linux-kernel - Re: [PATCH v2] KVM: x86: Check for rmaps allocation

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Date:   Fri, 7 Jan 2022 18:08:50 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Nikunj A Dadhania <nikunj@....com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Peter Xu <peterx@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, vasant.hegde@....com,
        brijesh.singh@....com
Subject: Re: [PATCH v2] KVM: x86: Check for rmaps allocation

On 1/5/22 05:03, 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
> 
> Return early when rmaps are not present.
> 
> Reported-by: Vasant Hegde <vasant.hegde@....com>
> Tested-by: Vasant Hegde <vasant.hegde@....com>
> Signed-off-by: Nikunj A Dadhania <nikunj@....com>
> ---
> 
> v1: https://lore.kernel.org/kvm/20220104092814.11553-1-nikunj@amd.com/T/#u
> 
> Check the rmaps inside kvm_mmu_rmaps_stat_show() as the rmaps can be
> allocated dynamically (Peter Xu)
> 
>   arch/x86/kvm/debugfs.c | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/arch/x86/kvm/debugfs.c b/arch/x86/kvm/debugfs.c
> index 543a8c04025c..9240b3b7f8dd 100644
> --- a/arch/x86/kvm/debugfs.c
> +++ b/arch/x86/kvm/debugfs.c
> @@ -95,6 +95,9 @@ static int kvm_mmu_rmaps_stat_show(struct seq_file *m, void *v)
>   	unsigned int *log[KVM_NR_PAGE_SIZES], *cur;
>   	int i, j, k, l, ret;
>   
> +	if (!kvm_memslots_have_rmaps(kvm))
> +		return 0;
> +
>   	ret = -ENOMEM;
>   	memset(log, 0, sizeof(log));
>   	for (i = 0; i < KVM_NR_PAGE_SIZES; i++) {

Queued, thanks.

Paolo