Re: [PATCH v2 10/13] KVM: x86: Shrink the usercopy region of the emulation context

Vitaly Kuznetsov
2020-02-26T17:51:11Z

Sean Christopherson <sean.j.christopherson@intel.com> writes:

> Shuffle a few operand structs to the end of struct x86_emulate_ctxt and
> update the cache creation to whitelist only the region of the emulation
> context that is expected to be copied to/from user memory, e.g. the
> instruction operands, registers, and fetch/io/mem caches.
>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> ---
> arch/x86/kvm/kvm_emulate.h | 8 +++++---
> arch/x86/kvm/x86.c | 12 ++++++------
> 2 files changed, 11 insertions(+), 9 deletions(-)
>
> diff --git a/arch/x86/kvm/kvm_emulate.h b/arch/x86/kvm/kvm_emulate.h
> index 2f0a600efdff..82f712d5c692 100644
> --- a/arch/x86/kvm/kvm_emulate.h
> +++ b/arch/x86/kvm/kvm_emulate.h
> @@ -322,9 +322,6 @@ struct x86_emulate_ctxt {
> u8 intercept;
> u8 op_bytes;
> u8 ad_bytes;
> - struct operand src;
> - struct operand src2;
> - struct operand dst;
> int (*execute)(struct x86_emulate_ctxt *ctxt);
> int (*check_perm)(struct x86_emulate_ctxt *ctxt);
> /*
> @@ -349,6 +346,11 @@ struct x86_emulate_ctxt {
> u8 seg_override;
> u64 d;
> unsigned long _eip;
> +
> + /* Here begins the usercopy section. */
> + struct operand src;
> + struct operand src2;
> + struct operand dst;

Out of pure curiosity, how certain are we that this is going to be
enough for userspaces?

> struct operand memop;
> /* Fields above regs are cleared together. */
> unsigned long _regs[NR_VCPU_REGS];
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 370af9fe0f5b..e1eaca65756b 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -235,13 +235,13 @@ static struct kmem_cache *x86_emulator_cache;
>
> static struct kmem_cache *kvm_alloc_emulator_cache(void)
> {
> - return kmem_cache_create_usercopy("x86_emulator",
> - sizeof(struct x86_emulate_ctxt),
> + unsigned int useroffset = offsetof(struct x86_emulate_ctxt, src);
> + unsigned int size = sizeof(struct x86_emulate_ctxt);
> +
> + return kmem_cache_create_usercopy("x86_emulator", size,
> __alignof__(struct x86_emulate_ctxt),
> - SLAB_ACCOUNT,
> - 0,
> - sizeof(struct x86_emulate_ctxt),
> - NULL);
> + SLAB_ACCOUNT, useroffset,
> + size - useroffset, NULL);
> }
>
> static int emulator_fix_hypercall(struct x86_emulate_ctxt *ctxt);

--
Vitaly