Commit 87da7e66 authored by Xiao Guangrong's avatar Xiao Guangrong Committed by Marcelo Tosatti
Browse files

KVM: x86: fix vcpu->mmio_fragments overflow 



After commit b3356bf0 (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: default avatarXiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Signed-off-by: default avatarMarcelo Tosatti <mtosatti@redhat.com>
parent 35fd3dc5

arch/x86/kvm/x86.c

+34 −26
Original line number Diff line number Diff line
@@ -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);



	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 = now;



		gpa += now;

		val += now;

		bytes -= now;

	}

	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;
@@ -5522,6 +5515,7 @@ static int complete_emulated_pio(struct kvm_vcpu *vcpu) 
 *

 * read:

 *   for each fragment

 *     for each mmio piece in the fragment

 *       write gpa, len

 *       exit

 *       copy data
@@ -5529,6 +5523,7 @@ static int complete_emulated_pio(struct kvm_vcpu *vcpu) 
 *

 * write:

 *   for each fragment

 *     for each mmio piece in the fragment

 *       write gpa, len

 *       copy data

 *       exit
@@ -5537,13 +5532,27 @@ 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;

include/linux/kvm_host.h

+2 −13
Original line number Diff line number Diff line
@@ -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,

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