linux-kernel - Re: [PATCH 1/2] KVM: SVM: fix svn_pin_memory()'s use of get_user_pages

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Date:   Tue, 26 May 2020 09:33:27 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     John Hubbard <jhubbard@...dia.com>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     Souptick Joarder <jrdr.linux@...il.com>,
        John Hubbard <jhubbard@...dia.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        "H . Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH 1/2] KVM: SVM: fix svn_pin_memory()'s use of get_user_pages_fast()

John Hubbard <jhubbard@...dia.com> writes:

> There are two problems in svn_pin_memory():
>
> 1) The return value of get_user_pages_fast() is stored in an
> unsigned long, although the declared return value is of type int.
> This will not cause any symptoms, but it is misleading.
> Fix this by changing the type of npinned to "int".
>
> 2) The number of pages passed into get_user_pages_fast() is stored
> in an unsigned long, even though get_user_pages_fast() accepts an
> int. This means that it is possible to silently overflow the number
> of pages.
>
> Fix this by adding a WARN_ON_ONCE() and an early error return. The
> npages variable is left as an unsigned long for convenience in
> checking for overflow.
>
> Fixes: 89c505809052 ("KVM: SVM: Add support for KVM_SEV_LAUNCH_UPDATE_DATA command")
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Sean Christopherson <sean.j.christopherson@...el.com>
> Cc: Vitaly Kuznetsov <vkuznets@...hat.com>
> Cc: Wanpeng Li <wanpengli@...cent.com>
> Cc: Jim Mattson <jmattson@...gle.com>
> Cc: Joerg Roedel <joro@...tes.org>
> Cc: H. Peter Anvin <hpa@...or.com>
> Cc: x86@...nel.org
> Cc: kvm@...r.kernel.org
> Signed-off-by: John Hubbard <jhubbard@...dia.com>
> ---
>  arch/x86/kvm/svm/sev.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 89f7f3aebd31..9693db1af57c 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -313,7 +313,8 @@ static struct page **sev_pin_memory(struct kvm *kvm, unsigned long uaddr,
>  				    int write)
>  {
>  	struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
> -	unsigned long npages, npinned, size;
> +	unsigned long npages, size;
> +	int npinned;
>  	unsigned long locked, lock_limit;
>  	struct page **pages;
>  	unsigned long first, last;
> @@ -333,6 +334,9 @@ static struct page **sev_pin_memory(struct kvm *kvm, unsigned long uaddr,
>  		return NULL;
>  	}
>  
> +	if (WARN_ON_ONCE(npages > INT_MAX))
> +		return NULL;
> +

I bit unrelated to this patch, but callers of sev_pin_memory() treat
NULL differently:

sev_launch_secret()/svm_register_enc_region() return -ENOMEM
sev_dbg_crypt() returns -EFAULT

Should we switch to ERR_PTR() to preserve the error?

>  	/* Avoid using vmalloc for smaller buffers. */
>  	size = npages * sizeof(struct page *);
>  	if (size > PAGE_SIZE)

Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>

-- 
Vitaly