block: qcow: limit max nesting depth for backing file

Impose a limit on the maximum nesting of file formats that can open more
files. For example, a qcow2 file can have a backing file, which could be
another qcow2 file with a backing file (or even the same file as the
original), potentially causing unbounded recursion.

This commit is based on crosvm implementation:
eb1640e301

Fixes: #6472

Signed-off-by: Yu Li <liyu.yukiteru@bytedance.com>

block/src/qcow/mod.rs

@@ -31,13 +31,16 @@ use vmm_sys_util::{
 
 pub use crate::qcow::raw_file::RawFile;
 
+/// Nesting depth limit for disk formats that can open other disk files.
+const MAX_NESTING_DEPTH: u32 = 10;
+
 #[sorted]
 #[derive(Debug, Error)]
 pub enum Error {
     #[error("Backing file io error: {0}")]
     BackingFileIo(io::Error),
     #[error("Backing file open error: {0}")]
-    BackingFileOpen(Box<crate::Error>),
+    BackingFileOpen(Box<Error>),
     #[error("Backing file name is too long: {0} bytes over")]
     BackingFileTooLong(usize),
     #[error("Compressed blocks not supported")]
@@ -70,6 +73,8 @@ pub enum Error {
     InvalidRefcountTableOffset,
     #[error("Invalid refcount table size: {0}")]
     InvalidRefcountTableSize(u64),
+    #[error("Maximum disk nesting depth exceeded")]
+    MaxNestingDepthExceeded,
     #[error("No free clusters")]
     NoFreeClusters,
     #[error("No refcount clusters")]
@@ -434,12 +439,20 @@ pub struct QcowFile {
     // List of unreferenced clusters available to be used. unref clusters become available once the
     // removal of references to them have been synced to disk.
     avail_clusters: Vec<u64>,
-    backing_file: Option<Box<dyn BlockBackend>>,
+    backing_file: Option<Box<Self>>,
 }
 
 impl QcowFile {
     /// Creates a QcowFile from `file`. File must be a valid qcow2 image.
-    pub fn from(mut file: RawFile) -> Result<QcowFile> {
+    ///
+    /// Additionally, max nesting depth of this qcow2 image will be set to default value 10.
+    pub fn from(file: RawFile) -> Result<QcowFile> {
+        Self::from_with_nesting_depth(file, MAX_NESTING_DEPTH)
+    }
+
+    /// Creates a QcowFile from `file` and with a max nesting depth. File must be a valid qcow2
+    /// image.
+    pub fn from_with_nesting_depth(mut file: RawFile, max_nesting_depth: u32) -> Result<QcowFile> {
         let header = QcowHeader::new(&mut file)?;
 
         // Only v2 and v3 files are supported.
@@ -466,14 +479,20 @@ impl QcowFile {
         let direct_io = file.is_direct();
 
         let backing_file = if let Some(backing_file_path) = header.backing_file_path.as_ref() {
+            if max_nesting_depth == 0 {
+                return Err(Error::MaxNestingDepthExceeded);
+            }
             let path = backing_file_path.clone();
             let backing_raw_file = OpenOptions::new()
                 .read(true)
                 .open(path)
                 .map_err(Error::BackingFileIo)?;
-            let backing_file = crate::create_disk_file(backing_raw_file, direct_io)
+            let backing_file = Self::from_with_nesting_depth(
+                RawFile::new(backing_raw_file, direct_io),
+                max_nesting_depth - 1,
+            )
             .map_err(|e| Error::BackingFileOpen(Box::new(e)))?;
-            Some(backing_file)
+            Some(Box::new(backing_file))
         } else {
             None
         };
@@ -621,20 +640,22 @@ impl QcowFile {
         file: RawFile,
         version: u32,
         backing_file_name: &str,
+        backing_file_max_nesting_depth: u32,
     ) -> Result<QcowFile> {
         let direct_io = file.is_direct();
         let backing_raw_file = OpenOptions::new()
             .read(true)
             .open(backing_file_name)
             .map_err(Error::BackingFileIo)?;
-        let backing_file = crate::create_disk_file(backing_raw_file, direct_io)
-            .map_err(|e| Error::BackingFileOpen(Box::new(e)))?;
-        let size = backing_file
-            .size()
+        let backing_file = Self::from_with_nesting_depth(
+            RawFile::new(backing_raw_file, direct_io),
+            backing_file_max_nesting_depth,
+        )
         .map_err(|e| Error::BackingFileOpen(Box::new(e)))?;
+        let size = backing_file.virtual_size();
         let header = QcowHeader::create_for_size_and_path(version, size, Some(backing_file_name))?;
         let mut result = QcowFile::new_from_header(file, header)?;
-        result.backing_file = Some(backing_file);
+        result.backing_file = Some(Box::new(backing_file));
         Ok(result)
     }
 
@@ -662,7 +683,7 @@ impl QcowFile {
         Ok(qcow)
     }
 
-    pub fn set_backing_file(&mut self, backing: Option<Box<dyn BlockBackend>>) {
+    pub fn set_backing_file(&mut self, backing: Option<Box<Self>>) {
         self.backing_file = backing;
     }
 
@@ -1778,11 +1799,17 @@ where
 /// Copy the contents of a disk image in `src_file` into `dst_file`.
 /// The type of `src_file` is automatically detected, and the output file type is
 /// determined by `dst_type`.
-pub fn convert(mut src_file: RawFile, dst_file: RawFile, dst_type: ImageType) -> Result<()> {
+pub fn convert(
+    mut src_file: RawFile,
+    dst_file: RawFile,
+    dst_type: ImageType,
+    src_max_nesting_depth: u32,
+) -> Result<()> {
     let src_type = detect_image_type(&mut src_file)?;
     match src_type {
         ImageType::Qcow2 => {
-            let mut src_reader = QcowFile::from(src_file)?;
+            let mut src_reader =
+                QcowFile::from_with_nesting_depth(src_file, src_max_nesting_depth)?;
             convert_reader(&mut src_reader, dst_file, dst_type)
         }
         ImageType::Raw => {