Re: [PATCH V2] x86/resctrl: Fix uninitialized memory read when last CPU of domain goes offline

[Moger, Babu]

On 4/1/24 13:16, 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().
> 
> Replace the IS_ENABLED(CONFIG_NO_HZ_FULL) with tick_nohz_full_enabled().
> The latter ensures tick_nohz_full_mask can be accessed safely and can be
> used whether kernel is built with CONFIG_NO_HZ_FULL enabled or not.
> 
> Fixes: a4846aaf3945 ("x86/resctrl: Add cpumask_any_housekeeping() for limbo/overflow")
> Reported-by: Tony Luck <tony.luck@xxxxxxxxx>
> Closes: https://lore.kernel.org/lkml/ZgIFT5gZgIQ9A9G7@agluck-desk3/
> Suggested-by: Ingo Molnar <mingo@xxxxxxxxxx>
> Signed-off-by: Reinette Chatre <reinette.chatre@xxxxxxxxx>

Reviewed-by: Babu Moger <babu.moger@xxxxxxx>


-- 
Thanks
Babu Moger