Fix deadlock in /proc/[pid]/fd/[num]

In order to resolve path names, fsSymlink.Readlink() may need to reenter
kernfs. Change the code so that kernfs.Inode.Readlink() is called without
locks and document the new contract.

PiperOrigin-RevId: 371770222

pkg/sentry/fsimpl/kernfs/filesystem.go

@@ -612,16 +612,24 @@ afterTrailingSymlink:
 
 // ReadlinkAt implements vfs.FilesystemImpl.ReadlinkAt.
 func (fs *Filesystem) ReadlinkAt(ctx context.Context, rp *vfs.ResolvingPath) (string, error) {
-	fs.mu.RLock()
 	defer fs.processDeferredDecRefs(ctx)
-	defer fs.mu.RUnlock()
+
+	fs.mu.RLock()
 	d, err := fs.walkExistingLocked(ctx, rp)
 	if err != nil {
+		fs.mu.RUnlock()
 		return "", err
 	}
 	if !d.isSymlink() {
+		fs.mu.RUnlock()
 		return "", syserror.EINVAL
 	}
+
+	// Inode.Readlink() cannot be called holding fs locks.
+	d.IncRef()
+	defer d.DecRef(ctx)
+	fs.mu.RUnlock()
+
 	return d.inode.Readlink(ctx, rp.Mount())
 }
 

pkg/sentry/fsimpl/kernfs/kernfs.go

@@ -534,6 +534,9 @@ func (d *Dentry) FSLocalPath() string {
 // - Checking that dentries passed to methods are of the appropriate file type.
 // - Checking permissions.
 //
+// Inode functions may be called holding filesystem wide locks and are not
+// allowed to call vfs functions that may reenter, unless otherwise noted.
+//
 // Specific responsibilities of implementations are documented below.
 type Inode interface {
 	// Methods related to reference counting. A generic implementation is
@@ -680,6 +683,9 @@ type inodeDirectory interface {
 type inodeSymlink interface {
 	// Readlink returns the target of a symbolic link. If an inode is not a
 	// symlink, the implementation should return EINVAL.
+	//
+	// Readlink is called with no kernfs locks held, so it may reenter if needed
+	// to resolve symlink targets.
 	Readlink(ctx context.Context, mnt *vfs.Mount) (string, error)
 
 	// Getlink returns the target of a symbolic link, as used by path

pkg/sentry/fsimpl/proc/task_fds.go

@@ -221,6 +221,8 @@ func (s *fdSymlink) Readlink(ctx context.Context, _ *vfs.Mount) (string, error)
 	defer file.DecRef(ctx)
 	root := vfs.RootFromContext(ctx)
 	defer root.DecRef(ctx)
+
+	// Note: it's safe to reenter kernfs from Readlink if needed to resolve path.
 	return s.task.Kernel().VFS().PathnameWithDeleted(ctx, root, file.VirtualDentry())
 }
 

test/syscalls/linux/proc.cc

@@ -2698,6 +2698,14 @@ TEST(Proc, Statfs) {
   EXPECT_EQ(st.f_namelen, NAME_MAX);
 }
 
+// Tests that /proc/[pid]/fd/[num] can resolve to a path inside /proc.
+TEST(Proc, ResolveSymlinkToProc) {
+  const auto proc = ASSERT_NO_ERRNO_AND_VALUE(Open("/proc/self/cmdline", 0));
+  const auto path = JoinPath("/proc/self/fd/", absl::StrCat(proc.get()));
+  const auto target = ASSERT_NO_ERRNO_AND_VALUE(ReadLink(path));
+  EXPECT_EQ(target, JoinPath("/proc/", absl::StrCat(getpid()), "/cmdline"));
+}
+
 }  // namespace
 }  // namespace testing
 }  // namespace gvisor