Handle NUL-only paths in exec

The path in execve(2), interpreter script, and ELF interpreter may all
be no more than a NUL-byte. Handle each of those cases.

PiperOrigin-RevId: 203155745
Change-Id: I1c8b1b387924b23b2cf942341dfc76c9003da959

pkg/sentry/loader/elf.go

@@ -405,6 +405,10 @@ func loadParsedELF(ctx context.Context, m *mm.MemoryManager, f *fs.File, info el
 			}
 
 		case elf.PT_INTERP:
+			if phdr.Filesz < 2 {
+				ctx.Infof("PT_INTERP path too small: %v", phdr.Filesz)
+				return loadedELF{}, syserror.ENOEXEC
+			}
 			if phdr.Filesz > syscall.PathMax {
 				ctx.Infof("PT_INTERP path too big: %v", phdr.Filesz)
 				return loadedELF{}, syserror.ENOEXEC
@@ -423,8 +427,26 @@ func loadParsedELF(ctx context.Context, m *mm.MemoryManager, f *fs.File, info el
 				return loadedELF{}, syserror.ENOEXEC
 			}
 
-			// Strip NUL-terminator from string.
-			interpreter = string(path[:len(path)-1])
+			// Strip NUL-terminator and everything beyond from
+			// string. Note that there may be a NUL-terminator
+			// before len(path)-1.
+			interpreter = string(path[:bytes.IndexByte(path, '\x00')])
+			if interpreter == "" {
+				// Linux actually attempts to open_exec("\0").
+				// open_exec -> do_open_execat fails to check
+				// that name != '\0' before calling
+				// do_filp_open, which thus opens the working
+				// directory.  do_open_execat returns EACCES
+				// because the directory is not a regular file.
+				//
+				// We bypass that nonsense and simply
+				// short-circuit with EACCES. Those this does
+				// mean that there may be some edge cases where
+				// the open path would return a different
+				// error.
+				ctx.Infof("PT_INTERP path is empty: %v", path)
+				return loadedELF{}, syserror.EACCES
+			}
 		}
 	}
 

pkg/sentry/loader/interpreter.go

@@ -82,6 +82,11 @@ func parseInterpreterScript(ctx context.Context, filename string, f *fs.File, ar
 		}
 	}
 
+	if string(interp) == "" {
+		ctx.Infof("Interpreter script contains no interpreter: %v", line)
+		return "", []string{}, syserror.ENOEXEC
+	}
+
 	// Build the new argument list:
 	//
 	// 1. The interpreter.

pkg/sentry/loader/loader.go

@@ -55,6 +55,11 @@ func readFull(ctx context.Context, f *fs.File, dst usermem.IOSequence, offset in
 //
 // name must be a readable, executable, regular file.
 func openPath(ctx context.Context, mm *fs.MountNamespace, root, wd *fs.Dirent, maxTraversals uint, name string) (*fs.Dirent, *fs.File, error) {
+	if name == "" {
+		ctx.Infof("cannot open empty name")
+		return nil, nil, syserror.ENOENT
+	}
+
 	d, err := mm.FindInode(ctx, root, wd, name, maxTraversals)
 	if err != nil {
 		return nil, nil, err