Re: [PATCH] scs: Release kasan vmalloc poison in scs_free process

[Sami Tolvanen]

Hi,

On Thu, Sep 23, 2021 at 2:53 AM <yee.lee@xxxxxxxxxxxx> wrote:
>
> From: Yee Lee <yee.lee@xxxxxxxxxxxx>
>
> Since scs allocation has been moved to vmalloc region, the
> shadow stack is protected by kasan_posion_vmalloc.
> However, the vfree_atomic operation needs to access
> its context for scs_free process and causes kasan error
> as the dump info below.
>
> This patch Adds kasan_unpoison_vmalloc() before vfree_atomic,
> which aligns to the prior flow as using kmem_cache.
> The vmalloc region will go back posioned in the following
> vumap() operations.
>
>  ==================================================================
>  BUG: KASAN: vmalloc-out-of-bounds in llist_add_batch+0x60/0xd4
>  Write of size 8 at addr ffff8000100b9000 by task kthreadd/2
>
>  CPU: 0 PID: 2 Comm: kthreadd Not tainted 5.15.0-rc2-11681-(skip)
>  Hardware name: linux,dummy-virt (DT)
>  Call trace:
>   dump_backtrace+0x0/0x43c
>   show_stack+0x1c/0x2c
>   dump_stack_lvl+0x68/0x84
>   print_address_description+0x80/0x394
>   kasan_report+0x180/0x1dc
>   __asan_report_store8_noabort+0x48/0x58
>   llist_add_batch+0x60/0xd4
>   vfree_atomic+0x60/0xe0
>   scs_free+0x1dc/0x1fc
>   scs_release+0xa4/0xd4
>   free_task+0x30/0xe4
>   __put_task_struct+0x1ec/0x2e0
>   delayed_put_task_struct+0x5c/0xa0
>   rcu_do_batch+0x62c/0x8a0
>   rcu_core+0x60c/0xc14
>   rcu_core_si+0x14/0x24
>   __do_softirq+0x19c/0x68c
>   irq_exit+0x118/0x2dc
>   handle_domain_irq+0xcc/0x134
>   gic_handle_irq+0x7c/0x1bc
>   call_on_irq_stack+0x40/0x70
>   do_interrupt_handler+0x78/0x9c
>   el1_interrupt+0x34/0x60
>   el1h_64_irq_handler+0x1c/0x2c
>   el1h_64_irq+0x78/0x7c
>   _raw_spin_unlock_irqrestore+0x40/0xcc
>   sched_fork+0x4f0/0xb00
>   copy_process+0xacc/0x3648
>   kernel_clone+0x168/0x534
>   kernel_thread+0x13c/0x1b0
>   kthreadd+0x2bc/0x400
>   ret_from_fork+0x10/0x20
>
>  Memory state around the buggy address:
>   ffff8000100b8f00: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
>   ffff8000100b8f80: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
>  >ffff8000100b9000: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
>                     ^
>   ffff8000100b9080: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
>   ffff8000100b9100: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
>  ==================================================================
>
> CC: Matthias Brugger <matthias.bgg@xxxxxxxxx>
> CC: Will Deacon <will@xxxxxxxxxx>
> CC: Sami Tolvanen <samitolvanen@xxxxxxxxxx>
> Signed-off-by: Yee Lee <yee.lee@xxxxxxxxxxxx>
> ---
>  kernel/scs.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/kernel/scs.c b/kernel/scs.c
> index e2a71fc82fa0..25c0d8e416e6 100644
> --- a/kernel/scs.c
> +++ b/kernel/scs.c
> @@ -68,6 +68,7 @@ void scs_free(void *s)
>
>         __scs_account(s, -1);
>
> +       kasan_unpoison_vmalloc(s, SCS_SIZE);

Thanks for the patch! I believe I had this in the original vmap SCS
series, but Will pointed out that unpoisoning shouldn't be necessary
before vfree_atomic() and I don't remember seeing this KASAN error
back then. I wonder if something changed there? Either way, the patch
looks good to me.

Reviewed-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx>

Sami