[PATCH] KVM: x86: Hoist input checks in kvm_add_msr_filter()

Siddharth Chandrasekaran
2021-05-03T12:21:49Z

In ioctl KVM_X86_SET_MSR_FILTER, input from user space is validated
after a memdup_user(). For invalid inputs we'd memdup and then call
kfree unnecessarily. Hoist input validation to avoid kfree altogether.

Signed-off-by: Siddharth Chandrasekaran <sidcha@amazon.de>
---
arch/x86/kvm/x86.c | 21 ++++++---------------
1 file changed, 6 insertions(+), 15 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index ee0dc58ac3a5..15c20b31cc91 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5393,11 +5393,16 @@ static int kvm_add_msr_filter(struct kvm_x86_msr_filter *msr_filter,
struct msr_bitmap_range range;
unsigned long *bitmap = NULL;
size_t bitmap_size;
- int r;

if (!user_range->nmsrs)
return 0;

+ if (user_range->flags & ~(KVM_MSR_FILTER_READ | KVM_MSR_FILTER_WRITE))
+ return -EINVAL;
+
+ if (!user_range->flags)
+ return -EINVAL;
+
bitmap_size = BITS_TO_LONGS(user_range->nmsrs) * sizeof(long);
if (!bitmap_size || bitmap_size > KVM_MSR_FILTER_MAX_BITMAP_SIZE)
return -EINVAL;
@@ -5413,24 +5418,10 @@ static int kvm_add_msr_filter(struct kvm_x86_msr_filter *msr_filter,
.bitmap = bitmap,
};

- if (range.flags & ~(KVM_MSR_FILTER_READ | KVM_MSR_FILTER_WRITE)) {
- r = -EINVAL;
- goto err;
- }
-
- if (!range.flags) {
- r = -EINVAL;
- goto err;
- }
-
- /* Everything ok, add this range identifier. */
msr_filter->ranges[msr_filter->count] = range;
msr_filter->count++;

return 0;
-err:
- kfree(bitmap);
- return r;
}

static int kvm_vm_ioctl_set_msr_filter(struct kvm *kvm, void __user *argp)
--
2.17.1

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