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SubjectRe: [PATCH] KVM: Use kvm_pfn_t for local PFN variable in hva_to_pfn_remapped()
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On 08/02/21 21:19, Sean Christopherson wrote:
> Use kvm_pfn_t, a.k.a. u64, for the local 'pfn' variable when retrieving
> a so called "remapped" hva/pfn pair. In theory, the hva could resolve to
> a pfn in high memory on a 32-bit kernel.
>
> This bug was inadvertantly exposed by commit bd2fae8da794 ("KVM: do not
> assume PTE is writable after follow_pfn"), which added an error PFN value
> to the mix, causing gcc to comlain about overflowing the unsigned long.
>
> arch/x86/kvm/../../../virt/kvm/kvm_main.c: In function ‘hva_to_pfn_remapped’:
> include/linux/kvm_host.h:89:30: error: conversion from ‘long long unsigned int’
> to ‘long unsigned int’ changes value from
> ‘9218868437227405314’ to ‘2’ [-Werror=overflow]
> 89 | #define KVM_PFN_ERR_RO_FAULT (KVM_PFN_ERR_MASK + 2)
> | ^
> virt/kvm/kvm_main.c:1935:9: note: in expansion of macro ‘KVM_PFN_ERR_RO_FAULT’
>
> Cc: stable@vger.kernel.org
> Fixes: add6a0cd1c5b ("KVM: MMU: try to fix up page faults before giving up")
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>
> I don't actually know that it's possible for a remapped pfn to be a 64-bit
> value on a stock kernel. But, backporting a one-liner is far easier and
> safer than trying to audit all possible flows. :-)
>
> virt/kvm/kvm_main.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index ee4ac2618ec5..001b9de4e727 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1906,7 +1906,7 @@ static int hva_to_pfn_remapped(struct vm_area_struct *vma,
> bool write_fault, bool *writable,
> kvm_pfn_t *p_pfn)
> {
> - unsigned long pfn;
> + kvm_pfn_t pfn;
> pte_t *ptep;
> spinlock_t *ptl;
> int r;
>

Queued, thanks.

Paolo

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