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>

1 files changed, 34 insertions, 26 deletions

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 1eefebe5d727..224a7e78cb6c 100644
--- 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;