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Date:   Fri, 17 Jul 2020 12:04:18 -0700
From:   Scott Branden <scott.branden@...adcom.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Mimi Zohar <zohar@...ux.ibm.com>,
        Matthew Wilcox <willy@...radead.org>,
        James Morris <jmorris@...ei.org>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Jessica Yu <jeyu@...nel.org>,
        Dmitry Kasatkin <dmitry.kasatkin@...il.com>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Casey Schaufler <casey@...aufler-ca.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Matthew Garrett <matthewgarrett@...gle.com>,
        David Howells <dhowells@...hat.com>,
        Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        KP Singh <kpsingh@...gle.com>, Dave Olsthoorn <dave@...aar.me>,
        Hans de Goede <hdegoede@...hat.com>,
        Peter Jones <pjones@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Stephen Boyd <stephen.boyd@...aro.org>,
        Paul Moore <paul@...l-moore.com>,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        linux-security-module@...r.kernel.org,
        linux-integrity@...r.kernel.org, selinux@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, kexec@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 06/13] fs/kernel_read_file: Remove redundant size argument

Hi Kees,

On 2020-07-17 10:43 a.m., Kees Cook wrote:
> In preparation for refactoring kernel_read_file*(), remove the redundant
> "size" argument which is not needed: it can be included in the return
I don't think the size argument is redundant though.
The existing kernel_read_file functions always read the whole file.
Now, what happens if the file is bigger than the buffer.
How does kernel_read_file know it read the whole file by looking at the 
return value?
> code, with callers adjusted. (VFS reads already cannot be larger than
> INT_MAX.)
>
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
>   drivers/base/firmware_loader/main.c |  8 ++++----
>   fs/kernel_read_file.c               | 20 +++++++++-----------
>   include/linux/kernel_read_file.h    |  8 ++++----
>   kernel/kexec_file.c                 | 13 ++++++-------
>   kernel/module.c                     |  7 +++----
>   security/integrity/digsig.c         |  5 +++--
>   security/integrity/ima/ima_fs.c     |  5 +++--
>   7 files changed, 32 insertions(+), 34 deletions(-)
>
> diff --git a/drivers/base/firmware_loader/main.c b/drivers/base/firmware_loader/main.c
> index d4a413ea48ce..ea419c7d3d34 100644
> --- a/drivers/base/firmware_loader/main.c
> +++ b/drivers/base/firmware_loader/main.c
> @@ -462,7 +462,7 @@ fw_get_filesystem_firmware(struct device *device, struct fw_priv *fw_priv,
>   					     size_t in_size,
>   					     const void *in_buffer))
>   {
> -	loff_t size;
> +	size_t size;
>   	int i, len;
>   	int rc = -ENOENT;
>   	char *path;
> @@ -494,10 +494,9 @@ fw_get_filesystem_firmware(struct device *device, struct fw_priv *fw_priv,
>   		fw_priv->size = 0;
>   
>   		/* load firmware files from the mount namespace of init */
> -		rc = kernel_read_file_from_path_initns(path, &buffer,
> -						       &size, msize,
> +		rc = kernel_read_file_from_path_initns(path, &buffer, msize,
>   						       READING_FIRMWARE);
> -		if (rc) {
> +		if (rc < 0) {
>   			if (rc != -ENOENT)
>   				dev_warn(device, "loading %s failed with error %d\n",
>   					 path, rc);
> @@ -506,6 +505,7 @@ fw_get_filesystem_firmware(struct device *device, struct fw_priv *fw_priv,
>   					 path);
>   			continue;
>   		}
> +		size = rc;
>   		dev_dbg(device, "Loading firmware from %s\n", path);
>   		if (decompress) {
>   			dev_dbg(device, "f/w decompressing %s\n",
> diff --git a/fs/kernel_read_file.c b/fs/kernel_read_file.c
> index 54d972d4befc..dc28a8def597 100644
> --- a/fs/kernel_read_file.c
> +++ b/fs/kernel_read_file.c
> @@ -5,7 +5,7 @@
>   #include <linux/security.h>
>   #include <linux/vmalloc.h>
>   
> -int kernel_read_file(struct file *file, void **buf, loff_t *size,
> +int kernel_read_file(struct file *file, void **buf,
>   		     loff_t max_size, enum kernel_read_file_id id)
>   {
>   	loff_t i_size, pos;
> @@ -29,7 +29,7 @@ int kernel_read_file(struct file *file, void **buf, loff_t *size,
>   		ret = -EINVAL;
>   		goto out;
>   	}
> -	if (i_size > SIZE_MAX || (max_size > 0 && i_size > max_size)) {
> +	if (i_size > INT_MAX || (max_size > 0 && i_size > max_size)) {
Should this be SSIZE_MAX?
>   		ret = -EFBIG;
>   		goto out;
>   	}
> @@ -59,8 +59,6 @@ int kernel_read_file(struct file *file, void **buf, loff_t *size,
>   	}
>   
>   	ret = security_kernel_post_read_file(file, *buf, i_size, id);
> -	if (!ret)
> -		*size = pos;
>   
>   out_free:
>   	if (ret < 0) {
> @@ -72,11 +70,11 @@ int kernel_read_file(struct file *file, void **buf, loff_t *size,
>   
>   out:
>   	allow_write_access(file);
> -	return ret;
> +	return ret == 0 ? pos : ret;
>   }
>   EXPORT_SYMBOL_GPL(kernel_read_file);
>   
> -int kernel_read_file_from_path(const char *path, void **buf, loff_t *size,
> +int kernel_read_file_from_path(const char *path, void **buf,
>   			       loff_t max_size, enum kernel_read_file_id id)
>   {
>   	struct file *file;
> @@ -89,14 +87,14 @@ int kernel_read_file_from_path(const char *path, void **buf, loff_t *size,
>   	if (IS_ERR(file))
>   		return PTR_ERR(file);
>   
> -	ret = kernel_read_file(file, buf, size, max_size, id);
> +	ret = kernel_read_file(file, buf, max_size, id);
>   	fput(file);
>   	return ret;
>   }
>   EXPORT_SYMBOL_GPL(kernel_read_file_from_path);
>   
>   int kernel_read_file_from_path_initns(const char *path, void **buf,
> -				      loff_t *size, loff_t max_size,
> +				      loff_t max_size,
>   				      enum kernel_read_file_id id)
>   {
>   	struct file *file;
> @@ -115,13 +113,13 @@ int kernel_read_file_from_path_initns(const char *path, void **buf,
>   	if (IS_ERR(file))
>   		return PTR_ERR(file);
>   
> -	ret = kernel_read_file(file, buf, size, max_size, id);
> +	ret = kernel_read_file(file, buf, max_size, id);
>   	fput(file);
>   	return ret;
>   }
>   EXPORT_SYMBOL_GPL(kernel_read_file_from_path_initns);
>   
> -int kernel_read_file_from_fd(int fd, void **buf, loff_t *size, loff_t max_size,
> +int kernel_read_file_from_fd(int fd, void **buf, loff_t max_size,
>   			     enum kernel_read_file_id id)
>   {
>   	struct fd f = fdget(fd);
> @@ -130,7 +128,7 @@ int kernel_read_file_from_fd(int fd, void **buf, loff_t *size, loff_t max_size,
>   	if (!f.file)
>   		goto out;
>   
> -	ret = kernel_read_file(f.file, buf, size, max_size, id);
> +	ret = kernel_read_file(f.file, buf, max_size, id);
>   out:
>   	fdput(f);
>   	return ret;
> diff --git a/include/linux/kernel_read_file.h b/include/linux/kernel_read_file.h
> index 78cf3d7dc835..0ca0bdbed1bd 100644
> --- a/include/linux/kernel_read_file.h
> +++ b/include/linux/kernel_read_file.h
> @@ -36,16 +36,16 @@ static inline const char *kernel_read_file_id_str(enum kernel_read_file_id id)
>   }
>   
>   int kernel_read_file(struct file *file,
> -		     void **buf, loff_t *size, loff_t max_size,
> +		     void **buf, loff_t max_size,
>   		     enum kernel_read_file_id id);
>   int kernel_read_file_from_path(const char *path,
> -			       void **buf, loff_t *size, loff_t max_size,
> +			       void **buf, loff_t max_size,
>   			       enum kernel_read_file_id id);
>   int kernel_read_file_from_path_initns(const char *path,
> -				      void **buf, loff_t *size, loff_t max_size,
> +				      void **buf, loff_t max_size,
>   				      enum kernel_read_file_id id);
>   int kernel_read_file_from_fd(int fd,
> -			     void **buf, loff_t *size, loff_t max_size,
> +			     void **buf, loff_t max_size,
>   			     enum kernel_read_file_id id);
>   
>   #endif /* _LINUX_KERNEL_READ_FILE_H */
> diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
> index 1358069ce9e9..a201bbb19158 100644
> --- a/kernel/kexec_file.c
> +++ b/kernel/kexec_file.c
> @@ -220,13 +220,12 @@ kimage_file_prepare_segments(struct kimage *image, int kernel_fd, int initrd_fd,
>   {
>   	int ret;
>   	void *ldata;
> -	loff_t size;
>   
>   	ret = kernel_read_file_from_fd(kernel_fd, &image->kernel_buf,
> -				       &size, INT_MAX, READING_KEXEC_IMAGE);
> -	if (ret)
> +				       INT_MAX, READING_KEXEC_IMAGE);
> +	if (ret < 0)
>   		return ret;
> -	image->kernel_buf_len = size;
> +	image->kernel_buf_len = ret;
>   
>   	/* Call arch image probe handlers */
>   	ret = arch_kexec_kernel_image_probe(image, image->kernel_buf,
> @@ -243,11 +242,11 @@ kimage_file_prepare_segments(struct kimage *image, int kernel_fd, int initrd_fd,
>   	/* It is possible that there no initramfs is being loaded */
>   	if (!(flags & KEXEC_FILE_NO_INITRAMFS)) {
>   		ret = kernel_read_file_from_fd(initrd_fd, &image->initrd_buf,
> -					       &size, INT_MAX,
> +					       INT_MAX,
>   					       READING_KEXEC_INITRAMFS);
> -		if (ret)
> +		if (ret < 0)
>   			goto out;
> -		image->initrd_buf_len = size;
> +		image->initrd_buf_len = ret;
>   	}
>   
>   	if (cmdline_len) {
> diff --git a/kernel/module.c b/kernel/module.c
> index e9765803601b..b6fd4f51cc30 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -3988,7 +3988,6 @@ SYSCALL_DEFINE3(init_module, void __user *, umod,
>   SYSCALL_DEFINE3(finit_module, int, fd, const char __user *, uargs, int, flags)
>   {
>   	struct load_info info = { };
> -	loff_t size;
>   	void *hdr = NULL;
>   	int err;
>   
> @@ -4002,12 +4001,12 @@ SYSCALL_DEFINE3(finit_module, int, fd, const char __user *, uargs, int, flags)
>   		      |MODULE_INIT_IGNORE_VERMAGIC))
>   		return -EINVAL;
>   
> -	err = kernel_read_file_from_fd(fd, &hdr, &size, INT_MAX,
> +	err = kernel_read_file_from_fd(fd, &hdr, INT_MAX,
>   				       READING_MODULE);
> -	if (err)
> +	if (err < 0)
>   		return err;
>   	info.hdr = hdr;
> -	info.len = size;
> +	info.len = err;
>   
>   	return load_module(&info, uargs, flags);
>   }
> diff --git a/security/integrity/digsig.c b/security/integrity/digsig.c
> index f8869be45d8f..97661ffabc4e 100644
> --- a/security/integrity/digsig.c
> +++ b/security/integrity/digsig.c
> @@ -171,16 +171,17 @@ int __init integrity_add_key(const unsigned int id, const void *data,
>   int __init integrity_load_x509(const unsigned int id, const char *path)
>   {
>   	void *data = NULL;
> -	loff_t size;
> +	size_t size;
>   	int rc;
>   	key_perm_t perm;
>   
> -	rc = kernel_read_file_from_path(path, &data, &size, 0,
> +	rc = kernel_read_file_from_path(path, &data, 0,
>   					READING_X509_CERTIFICATE);
>   	if (rc < 0) {
>   		pr_err("Unable to open file: %s (%d)", path, rc);
>   		return rc;
>   	}
> +	size = rc;
>   
>   	perm = (KEY_POS_ALL & ~KEY_POS_SETATTR) | KEY_USR_VIEW | KEY_USR_READ;
>   
> diff --git a/security/integrity/ima/ima_fs.c b/security/integrity/ima/ima_fs.c
> index e13ffece3726..9ba145d3d6d9 100644
> --- a/security/integrity/ima/ima_fs.c
> +++ b/security/integrity/ima/ima_fs.c
> @@ -275,7 +275,7 @@ static ssize_t ima_read_policy(char *path)
>   {
>   	void *data = NULL;
>   	char *datap;
> -	loff_t size;
> +	size_t size;
>   	int rc, pathlen = strlen(path);
>   
>   	char *p;
> @@ -284,11 +284,12 @@ static ssize_t ima_read_policy(char *path)
>   	datap = path;
>   	strsep(&datap, "\n");
>   
> -	rc = kernel_read_file_from_path(path, &data, &size, 0, READING_POLICY);
> +	rc = kernel_read_file_from_path(path, &data, 0, READING_POLICY);
>   	if (rc < 0) {
>   		pr_err("Unable to open file: %s (%d)", path, rc);
>   		return rc;
>   	}
> +	size = rc;
>   
>   	datap = data;
>   	while (size > 0 && (p = strsep(&datap, "\n"))) {

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