct cpumask *clear_user_cpus_ptr(struct task_struct *p)
> -{
> -       struct cpumask *user_mask = NULL;
> -
> -       swap(p->user_cpus_ptr, user_mask);
> -
> -       return user_mask;
> -}
> -
>  void release_user_cpus_ptr(struct task_struct *p)
>  {
>         kfree(clear_user_cpus_ptr(p));
> --
> 2.38.1
> 
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