executor: move syz_mount_image's sanity checks to syz-fuzzer

It will simplify the C code and let us extract the raw images in a more
convenient way.

1 files changed, 176 insertions, 0 deletions

diff --git a/sys/linux/init_images.go b/sys/linux/init_images.go
new file mode 100644
index 000000000..e914b8b4d
--- /dev/null
+++ b/sys/linux/init_images.go
@@ -0,0 +1,176 @@
+// Copyright 2022 syzkaller project authors. All rights reserved.
+// Use of this source code is governed by Apache 2 LICENSE that can be found in the LICENSE file.
+
+package linux
+
+import (
+	"fmt"
+	"io"
+
+	"github.com/google/syzkaller/prog"
+)
+
+type zeroReader struct {
+	left int
+}
+
+func (zr *zeroReader) Read(p []byte) (n int, err error) {
+	if zr.left == 0 {
+		return 0, io.EOF
+	}
+	toRead := zr.left
+	if toRead > len(p) {
+		toRead = len(p)
+	}
+	for i := 0; i < toRead; i++ {
+		p[i] = 0
+	}
+	return toRead, nil
+}
+
+func newZeroReader(size int) io.Reader {
+	return &zeroReader{left: size}
+}
+
+const imageMaxSize = 129 << 20
+
+func fixUpImageSegments(parsed *mountImageArgs) {
+	parsed.filterSegments(func(i int, segment *mountImageSegment) bool {
+		if segment.parseError != nil {
+			return false
+		}
+		return segment.offset.Val < imageMaxSize && segment.size.Val < imageMaxSize
+	})
+	newSize := parsed.size.Val
+	for _, segment := range parsed.segments {
+		actualSize := uint64(len(segment.data.Data()))
+		if segment.size.Val > actualSize {
+			segment.size.Val = actualSize
+		}
+		if segment.offset.Val+segment.size.Val > imageMaxSize {
+			segment.offset.Val = imageMaxSize - segment.size.Val
+		}
+		if segment.offset.Val+segment.size.Val > newSize {
+			newSize = segment.offset.Val + segment.size.Val
+		}
+	}
+	if newSize > imageMaxSize {
+		newSize = imageMaxSize
+	}
+	parsed.size.Val = newSize
+}
+
+func (arch *arch) fixUpSyzMountImage(c *prog.Call) {
+	// Previously we did such a sanitization right in the common_linux.h, but this was problematic
+	// for two reasons:
+	// 1) It further complicates the already complicated executor code.
+	// 2) We'd need to duplicate the logic in Go for raw image extraction.
+	// So now we do all the initialization in Go and let the C code only interpret the commands.
+	ret, err := parseSyzMountImage(c)
+	if err != nil {
+		deactivateSyzMountImage(c)
+		return
+	}
+	const maxImageSegments = 4096
+	ret.filterSegments(func(i int, _ *mountImageSegment) bool {
+		return i < maxImageSegments
+	})
+	fixUpImageSegments(ret)
+}
+
+type mountImageArgs struct {
+	size          *prog.ConstArg
+	segmentsCount *prog.ConstArg
+	segmentsGroup *prog.GroupArg
+	segments      []*mountImageSegment
+}
+
+func (m *mountImageArgs) filterSegments(filter func(int, *mountImageSegment) bool) {
+	newArgs := []prog.Arg{}
+	newSegments := []*mountImageSegment{}
+	for i, segment := range m.segments {
+		if filter(i, segment) {
+			newSegments = append(newSegments, segment)
+			newArgs = append(newArgs, m.segmentsGroup.Inner[i])
+		}
+	}
+	m.segments = newSegments
+	m.segmentsGroup.Inner = newArgs
+	m.segmentsCount.Val = uint64(len(newArgs))
+}
+
+type mountImageSegment struct {
+	data       *prog.DataArg
+	offset     *prog.ConstArg
+	size       *prog.ConstArg
+	parseError error
+}
+
+func parseImageSegment(segmentArg prog.Arg) *mountImageSegment {
+	ret := &mountImageSegment{}
+	segmentFields, ok := segmentArg.(*prog.GroupArg)
+	if segmentFields == nil || !ok {
+		return &mountImageSegment{parseError: fmt.Errorf("it is not a group")}
+	}
+	if len(segmentFields.Inner) != 3 {
+		return &mountImageSegment{parseError: fmt.Errorf("invalid number of nested fields")}
+	}
+	dataPtr, ok := segmentFields.Inner[0].(*prog.PointerArg)
+	if dataPtr == nil || dataPtr.Res == nil || !ok {
+		return &mountImageSegment{parseError: fmt.Errorf("invalid data field ptr")}
+	}
+	ret.data, ok = dataPtr.Res.(*prog.DataArg)
+	if ret.data == nil || !ok {
+		return &mountImageSegment{parseError: fmt.Errorf("invalid data arg")}
+	}
+	ret.size, ok = segmentFields.Inner[1].(*prog.ConstArg)
+	if ret.size == nil || !ok {
+		return &mountImageSegment{parseError: fmt.Errorf("invalid size arg")}
+	}
+	ret.offset, ok = segmentFields.Inner[2].(*prog.ConstArg)
+	if ret.offset == nil || !ok {
+		return &mountImageSegment{parseError: fmt.Errorf("invalid offset arg")}
+	}
+	return ret
+}
+
+func deactivateSyzMountImage(c *prog.Call) {
+	groupArg := c.Args[4]
+	newArg := groupArg.Type().DefaultArg(groupArg.Dir())
+	prog.RemoveArg(groupArg)
+	c.Args[4] = newArg
+	// Also set the segments count field to 0.
+	c.Args[3].(*prog.ConstArg).Val = 0
+}
+
+func parseSyzMountImage(c *prog.Call) (*mountImageArgs, error) {
+	if len(c.Args) < 5 {
+		panic("invalid number of arguments in syz_mount_image")
+	}
+	segmentsCountArg, ok := c.Args[3].(*prog.ConstArg)
+	if !ok {
+		panic("syz_mount_image's segment count was expected to be const")
+	}
+	sizeArg, ok := c.Args[2].(*prog.ConstArg)
+	if !ok {
+		panic("syz_mount_image's size arg is not const")
+	}
+	segmentsPtrArg, ok := c.Args[4].(*prog.PointerArg)
+	if !ok {
+		return nil, fmt.Errorf("invalid segments arg")
+	}
+	segmentsGroup, ok := segmentsPtrArg.Res.(*prog.GroupArg)
+	if segmentsGroup == nil || !ok {
+		return nil, fmt.Errorf("segments are not a group")
+	}
+	ret := &mountImageArgs{
+		segmentsCount: segmentsCountArg,
+		segmentsGroup: segmentsGroup,
+		size:          sizeArg,
+	}
+	for _, segmentArg := range segmentsGroup.Inner {
+		parsed := parseImageSegment(segmentArg)
+		ret.segments = append(ret.segments, parsed)
+	}
+	return ret, nil
+}