linux-kernel - Re: [PATCH] KVM: x86/vmx: Do not skip segment attributes if unusable bit is set

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Date:   Sun, 22 Jan 2023 10:15:13 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Hendrik Borghorst <hborghor@...zon.de>
Cc:     graf@...zon.de, Sean Christopherson <seanjc@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86/vmx: Do not skip segment attributes if unusable
 bit is set

On 11/14/22 17:48, Hendrik Borghorst wrote:
> When serializing and deserializing kvm_sregs, attributes of the segment
> descriptors are stored by user space. For unusable segments,
> vmx_segment_access_rights skips all attributes and sets them to 0.
> 
> This means we zero out the DPL (Descriptor Privilege Level) for unusable
> entries.
> 
> Unusable segments are - contrary to their name - usable in 64bit mode and
> are used by guests to for example create a linear map through the
> NULL selector.
> 
> VMENTER checks if SS.DPL is correct depending on the CS segment type.
> For types 9 (Execute Only) and 11 (Execute Read), CS.DPL must be equal to
> SS.DPL [1].
> 
> We have seen real world guests setting CS to a usable segment with DPL=3
> and SS to an unusable segment with DPL=3. Once we go through an sregs
> get/set cycle, SS.DPL turns to 0. This causes the virtual machine to crash
> reproducibly.
> 
> This commit changes the attribute logic to always preserve attributes for
> unusable segments. According to [2] SS.DPL is always saved on VM exits,
> regardless of the unusable bit so user space applications should have saved
> the information on serialization correctly.
> 
> [3] specifies that besides SS.DPL the rest of the attributes of the
> descriptors are undefined after VM entry if unusable bit is set. So, there
> should be no harm in setting them all to the previous state.
> 
> [1] Intel SDM Vol 3C 26.3.1.2 Checks on Guest Segment Registers
> [2] Intel SDM Vol 3C 27.3.2 Saving Segment Registers and Descriptor-Table
> Registers
> [3] Intel SDM Vol 3C 26.3.2.2 Loading Guest Segment Registers and
> Descriptor-Table Registers
> 
> Cc: Alexander Graf <graf@...zon.de>
> Signed-off-by: Hendrik Borghorst <hborghor@...zon.de>
> ---
>   arch/x86/kvm/vmx/vmx.c | 21 +++++++++------------
>   1 file changed, 9 insertions(+), 12 deletions(-)

Hi Hendrik,

thanks for the patch!  I have queued it now for 6.2-rc6, and added Cc: 
stable.

Would you mind providing a test in the form of a patch for 
tools/testing/selftests/kvm?  I think a kvm-unit-tests testcase would be 
harder to do because there's no easy way to force 
KVM_GET_SREGS/KVM_SET_SREGS.

Thanks,

Paolo