ima: open a new file instance if no read permissions

Open a new file instance as opposed to changing file->f_mode when
the file is not readable.  This is done to accomodate overlayfs
stacked file operations change.  The real struct file is hidden
behind the overlays struct file.  So, any file->f_mode manipulations are
not reflected on the real struct file.  Open the file again in read mode
if original file cannot be read, read and calculate the hash.

Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
Cc: stable@vger.kernel.org (linux-4.19)
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

security/integrity/ima/ima_crypto.c

@@ -210,7 +210,7 @@ static int ima_calc_file_hash_atfm(struct file *file,
 {
 	loff_t i_size, offset;
 	char *rbuf[2] = { NULL, };
-	int rc, read = 0, rbuf_len, active = 0, ahash_rc = 0;
+	int rc, rbuf_len, active = 0, ahash_rc = 0;
 	struct ahash_request *req;
 	struct scatterlist sg[1];
 	struct crypto_wait wait;
@@ -257,11 +257,6 @@ static int ima_calc_file_hash_atfm(struct file *file,
 					  &rbuf_size[1], 0);
 	}
 
-	if (!(file->f_mode & FMODE_READ)) {
-		file->f_mode |= FMODE_READ;
-		read = 1;
-	}
-
 	for (offset = 0; offset < i_size; offset += rbuf_len) {
 		if (!rbuf[1] && offset) {
 			/* Not using two buffers, and it is not the first
@@ -300,8 +295,6 @@ static int ima_calc_file_hash_atfm(struct file *file,
 	/* wait for the last update request to complete */
 	rc = ahash_wait(ahash_rc, &wait);
 out3:
-	if (read)
-		file->f_mode &= ~FMODE_READ;
 	ima_free_pages(rbuf[0], rbuf_size[0]);
 	ima_free_pages(rbuf[1], rbuf_size[1]);
 out2:
@@ -336,7 +329,7 @@ static int ima_calc_file_hash_tfm(struct file *file,
 {
 	loff_t i_size, offset = 0;
 	char *rbuf;
-	int rc, read = 0;
+	int rc;
 	SHASH_DESC_ON_STACK(shash, tfm);
 
 	shash->tfm = tfm;
@@ -357,11 +350,6 @@ static int ima_calc_file_hash_tfm(struct file *file,
 	if (!rbuf)
 		return -ENOMEM;
 
-	if (!(file->f_mode & FMODE_READ)) {
-		file->f_mode |= FMODE_READ;
-		read = 1;
-	}
-
 	while (offset < i_size) {
 		int rbuf_len;
 
@@ -378,8 +366,6 @@ static int ima_calc_file_hash_tfm(struct file *file,
 		if (rc)
 			break;
 	}
-	if (read)
-		file->f_mode &= ~FMODE_READ;
 	kfree(rbuf);
 out:
 	if (!rc)
@@ -420,6 +406,8 @@ int ima_calc_file_hash(struct file *file, struct ima_digest_data *hash)
 {
 	loff_t i_size;
 	int rc;
+	struct file *f = file;
+	bool new_file_instance = false, modified_flags = false;
 
 	/*
 	 * For consistency, fail file's opened with the O_DIRECT flag on
@@ -431,15 +419,41 @@ int ima_calc_file_hash(struct file *file, struct ima_digest_data *hash)
 		return -EINVAL;
 	}
 
-	i_size = i_size_read(file_inode(file));
+	/* Open a new file instance in O_RDONLY if we cannot read */
+	if (!(file->f_mode & FMODE_READ)) {
+		int flags = file->f_flags & ~(O_WRONLY | O_APPEND |
+				O_TRUNC | O_CREAT | O_NOCTTY | O_EXCL);
+		flags |= O_RDONLY;
+		f = dentry_open(&file->f_path, flags, file->f_cred);
+		if (IS_ERR(f)) {
+			/*
+			 * Cannot open the file again, lets modify f_flags
+			 * of original and continue
+			 */
+			pr_info_ratelimited("Unable to reopen file for reading.\n");
+			f = file;
+			f->f_flags |= FMODE_READ;
+			modified_flags = true;
+		} else {
+			new_file_instance = true;
+		}
+	}
+
+	i_size = i_size_read(file_inode(f));
 
 	if (ima_ahash_minsize && i_size >= ima_ahash_minsize) {
-		rc = ima_calc_file_ahash(file, hash);
+		rc = ima_calc_file_ahash(f, hash);
 		if (!rc)
-			return 0;
+			goto out;
 	}
 
-	return ima_calc_file_shash(file, hash);
+	rc = ima_calc_file_shash(f, hash);
+out:
+	if (new_file_instance)
+		fput(f);
+	else if (modified_flags)
+		f->f_flags &= ~FMODE_READ;
+	return rc;
 }
 
 /*