Fixed /proc/cpuinfo permissions

This also applies these permissions to other static proc files.

Change-Id: I4167e585fed49ad271aa4e1f1260babb3239a73d
PiperOrigin-RevId: 242898575

pkg/sentry/fs/fsutil/inode.go

@@ -452,3 +452,15 @@ type InodeGenericChecker struct{}
 func (InodeGenericChecker) Check(ctx context.Context, inode *fs.Inode, p fs.PermMask) bool {
 	return fs.ContextCanAccessFile(ctx, inode, p)
 }
+
+// InodeDenyWriteChecker implements fs.InodeOperations.Check which denies all
+// write operations.
+type InodeDenyWriteChecker struct{}
+
+// Check implements fs.InodeOperations.Check.
+func (InodeDenyWriteChecker) Check(ctx context.Context, inode *fs.Inode, p fs.PermMask) bool {
+	if p.Write {
+		return false
+	}
+	return fs.ContextCanAccessFile(ctx, inode, p)
+}

pkg/sentry/fs/proc/inode.go

@@ -53,7 +53,7 @@ func (i *taskOwnedInodeOps) UnstableAttr(ctx context.Context, inode *fs.Inode) (
 //
 // +stateify savable
 type staticFileInodeOps struct {
-	fsutil.InodeGenericChecker       `state:"nosave"`
+	fsutil.InodeDenyWriteChecker     `state:"nosave"`
 	fsutil.InodeNoExtendedAttributes `state:"nosave"`
 	fsutil.InodeNoopRelease          `state:"nosave"`
 	fsutil.InodeNoopTruncate         `state:"nosave"`

test/syscalls/linux/proc.cc

@@ -725,6 +725,10 @@ TEST(ProcCpuinfo, RequiredFieldsArePresent) {
   }
 }
 
+TEST(ProcCpuinfo, DeniesWrite) {
+  EXPECT_THAT(open("/proc/cpuinfo", O_WRONLY), SyscallFailsWithErrno(EACCES));
+}
+
 // Sanity checks that uptime is present.
 TEST(ProcUptime, IsPresent) {
   std::string proc_uptime = ASSERT_NO_ERRNO_AND_VALUE(GetContents("/proc/uptime"));