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    SubjectRe: [PATCH 8/9] x86: kvm_hv_set_msr(): use __put_user() instead of 32bit __clear_user()
    On Sat, May 30, 2020 at 12:20:54PM -0700, Linus Torvalds wrote:
    > On Sat, May 30, 2020 at 12:14 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
    > >
    > > > And none of that code verifies that the end result is a user address.
    > >
    > > kvm_is_error_hva() is
    > > return addr >= PAGE_OFFSET;
    >
    > Ahh, that's what I missed. It won't work on other architectures, but
    > within x86 it's fine.

    FWIW, we use virt/kvm on x86, powerpc, mips, s390 and arm64.

    For x86 and powerpc the check is, AFAICS, OK (ppc kernel might start
    higher than PAGE_OFFSET, but not lower than it). For arm64... not
    sure - I'm not familiar with the virtual address space layout we use
    there. mips does *NOT* get that protection at all - there kvm_is_error_hva()
    is IS_ERR_VALUE() (thus the "at least on non-mips" upthread). And
    for s390 it's also IS_ERR_VALUE(), but that's an separate can of worms -
    there access_ok() is constant true; if we ever hit any of that code in
    virt/kvm while under KERNEL_DS, we are well and truly fucked there.

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