Re: [PATCH] x86/resctrl: Fix uninitialized memory read when last CPU of domain goes offline

["Moger, Babu" <babu.moger@amd.com>]

Hi Reinette,

On 3/28/24 16:12, Reinette Chatre wrote:
> Tony encountered the OOPS below when the last CPU of a domain goes
> offline while running a kernel built with CONFIG_NO_HZ_FULL:
> 
>     BUG: kernel NULL pointer dereference, address: 0000000000000000
>     #PF: supervisor read access in kernel mode
>     #PF: error_code(0x0000) - not-present page
>     PGD 0
>     Oops: 0000 [#1] PREEMPT SMP NOPTI
>     ...
>     RIP: 0010:__find_nth_andnot_bit+0x66/0x110
>     ...
>     Call Trace:
>      <TASK>
>      ? __die+0x1f/0x60
>      ? page_fault_oops+0x176/0x5a0
>      ? exc_page_fault+0x7f/0x260
>      ? asm_exc_page_fault+0x22/0x30
>      ? __pfx_resctrl_arch_offline_cpu+0x10/0x10
>      ? __find_nth_andnot_bit+0x66/0x110
>      ? __cancel_work+0x7d/0xc0
>      cpumask_any_housekeeping+0x55/0x110
>      mbm_setup_overflow_handler+0x40/0x70
>      resctrl_offline_cpu+0x101/0x110
>      resctrl_arch_offline_cpu+0x19/0x260
>      cpuhp_invoke_callback+0x156/0x6b0
>      ? cpuhp_thread_fun+0x5f/0x250
>      cpuhp_thread_fun+0x1ca/0x250
>      ? __pfx_smpboot_thread_fn+0x10/0x10
>      smpboot_thread_fn+0x184/0x220
>      kthread+0xe0/0x110
>      ? __pfx_kthread+0x10/0x10
>      ret_from_fork+0x2d/0x50
>      ? __pfx_kthread+0x10/0x10
>      ret_from_fork_asm+0x1a/0x30
>      </TASK>
> 
> The NULL pointer dereference is encountered while searching for another
> online CPU in the domain (of which there are none) that can be used to
> run the MBM overflow handler.
> 
> Because the kernel is configured with CONFIG_NO_HZ_FULL the search for
> another CPU (in its effort to prefer those CPUs that aren't marked
> nohz_full) consults the mask representing the nohz_full CPUs,
> tick_nohz_full_mask. On a kernel with CONFIG_CPUMASK_OFFSTACK=y
> tick_nohz_full_mask is not allocated unless the kernel is booted with
> the "nohz_full=" parameter and because of that any access to
> tick_nohz_full_mask needs to be guarded with tick_nohz_full_enabled().
> 
> Add a tick_nohz_full_enabled() check to ensure that tick_nohz_full_mask
> has been initialized and can thus be accessed safely.
> 
> Fixes: a4846aaf3945 ("x86/resctrl: Add cpumask_any_housekeeping() for limbo/overflow")
> Reported-by: Tony Luck <tony.luck@intel.com>
> Closes: https://lore.kernel.org/lkml/ZgIFT5gZgIQ9A9G7@agluck-desk3/
> Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
> ---
>  arch/x86/kernel/cpu/resctrl/internal.h | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/x86/kernel/cpu/resctrl/internal.h b/arch/x86/kernel/cpu/resctrl/internal.h
> index c99f26ebe7a6..4f9ef35626a7 100644
> --- a/arch/x86/kernel/cpu/resctrl/internal.h
> +++ b/arch/x86/kernel/cpu/resctrl/internal.h
> @@ -85,6 +85,10 @@ cpumask_any_housekeeping(const struct cpumask *mask, int exclude_cpu)
>  	if (cpu < nr_cpu_ids && !tick_nohz_full_cpu(cpu))
>  		return cpu;
>  
> +	/* Only continue if tick_nohz_full_mask has been initialized. */
> +	if (!tick_nohz_full_enabled())
> +		return cpu;
> +

I am curious why this below check didn't fail?

if (cpu < nr_cpu_ids && !tick_nohz_full_cpu(cpu))
  		return cpu;

The tick_nohz_full_cpu() already checks tick_nohz_full_enabled().

It should returned 'false' and  returned cpu already.

Did i miss something?

-- 
Thanks
Babu Moger