fix: robust stsc allocations by early size consistency check

mp4/stsc.go

@@ -7,10 +7,6 @@ import (
 	"github.com/Eyevinn/mp4ff/bits"
 )
 
-const (
-	safeAllocationSize = 512
-)
-
 // StscBox is Sample To Chunk Box in progressive file.
 //
 // A chunk contains samples. This table defines to which chunk a sample is associated.
@@ -53,9 +49,13 @@ func DecodeStscSR(hdr BoxHeader, startPos uint64, sr bits.SliceReader) (Box, err
 	b := StscBox{
 		Version: byte(versionAndFlags >> 24),
 		Flags:   versionAndFlags & flagsMask,
-		Entries: make([]StscEntry, 0, limitedInitialSize(entryCount)),
+	}
+	if hdr.Size != b.expectedSize(int(entryCount)) {
+		return nil, fmt.Errorf("invalid stsc box size")
 	}
 
+	b.Entries = make([]StscEntry, 0, entryCount)
+
 	var accSampleNr uint32 = 1
 
 	for i := 0; i < int(entryCount); i++ {
@@ -74,7 +74,7 @@ func DecodeStscSR(hdr BoxHeader, startPos uint64, sr bits.SliceReader) (Box, err
 		} else {
 			if sdi != b.singleSampleDescriptionID {
 				if b.singleSampleDescriptionID != 0 {
-					b.SampleDescriptionID = make([]uint32, 0, limitedInitialSize(entryCount))
+					b.SampleDescriptionID = make([]uint32, 0, entryCount)
 					for j := 0; j < i; j++ {
 						b.SampleDescriptionID = append(b.SampleDescriptionID, b.singleSampleDescriptionID)
 					}
@@ -94,7 +94,11 @@ func (b *StscBox) Type() string {
 
 // Size - box-specific size
 func (b *StscBox) Size() uint64 {
-	return uint64(boxHeaderSize + 8 + len(b.Entries)*12)
+	return b.expectedSize(len(b.Entries))
+}
+
+func (b *StscBox) expectedSize(nrEntries int) uint64 {
+	return uint64(boxHeaderSize + 8 + nrEntries*12)
 }
 
 // Encode - write box to w
@@ -288,11 +292,3 @@ func (b *StscBox) FindEntryNrForSampleNr(sampleNr, lowEntryIdx uint32) uint32 {
 	}
 	return low - 1
 }
-
-// limitedInitialSize returns a limited size to prevent from excessive initial allocation.
-func limitedInitialSize(a uint32) uint32 {
-	if a > safeAllocationSize {
-		return safeAllocationSize
-	}
-	return a
-}