Re: [PATCH] KVM: arm64: Filter out invalid core register IDs in KVM_GET_REG_LIST

[Dave Martin <Dave.Martin@arm.com>]

On Tue, Jun 04, 2019 at 11:23:01AM +0200, Andrew Jones wrote:
> On Mon, Jun 03, 2019 at 05:52:07PM +0100, Dave Martin wrote:
> > Since commit d26c25a9d19b ("arm64: KVM: Tighten guest core register
> > access from userspace"), KVM_{GET,SET}_ONE_REG rejects register IDs
> > that do not correspond to a single underlying architectural register.
> > 
> > KVM_GET_REG_LIST was not changed to match however: instead, it
> > simply yields a list of 32-bit register IDs that together cover the
> > whole kvm_regs struct.  This means that if userspace tries to use
> > the resulting list of IDs directly to drive calls to KVM_*_ONE_REG,
> > some of those calls will now fail.
> > 
> > This was not the intention.  Instead, iterating KVM_*_ONE_REG over
> > the list of IDs returned by KVM_GET_REG_LIST should be guaranteed
> > to work.
> > 
> > This patch fixes the problem by splitting validate_core_offset()
> > into a backend core_reg_size_from_offset() which does all of the
> > work except for checking that the size field in the register ID
> > matches, and kvm_arm_copy_reg_indices() and num_core_regs() are
> > converted to use this to enumerate the valid offsets.
> > 
> > kvm_arm_copy_reg_indices() now also sets the register ID size field
> > appropriately based on the value returned, so the register ID
> > supplied to userspace is fully qualified for use with the register
> > access ioctls.
> 
> Ah yes, I've seen this issue, but hadn't gotten around to fixing it.
> 
> > 
> > Cc: stable@vger.kernel.org
> > Fixes: d26c25a9d19b ("arm64: KVM: Tighten guest core register access from userspace")
> > Signed-off-by: Dave Martin <Dave.Martin@arm.com>
> > 
> > ---
> > 
> > Changes since v3:
> 
> Hmm, I didn't see a v1-v3.

Looks like I didn't mark v3 as such when posting [1], but this has been
knocking around for a while.  It was rather low-priority and I hadn't
got around to testing it previously...


[1] [PATCH] KVM: arm64: Filter out invalid core register IDs in KVM_GET_REG_LIST
https://lists.cs.columbia.edu/pipermail/kvmarm/2019-April/035417.html

> > 
> >  * Rebased onto v5.2-rc1.
> > 
> >  * Tested with qemu by migrating from one qemu instance to another on
> >    ThunderX2.
> 
> One of the reasons I was slow to fix this is because QEMU doesn't care
> about the core registers when it uses KVM_GET_REG_LIST. It just completely
> skips all core reg indices, so it never finds out that they're invalid.
> And kvmtool doesn't use KVM_GET_REG_LIST at all. But it's certainly good
> to fix this.

[...]

> Reviewed-by: Andrew Jones <drjones@redhat.com>
> 
> I've also tested this using a kvm selftests test I wrote. I haven't posted
> that test yet because it needs some cleanup and I planned on getting back
> to that when getting back to fixing this issue. Anyway, before this patch
> every other 64-bit core reg index is invalid (because its indexing 32-bits
> but claiming a size of 64), all fp regs are invalid, and we were even
> providing a couple indices that mapped to struct padding. After this patch
> everything is right with the world.
> 
> Tested-by: Andrew Jones <drjones@redhat.com>

Thanks
---Dave
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