linux-kernel - Re: [PATCH 16/19] kvm: x86: Introduce KVM_{G|S}ET

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Message-ID: <874k7cvfsz.ffs@tglx>
Date:   Mon, 13 Dec 2021 13:40:28 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Yang Zhong <yang.zhong@...el.com>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com
Cc:     seanjc@...gle.com, jun.nakajima@...el.com, kevin.tian@...el.com,
        jing2.liu@...ux.intel.com, jing2.liu@...el.com
Subject: Re: [PATCH 16/19] kvm: x86: Introduce KVM_{G|S}ET_XSAVE2 ioctl

On Mon, Dec 13 2021 at 11:43, Paolo Bonzini wrote:
> On 12/13/21 11:10, Thomas Gleixner wrote:
>> On Fri, Dec 10 2021 at 17:30, Paolo Bonzini wrote:
>>> I think fpu_copy_uabi_to_guest_fpstate (and therefore
>>> copy_uabi_from_kernel_to_xstate) needs to check that the size is
>>> compatible with the components in the input.
>> 
>> fpu_copy_uabi_to_guest_fpstate() expects that the input buffer is
>> correctly sized. We surely can add a size check there.
>
> fpu_copy_guest_fpstate_to_uabi is more problematic because that one
> writes memory.  For fpu_copy_uabi_to_guest_fpstate, we know the input
> buffer size from the components and we can use it to do a properly-sized
> memdup_user.
>
> For fpu_copy_guest_fpstate_to_uabi we can just decide that KVM_GET_XSAVE
> will only save up to the first 4K.  Something like the following might
> actually be good for 5.16-rc; right now, header.xfeatures might lead
> userspace into reading uninitialized or unmapped memory:

If user space supplies a 4k buffer and reads beyond the end of the
buffer then it's hardly a kernel problem.

That function allows to provide a short buffer and fill it up to the
point where the buffer ends with the real information.

Thanks,

        tglx