Re: [PATCH v2] x86/mtrr: allocate the cache map before taking mtrr_mutex

["Jürgen Groß" <jgross@suse.com>]

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On 05.10.26 15:02, Yogesh Gaur wrote:
> mtrr_copy_map() does a GFP_KERNEL allocation with mtrr_mutex held.
> It runs once at boot from mtrr_init_finalize(), but lockdep keeps
> the mtrr_mutex -> fs_reclaim dependency it records there for the
> life of the system. Once another path gives lockdep the rest of a
> cycle back to a lock held around mtrr_mutex, the next MTRR ioctl
> reports a circular dependency. syzbot found this through nbd, which
> takes cpu_hotplug_lock (via sk_set_memalloc() -> static_key_slow_inc())
> under its tx_lock, while mtrr_del_page() takes mtrr_mutex under
> cpu_hotplug_lock:
> 
>    WARNING: possible circular locking dependency detected
>    syz.9.5848/21744 is trying to acquire lock:
>     (mtrr_mutex), at: mtrr_del_page arch/x86/kernel/cpu/mtrr/mtrr.c:408
>    but task is already holding lock:
>     (cpu_hotplug_lock), at: mtrr_del_page arch/x86/kernel/cpu/mtrr/mtrr.c:407
>    -> #1 (fs_reclaim):
>           fs_reclaim_acquire
>           might_alloc
>           slab_pre_alloc_hook
>           __kmalloc_noprof
>           mtrr_copy_map arch/x86/kernel/cpu/mtrr/generic.c:413
>           mtrr_init_finalize arch/x86/kernel/cpu/mtrr/mtrr.c:618
>    Chain exists of:
>      mtrr_mutex --> &nsock->tx_lock --> cpu_hotplug_lock
> 
> The allocation does not need the mutex; only publishing the new map
> does. Allocate first and assign cache_map once mtrr_mutex is held.
> The rest of the function is unchanged.
> 
> Fixes: 061b984aab58 ("x86/mtrr: Construct a memory map with cache modes")
> Reported-by: syzbot+342762971f666337474e@syzkaller.appspotmail.com
> Assisted-by: LLM
> Signed-off-by: Yogesh Gaur <yogeshgaur.83@gmail.com>

Reviewed-by: Juergen Gross <jgross@suse.com>


Juergen

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