Re: [PATCH v5 2/4] KVM: stats: Add fd-based API to read binary stats data

On Mon, May 17, 2021 at 02:53:12PM +0000, Jing Zhang wrote:

Provides a file descriptor per VM to read VM stats info/data. Provides a file descriptor per vCPU to read vCPU stats info/data.

Signed-off-by: Jing Zhang jingzhangos@google.com

arch/arm64/kvm/guest.c | 26 +++++ arch/mips/kvm/mips.c | 52 +++++++++ arch/powerpc/kvm/book3s.c | 52 +++++++++ arch/powerpc/kvm/booke.c | 45 ++++++++ arch/s390/kvm/kvm-s390.c | 117 ++++++++++++++++++++ arch/x86/kvm/x86.c | 53 +++++++++ include/linux/kvm_host.h | 127 ++++++++++++++++++++++ include/uapi/linux/kvm.h | 50 +++++++++ virt/kvm/kvm_main.c | 223 ++++++++++++++++++++++++++++++++++++++ 9 files changed, 745 insertions(+)

+static ssize_t kvm_vcpu_stats_read(struct file *file, char __user *user_buffer,

• 	      size_t size, loff_t *offset)
  

+{

• char id[KVM_STATS_ID_MAXLEN];
• struct kvm_vcpu *vcpu = file->private_data;
• ssize_t copylen, len, remain = size;
• size_t size_header, size_desc, size_stats;
• loff_t pos = *offset;
• char __user *dest = user_buffer;
• void *src;

Nit. Better to do pointer arithmetic on a "char *". Note that gcc and clang will do the expected thing.

• snprintf(id, sizeof(id), "kvm-%d/vcpu-%d",

• 	task_pid_nr(current), vcpu->vcpu_id);
  

• size_header = sizeof(kvm_vcpu_stats_header);
• size_desc =

• kvm_vcpu_stats_header.count * sizeof(struct _kvm_stats_desc);
  

• size_stats = sizeof(vcpu->stat);
• len = sizeof(id) + size_header + size_desc + size_stats - pos;
• len = min(len, remain);
• if (len <= 0)

• return 0;
  

• remain = len;

If 'desc_offset' is not right after the header, then the 'len' calculation is missing the gap into account. For example, assuming there is a gap of 0x1000000 between the header and the descriptors:

desc_offset = sizeof(id) + size_header + 0x1000000

and the user calls the ioctl with enough space for the whole file, including the gap:

*offset = 0 size = sizeof(id) + size_header + size_desc + size_stats + 0x1000000

then 'remain' gets the wrong size:

remain = sizeof(id) + size_header + size_desc + size_stats

and ... (more below)

• /* Copy kvm vcpu stats header id string */
• copylen = sizeof(id) - pos;
• copylen = min(copylen, remain);
• if (copylen > 0) {

• src = (void *)id + pos;
  

• if (copy_to_user(dest, src, copylen))
  

• 	return -EFAULT;
  

• remain -= copylen;
  

• pos += copylen;
  

• dest += copylen;
  

• }
• /* Copy kvm vcpu stats header */
• copylen = sizeof(id) + size_header - pos;
• copylen = min(copylen, remain);
• if (copylen > 0) {

• src = (void *)&kvm_vcpu_stats_header;
  

• src += pos - sizeof(id);
  

• if (copy_to_user(dest, src, copylen))
  

• 	return -EFAULT;
  

• remain -= copylen;
  

• pos += copylen;
  

• dest += copylen;
  

• }
• /* Copy kvm vcpu stats descriptors */
• copylen = kvm_vcpu_stats_header.desc_offset + size_desc - pos;

This would be the state at this point:

pos = sizeof(id) + size_header copylen = sizeof(id) + size_header + 0x1000000 + size_desc - (sizeof(id) + size_header) = 0x1000000 + size_desc remain = size_desc + size_stats

• copylen = min(copylen, remain);

copylen = size_desc + size_stats

which is not enough to copy the descriptors (and the data).

• if (copylen > 0) {

• src = (void *)&kvm_vcpu_stats_desc;
  

• src += pos - kvm_vcpu_stats_header.desc_offset;
  

Moreover, src also needs to take the gap into account.

src = &kvm_vcpu_stats_desc + (sizeof(id) + size_header) - (sizeof(id) + size_header + 0x1000000) = &kvm_vcpu_stats_desc - 0x1000000

Otherwise, src ends up pointing at the wrong place.

• if (copy_to_user(dest, src, copylen))
  

• 	return -EFAULT;
  

• remain -= copylen;
  

• pos += copylen;
  

• dest += copylen;
  

• }
• /* Copy kvm vcpu stats values */
• copylen = kvm_vcpu_stats_header.data_offset + size_stats - pos;

The same problem occurs here. There is a potential gap before data_offset that needs to be taken into account for src and len.

Would it be possible to just ensure that there is no gap? maybe even remove data_offset and desc_offset and always place them adjacent, and have the descriptors right after the header.

• copylen = min(copylen, remain);
• if (copylen > 0) {

• src = (void *)&vcpu->stat;
  

• src += pos - kvm_vcpu_stats_header.data_offset;
  

• if (copy_to_user(dest, src, copylen))
  

• 	return -EFAULT;
  

• remain -= copylen;
  

• pos += copylen;
  

• dest += copylen;
  

• }
• *offset = pos;
• return len;

+}

+static ssize_t kvm_vm_stats_read(struct file *file, char __user *user_buffer,

• 	      size_t size, loff_t *offset)
  

+{

Consider moving the common code between kvm_vcpu_stats_read and this one into some function that takes pointers to header, desc, and data. Unless there is something vcpu or vm specific besides that.

• char id[KVM_STATS_ID_MAXLEN];
• struct kvm *kvm = file->private_data;
• ssize_t copylen, len, remain = size;
• size_t size_header, size_desc, size_stats;
• loff_t pos = *offset;
• char __user *dest = user_buffer;
• void *src;
• snprintf(id, sizeof(id), "kvm-%d", task_pid_nr(current));
• size_header = sizeof(kvm_vm_stats_header);
• size_desc = kvm_vm_stats_header.count * sizeof(struct _kvm_stats_desc);
• size_stats = sizeof(kvm->stat);
• len = sizeof(id) + size_header + size_desc + size_stats - pos;
• len = min(len, remain);
• if (len <= 0)

• return 0;
  

• remain = len;
• /* Copy kvm vm stats header id string */
• copylen = sizeof(id) - pos;
• copylen = min(copylen, remain);
• if (copylen > 0) {

• src = (void *)id + pos;
  

• if (copy_to_user(dest, src, copylen))
  

• 	return -EFAULT;
  

• remain -= copylen;
  

• pos += copylen;
  

• dest += copylen;
  

• }
• /* Copy kvm vm stats header */
• copylen = sizeof(id) + size_header - pos;
• copylen = min(copylen, remain);
• if (copylen > 0) {

• src = (void *)&kvm_vm_stats_header;
  

• src += pos - sizeof(id);
  

• if (copy_to_user(dest, src, copylen))
  

• 	return -EFAULT;
  

• remain -= copylen;
  

• pos += copylen;
  

• dest += copylen;
  

• }
• /* Copy kvm vm stats descriptors */
• copylen = kvm_vm_stats_header.desc_offset + size_desc - pos;
• copylen = min(copylen, remain);
• if (copylen > 0) {

• src = (void *)&kvm_vm_stats_desc;
  

• src += pos - kvm_vm_stats_header.desc_offset;
  

• if (copy_to_user(dest, src, copylen))
  

• 	return -EFAULT;
  

• remain -= copylen;
  

• pos += copylen;
  

• dest += copylen;
  

• }
• /* Copy kvm vm stats values */
• copylen = kvm_vm_stats_header.data_offset + size_stats - pos;
• copylen = min(copylen, remain);
• if (copylen > 0) {

• src = (void *)&kvm->stat;
  

• src += pos - kvm_vm_stats_header.data_offset;
  

• if (copy_to_user(dest, src, copylen))
  

• 	return -EFAULT;
  

• remain -= copylen;
  

• pos += copylen;
  

• dest += copylen;
  

• }
• *offset = pos;
• return len;

+}

-- 2.31.1.751.gd2f1c929bd-goog

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