On 11/17/2011 12:00 PM, Carsten Otte wrote:
> From: Christian Borntraeger <borntraeger@xxxxxxxxxx>
>
> There is a potential host deadlock in the tprot intercept handling.
> We must not hold the mmap semaphore while resolving the guest
> address. If userspace is remapping, then the memory detection in
> the guest is broken anyway so we can safely separate the
> address translation from walking the vmas.
>
> Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
> Signed-off-by: Carsten Otte <cotte@xxxxxxxxxx>
> ---
>
> arch/s390/kvm/priv.c | 10 ++++++++--
> 1 file changed, 8 insertions(+), 2 deletions(-)
>
> diff -urpN linux-2.6/arch/s390/kvm/priv.c linux-2.6-patched/arch/s390/kvm/priv.c
> --- linux-2.6/arch/s390/kvm/priv.c 2011-10-24 09:10:05.000000000 +0200
> +++ linux-2.6-patched/arch/s390/kvm/priv.c 2011-11-17 10:03:53.000000000 +0100
> @@ -336,6 +336,7 @@ static int handle_tprot(struct kvm_vcpu
> u64 address1 = disp1 + base1 ? vcpu->arch.guest_gprs[base1] : 0;
> u64 address2 = disp2 + base2 ? vcpu->arch.guest_gprs[base2] : 0;
> struct vm_area_struct *vma;
> + unsigned long user_address;
>
> vcpu->stat.instruction_tprot++;
>
> @@ -349,9 +350,14 @@ static int handle_tprot(struct kvm_vcpu
> return -EOPNOTSUPP;
>
>
> + /* we must resolve the address without holding the mmap semaphore.
> + * This is ok since the userspace hypervisor is not supposed to change
> + * the mapping while the guest queries the memory. Otherwise the guest
> + * might crash or get wrong info anyway. */
> + user_address = (unsigned long) __guestaddr_to_user(vcpu, address1);
> +
> down_read(¤t->mm->mmap_sem);
> - vma = find_vma(current->mm,
> - (unsigned long) __guestaddr_to_user(vcpu, address1));
> + vma = find_vma(current->mm, user_address);
> if (!vma) {
> up_read(¤t->mm->mmap_sem);
> return kvm_s390_inject_program_int(vcpu, PGM_ADDRESSING);
>
Unrelated to the patch, but I'm curious: it looks like __gmap_fault()
dereferences the guest page table? How can it assume that it is mapped?
I'm probably misreading the code.
A little closer to the patch, x86 handles the same issue by calling
get_user_pages_fast(). This should be more scalable than bouncing
mmap_sem, something to consider.
--
error compiling committee.c: too many arguments to function
--
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