linux-kernel - Re: [PATCH] KVM: x86: Do not create mmu_rmaps

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Message-ID: <YdQYK4oRDU2Dmfwe@xz-m1.local>
Date:   Tue, 4 Jan 2022 17:49:31 +0800
From:   Peter Xu <peterx@...hat.com>
To:     Nikunj A Dadhania <nikunj@....com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, vasant.hegde@....com,
        brijesh.singh@....com
Subject: Re: [PATCH] KVM: x86: Do not create mmu_rmaps_stat for TDP MMU

Hi, Nikunj,

On Tue, Jan 04, 2022 at 02:58:14PM +0530, Nikunj A Dadhania wrote:
> With TDP MMU being the default now, access to mmu_rmaps_stat debugfs
> file causes following oops:
> 
> BUG: kernel NULL pointer dereference, address: 0000000000000000
> PGD 0 P4D 0
> Oops: 0000 [#1] PREEMPT SMP NOPTI
> CPU: 7 PID: 3185 Comm: cat Not tainted 5.16.0-rc4+ #204
> RIP: 0010:pte_list_count+0x6/0x40
>  Call Trace:
>   <TASK>
>   ? kvm_mmu_rmaps_stat_show+0x15e/0x320
>   seq_read_iter+0x126/0x4b0
>   ? aa_file_perm+0x124/0x490
>   seq_read+0xf5/0x140
>   full_proxy_read+0x5c/0x80
>   vfs_read+0x9f/0x1a0
>   ksys_read+0x67/0xe0
>   __x64_sys_read+0x19/0x20
>   do_syscall_64+0x3b/0xc0
>   entry_SYSCALL_64_after_hwframe+0x44/0xae
>  RIP: 0033:0x7fca6fc13912
> 
> Create mmu_rmaps_stat debugfs file only when rmaps are created.
> 
> Reported-by: Vasant Hegde <vasant.hegde@....com>
> Tested-by: Vasant Hegde <vasant.hegde@....com>
> Signed-off-by: Nikunj A Dadhania <nikunj@....com>

Shall we put the check into kvm_mmu_rmaps_stat_show()?  As iiuc the rmap can be
allocated dynamically (shadow_root_allocated changing from 0->1).

Thanks,

-- 
Peter Xu