registry/{storage,handlers}: limit content sizes

Under certain circumstances, the use of `StorageDriver.GetContent` can
result in unbounded memory allocations. In particualr, this happens when
accessing a layer through the manifests endpoint.

This problem is mitigated by setting a 4MB limit when using to access
content that may have been accepted from a user. In practice, this means
setting the limit with the use of `BlobProvider.Get` by wrapping
`StorageDriver.GetContent` in a helper that uses `StorageDriver.Reader`
with a `limitReader` that returns an error.

When mitigating this security issue, we also noticed that the size of
manifests uploaded to the registry is also unlimited. We apply similar
logic to the request body of payloads that are full buffered.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

registry/storage/blobstore.go

@@ -27,7 +27,7 @@ func (bs *blobStore) Get(ctx context.Context, dgst digest.Digest) ([]byte, error
 		return nil, err
 	}
 
-	p, err := bs.driver.GetContent(ctx, bp)
+	p, err := getContent(ctx, bs.driver, bp)
 	if err != nil {
 		switch err.(type) {
 		case driver.PathNotFoundError:
@@ -37,7 +37,7 @@ func (bs *blobStore) Get(ctx context.Context, dgst digest.Digest) ([]byte, error
 		return nil, err
 	}
 
-	return p, err
+	return p, nil
 }
 
 func (bs *blobStore) Open(ctx context.Context, dgst digest.Digest) (distribution.ReadSeekCloser, error) {

registry/storage/io.go

@@ -0,0 +1,71 @@
+package storage
+
+import (
+	"errors"
+	"io"
+	"io/ioutil"
+
+	"github.com/docker/distribution/context"
+	"github.com/docker/distribution/registry/storage/driver"
+)
+
+const (
+	maxBlobGetSize = 4 << 20
+)
+
+func getContent(ctx context.Context, driver driver.StorageDriver, p string) ([]byte, error) {
+	r, err := driver.Reader(ctx, p, 0)
+	if err != nil {
+		return nil, err
+	}
+
+	return readAllLimited(r, maxBlobGetSize)
+}
+
+func readAllLimited(r io.Reader, limit int64) ([]byte, error) {
+	r = limitReader(r, limit)
+	return ioutil.ReadAll(r)
+}
+
+// limitReader returns a new reader limited to n bytes. Unlike io.LimitReader,
+// this returns an error when the limit reached.
+func limitReader(r io.Reader, n int64) io.Reader {
+	return &limitedReader{r: r, n: n}
+}
+
+// limitedReader implements a reader that errors when the limit is reached.
+//
+// Partially cribbed from net/http.MaxBytesReader.
+type limitedReader struct {
+	r   io.Reader // underlying reader
+	n   int64     // max bytes remaining
+	err error     // sticky error
+}
+
+func (l *limitedReader) Read(p []byte) (n int, err error) {
+	if l.err != nil {
+		return 0, l.err
+	}
+	if len(p) == 0 {
+		return 0, nil
+	}
+	// If they asked for a 32KB byte read but only 5 bytes are
+	// remaining, no need to read 32KB. 6 bytes will answer the
+	// question of the whether we hit the limit or go past it.
+	if int64(len(p)) > l.n+1 {
+		p = p[:l.n+1]
+	}
+	n, err = l.r.Read(p)
+
+	if int64(n) <= l.n {
+		l.n -= int64(n)
+		l.err = err
+		return n, err
+	}
+
+	n = int(l.n)
+	l.n = 0
+
+	l.err = errors.New("storage: read exceeds limit")
+	return n, l.err
+}