KVM: x86: fix vcpu->mmio_fragments overflow

After commit b3356bf0dbb349 (KVM: emulator: optimize "rep ins" handling),
the pieces of io data can be collected and write them to the guest memory
or MMIO together

Unfortunately, kvm splits the mmio access into 8 bytes and store them to
vcpu->mmio_fragments. If the guest uses "rep ins" to move large data, it
will cause vcpu->mmio_fragments overflow

The bug can be exposed by isapc (-M isapc):

[23154.818733] general protection fault: 0000 [#1] SMP DEBUG_PAGEALLOC
[ ......]
[23154.858083] Call Trace:
[23154.859874]  [<ffffffffa04f0e17>] kvm_get_cr8+0x1d/0x28 [kvm]
[23154.861677]  [<ffffffffa04fa6d4>] kvm_arch_vcpu_ioctl_run+0xcda/0xe45 [kvm]
[23154.863604]  [<ffffffffa04f5a1a>] ? kvm_arch_vcpu_load+0x17b/0x180 [kvm]

Actually, we can use one mmio_fragment to store a large mmio access then
split it when we pass the mmio-exit-info to userspace. After that, we only
need two entries to store mmio info for the cross-mmio pages access

Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 1eefebe5d72758873df0d13e4ae8c686a7d016ee..224a7e78cb6c40330dfebc943e315c0a7231efa1 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3779,7 +3779,7 @@ static int write_exit_mmio(struct kvm_vcpu *vcpu, gpa_t gpa,
 {
        struct kvm_mmio_fragment *frag = &vcpu->mmio_fragments[0];
 
-       memcpy(vcpu->run->mmio.data, frag->data, frag->len);
+       memcpy(vcpu->run->mmio.data, frag->data, min(8u, frag->len));
        return X86EMUL_CONTINUE;
 }
 
@@ -3832,18 +3832,11 @@ mmio:
        bytes -= handled;
        val += handled;
 
-       while (bytes) {
-               unsigned now = min(bytes, 8U);
-
-               frag = &vcpu->mmio_fragments[vcpu->mmio_nr_fragments++];
-               frag->gpa = gpa;
-               frag->data = val;
-               frag->len = now;
-
-               gpa += now;
-               val += now;
-               bytes -= now;
-       }
+       WARN_ON(vcpu->mmio_nr_fragments >= KVM_MAX_MMIO_FRAGMENTS);
+       frag = &vcpu->mmio_fragments[vcpu->mmio_nr_fragments++];
+       frag->gpa = gpa;
+       frag->data = val;
+       frag->len = bytes;
        return X86EMUL_CONTINUE;
 }
 
@@ -3890,7 +3883,7 @@ int emulator_read_write(struct x86_emulate_ctxt *ctxt, unsigned long addr,
        vcpu->mmio_needed = 1;
        vcpu->mmio_cur_fragment = 0;
 
-       vcpu->run->mmio.len = vcpu->mmio_fragments[0].len;
+       vcpu->run->mmio.len = min(8u, vcpu->mmio_fragments[0].len);
        vcpu->run->mmio.is_write = vcpu->mmio_is_write = ops->write;
        vcpu->run->exit_reason = KVM_EXIT_MMIO;
        vcpu->run->mmio.phys_addr = gpa;
@@ -5522,28 +5515,44 @@ static int complete_emulated_pio(struct kvm_vcpu *vcpu)
  *
  * read:
  *   for each fragment
- *     write gpa, len
- *     exit
- *     copy data
+ *     for each mmio piece in the fragment
+ *       write gpa, len
+ *       exit
+ *       copy data
  *   execute insn
  *
  * write:
  *   for each fragment
- *      write gpa, len
- *      copy data
- *      exit
+ *     for each mmio piece in the fragment
+ *       write gpa, len
+ *       copy data
+ *       exit
  */
 static int complete_emulated_mmio(struct kvm_vcpu *vcpu)
 {
        struct kvm_run *run = vcpu->run;
        struct kvm_mmio_fragment *frag;
+       unsigned len;
 
        BUG_ON(!vcpu->mmio_needed);
 
        /* Complete previous fragment */
-       frag = &vcpu->mmio_fragments[vcpu->mmio_cur_fragment++];
+       frag = &vcpu->mmio_fragments[vcpu->mmio_cur_fragment];
+       len = min(8u, frag->len);
        if (!vcpu->mmio_is_write)
-               memcpy(frag->data, run->mmio.data, frag->len);
+               memcpy(frag->data, run->mmio.data, len);
+
+       if (frag->len <= 8) {
+               /* Switch to the next fragment. */
+               frag++;
+               vcpu->mmio_cur_fragment++;
+       } else {
+               /* Go forward to the next mmio piece. */
+               frag->data += len;
+               frag->gpa += len;
+               frag->len -= len;
+       }
+
        if (vcpu->mmio_cur_fragment == vcpu->mmio_nr_fragments) {
                vcpu->mmio_needed = 0;
                if (vcpu->mmio_is_write)
@@ -5551,13 +5560,12 @@ static int complete_emulated_mmio(struct kvm_vcpu *vcpu)
                vcpu->mmio_read_completed = 1;
                return complete_emulated_io(vcpu);
        }
-       /* Initiate next fragment */
-       ++frag;
+
        run->exit_reason = KVM_EXIT_MMIO;
        run->mmio.phys_addr = frag->gpa;
        if (vcpu->mmio_is_write)
-               memcpy(run->mmio.data, frag->data, frag->len);
-       run->mmio.len = frag->len;
+               memcpy(run->mmio.data, frag->data, min(8u, frag->len));
+       run->mmio.len = min(8u, frag->len);
        run->mmio.is_write = vcpu->mmio_is_write;
        vcpu->arch.complete_userspace_io = complete_emulated_mmio;
        return 0;

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 93bfc9f9815c7fa178ad70b555e7a2afb86b3f02..ecc554374e440a73df8aaf5b5787f86705e647b0 100644 (file)
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -42,19 +42,8 @@
  */
 #define KVM_MEMSLOT_INVALID    (1UL << 16)
 
-/*
- * If we support unaligned MMIO, at most one fragment will be split into two:
- */
-#ifdef KVM_UNALIGNED_MMIO
-#  define KVM_EXTRA_MMIO_FRAGMENTS 1
-#else
-#  define KVM_EXTRA_MMIO_FRAGMENTS 0
-#endif
-
-#define KVM_USER_MMIO_SIZE 8
-
-#define KVM_MAX_MMIO_FRAGMENTS \
-       (KVM_MMIO_SIZE / KVM_USER_MMIO_SIZE + KVM_EXTRA_MMIO_FRAGMENTS)
+/* Two fragments for cross MMIO pages. */
+#define KVM_MAX_MMIO_FRAGMENTS 2
 
 /*
  * For the normal pfn, the highest 12 bits should be zero,