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KVM: Clean up coalesced MMIO ring full check

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Fold coalesced_mmio_has_room() into its sole caller, coalesced_mmio_write(),
as it's really just a single line of code, has a goofy return value, and
is unnecessarily brittle.

E.g. if coalesced_mmio_has_room() were to check ring->last directly, or
the caller failed to use READ_ONCE(), KVM would be susceptible to TOCTOU
attacks from userspace.

Opportunistically add a comment explaining why on earth KVM leaves one
entry free, which may not be obvious to readers that aren't familiar with
ring buffers.

No functional change intended.

Reviewed-by: Ilias Stamatis <ilstam@amazon.com>
Cc: Paul Durrant <paul@xen.org>
Link: https://lore.kernel.org/r/20240828181446.652474-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
This commit is contained in:
Sean Christopherson 2024-08-28 11:14:46 -07:00
parent 215b3cb7a8
commit e027ba1b83
1 changed files with 8 additions and 21 deletions
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29
virt/kvm/coalesced_mmio.c
View File

@ -40,25 +40,6 @@ static int coalesced_mmio_in_range(struct kvm_coalesced_mmio_dev *dev,
return 1;
}
static int coalesced_mmio_has_room(struct kvm_coalesced_mmio_dev *dev, u32 last)
{
struct kvm_coalesced_mmio_ring *ring;
/* Are we able to batch it ? */
/* last is the first free entry
* check if we don't meet the first used entry
* there is always one unused entry in the buffer
*/
ring = dev->kvm->coalesced_mmio_ring;
if ((last + 1) % KVM_COALESCED_MMIO_MAX == READ_ONCE(ring->first)) {
/* full */
return 0;
}
return 1;
}
static int coalesced_mmio_write(struct kvm_vcpu *vcpu,
struct kvm_io_device *this, gpa_t addr,
int len, const void *val)
@ -72,9 +53,15 @@ static int coalesced_mmio_write(struct kvm_vcpu *vcpu,
spin_lock(&dev->kvm->ring_lock);
/*
* last is the index of the entry to fill. Verify userspace hasn't
* set last to be out of range, and that there is room in the ring.
* Leave one entry free in the ring so that userspace can differentiate
* between an empty ring and a full ring.
*/
insert = READ_ONCE(ring->last);
if (!coalesced_mmio_has_room(dev, insert) ||
insert >= KVM_COALESCED_MMIO_MAX) {
if (insert >= KVM_COALESCED_MMIO_MAX ||
(insert + 1) % KVM_COALESCED_MMIO_MAX == READ_ONCE(ring->first)) {
spin_unlock(&dev->kvm->ring_lock);
return -EOPNOTSUPP;
}