On Tue, Oct 27, 2026 at 3:12 PM Sean Christopherson <[email protected]> wrote:
>
> Add a check on whether or not doing a user access is ok in KVM's APIs for
> accessing guest memory, *before* doing memslot lookup/validation, so that
> buggy KVM accesses to guest memory more likely to fail noisily. Because
"to guest memory [are] more likely"
> KVM installs dummy/empty memslots early in VM destruction, bugs in the VM
> teardown path will fail on the memslot checks without getting to the WARNs
> buried in kvm_copy_{to,from}_user() and friends.
>
> Signed-off-by: Sean Christopherson <[email protected]>
One somewhat minor comment below. Feel free to add:
Reviewed-by: James Houghton <[email protected]>
> ---
> virt/kvm/kvm_main.c | 15 ++++++++++-----
> 1 file changed, 10 insertions(+), 5 deletions(-)
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 1058b864d427..d8b914b76f90 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -3298,6 +3298,11 @@ static int next_segment(unsigned long len, int offset)
> return len;
> }
>
> +static bool kvm_is_guest_access_ok(struct kvm *kvm, int offset, int len, int
> size)
Hrm... I think you at least want `len` and `size` to be `unsigned
long`, and you probably want `offset` to be `unsigned int` or
`unsigned long`. The *_cached() functions are the interesting ones in
this case; I've left a note below at the first callsite.
> +{
> + return kvm_can_do_uaccess(kvm) && !WARN_ON_ONCE(offset + len > size);
> +}
> +
> /* Copy @len bytes from guest memory at '(@gfn * PAGE_SIZE) + @offset' to
> @data */
> static int __kvm_read_guest_page(struct kvm *kvm, struct kvm_memory_slot
> *slot,
> gfn_t gfn, void *data, int offset, int len)
> @@ -3305,7 +3310,7 @@ static int __kvm_read_guest_page(struct kvm *kvm,
> struct kvm_memory_slot *slot,
> int r;
> unsigned long addr;
>
> - if (WARN_ON_ONCE(offset + len > PAGE_SIZE))
> + if (!kvm_is_guest_access_ok(kvm, offset, len, PAGE_SIZE))
> return -EFAULT;
>
> addr = gfn_to_hva_memslot_prot(slot, gfn, NULL);
> @@ -3381,7 +3386,7 @@ static int __kvm_read_guest_atomic(struct kvm *kvm,
> struct kvm_memory_slot *slot
> int r;
> unsigned long addr;
>
> - if (WARN_ON_ONCE(offset + len > PAGE_SIZE))
> + if (!kvm_is_guest_access_ok(kvm, offset, len, PAGE_SIZE))
> return -EFAULT;
>
> addr = gfn_to_hva_memslot_prot(slot, gfn, NULL);
> @@ -3414,7 +3419,7 @@ static int __kvm_write_guest_page(struct kvm *kvm,
> int r;
> unsigned long addr;
>
> - if (WARN_ON_ONCE(offset + len > PAGE_SIZE))
> + if (!kvm_is_guest_access_ok(kvm, offset, len, PAGE_SIZE))
> return -EFAULT;
>
> addr = gfn_to_hva_memslot(memslot, gfn);
> @@ -3544,7 +3549,7 @@ int kvm_write_guest_offset_cached(struct kvm *kvm,
> struct gfn_to_hva_cache *ghc,
> int r;
> gpa_t gpa = ghc->gpa + offset;
>
> - if (WARN_ON_ONCE(len + offset > ghc->len))
> + if (!kvm_is_guest_access_ok(kvm, offset, len, ghc->len))
Currently, this truncates `len` and `ghc->len` from `unsigned long` to
`int`, and it will change `offset` from `unsigned int` to `int`.
> return -EINVAL;
>
> if (slots->generation != ghc->generation) {
> @@ -3582,7 +3587,7 @@ int kvm_read_guest_offset_cached(struct kvm *kvm,
> struct gfn_to_hva_cache *ghc,
> int r;
> gpa_t gpa = ghc->gpa + offset;
>
> - if (WARN_ON_ONCE(len + offset > ghc->len))
> + if (!kvm_is_guest_access_ok(kvm, offset, len, ghc->len))
> return -EINVAL;
>
> if (slots->generation != ghc->generation) {
> --
> 2.56.0.307.g5fe7bd680c-goog
>
