Re: [PATCH v10 2/9] fs: introduce kernel_pread_file* support

[Kees Cook <keescook@chromium.org>]

On Mon, Jul 06, 2020 at 04:23:02PM -0700, Scott Branden wrote:
> Add kernel_pread_file* support to kernel to allow for partial read
> of files with an offset into the file.
> 
> Signed-off-by: Scott Branden <scott.branden@broadcom.com>
> ---
>  fs/exec.c                        | 93 ++++++++++++++++++++++++--------
>  include/linux/kernel_read_file.h | 17 ++++++
>  2 files changed, 87 insertions(+), 23 deletions(-)
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index 4ea87db5e4d5..e6a8a65f7478 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -928,10 +928,14 @@ struct file *open_exec(const char *name)
>  }
>  EXPORT_SYMBOL(open_exec);
>  
> -int kernel_read_file(struct file *file, void **buf, loff_t *size,
> -		     loff_t max_size, enum kernel_read_file_id id)
> -{
> -	loff_t i_size, pos;
> +int kernel_pread_file(struct file *file, void **buf, loff_t *size,
> +		      loff_t max_size, loff_t pos,
> +		      enum kernel_read_file_id id)
> +{
> +	loff_t alloc_size;
> +	loff_t buf_pos;
> +	loff_t read_end;
> +	loff_t i_size;
>  	ssize_t bytes = 0;
>  	int ret;
>  
> @@ -951,21 +955,32 @@ 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)) {
> +
> +	/* Default read to end of file */
> +	read_end = i_size;
> +
> +	/* Allow reading partial portion of file */
> +	if ((id == READING_FIRMWARE_PARTIAL_READ) &&
> +	    (i_size > (pos + max_size)))
> +		read_end = pos + max_size;

There's no need to involve "id" here. There are other signals about
what's happening (i.e. pos != 0, max_size != i_size, etc).

> +
> +	alloc_size = read_end - pos;
> +	if (i_size > SIZE_MAX || (max_size > 0 && alloc_size > max_size)) {
>  		ret = -EFBIG;
>  		goto out;
>  	}
>  
> -	if (id != READING_FIRMWARE_PREALLOC_BUFFER)
> -		*buf = vmalloc(i_size);
> +	if ((id != READING_FIRMWARE_PARTIAL_READ) &&
> +	    (id != READING_FIRMWARE_PREALLOC_BUFFER))
> +		*buf = vmalloc(alloc_size);
>  	if (!*buf) {
>  		ret = -ENOMEM;
>  		goto out;
>  	}

The id usage here was a mistake in upstream, and the series I sent is
trying to clean that up.

Greg, it seems this series is going to end up in your tree due to it
being drivers/misc? I guess I need to direct my series to Greg then, but
get LSM folks Acks.

>  
> -	pos = 0;
> -	while (pos < i_size) {
> -		bytes = kernel_read(file, *buf + pos, i_size - pos, &pos);
> +	buf_pos = 0;
> +	while (pos < read_end) {
> +		bytes = kernel_read(file, *buf + buf_pos, read_end - pos, &pos);
>  		if (bytes < 0) {
>  			ret = bytes;
>  			goto out_free;
> @@ -973,20 +988,23 @@ int kernel_read_file(struct file *file, void **buf, loff_t *size,
>  
>  		if (bytes == 0)
>  			break;
> +
> +		buf_pos += bytes;
>  	}
>  
> -	if (pos != i_size) {
> +	if (pos != read_end) {
>  		ret = -EIO;
>  		goto out_free;
>  	}
>  
> -	ret = security_kernel_post_read_file(file, *buf, i_size, id);
> +	ret = security_kernel_post_read_file(file, *buf, alloc_size, id);
>  	if (!ret)
>  		*size = pos;

This call cannot be inside kernel_pread_file(): any future LSMs will see
a moving window of contents, etc. It'll need to be in kernel_read_file()
proper.

>  
>  out_free:
>  	if (ret < 0) {
> -		if (id != READING_FIRMWARE_PREALLOC_BUFFER) {
> +		if ((id != READING_FIRMWARE_PARTIAL_READ) &&
> +		    (id != READING_FIRMWARE_PREALLOC_BUFFER)) {
>  			vfree(*buf);
>  			*buf = NULL;
>  		}
> @@ -996,10 +1014,18 @@ int kernel_read_file(struct file *file, void **buf, loff_t *size,
>  	allow_write_access(file);
>  	return ret;
>  }
> +
> +int kernel_read_file(struct file *file, void **buf, loff_t *size,
> +		     loff_t max_size, enum kernel_read_file_id id)
> +{
> +	return kernel_pread_file(file, buf, size, max_size, 0, id);
> +}
>  EXPORT_SYMBOL_GPL(kernel_read_file);
>  
> -int kernel_read_file_from_path(const char *path, void **buf, loff_t *size,
> -			       loff_t max_size, enum kernel_read_file_id id)
> +int kernel_pread_file_from_path(const char *path, void **buf,
> +				loff_t *size,
> +				loff_t max_size, loff_t pos,
> +				enum kernel_read_file_id id)
>  {
>  	struct file *file;
>  	int ret;
> @@ -1011,15 +1037,22 @@ 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_pread_file(file, buf, size, max_size, pos, id);
>  	fput(file);
>  	return ret;
>  }
> +
> +int kernel_read_file_from_path(const char *path, void **buf, loff_t *size,
> +			       loff_t max_size, enum kernel_read_file_id id)
> +{
> +	return kernel_pread_file_from_path(path, buf, size, max_size, 0, id);
> +}
>  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,
> -				      enum kernel_read_file_id id)
> +int kernel_pread_file_from_path_initns(const char *path, void **buf,
> +				       loff_t *size,
> +				       loff_t max_size, loff_t pos,
> +				       enum kernel_read_file_id id)
>  {
>  	struct file *file;
>  	struct path root;
> @@ -1037,14 +1070,22 @@ 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_pread_file(file, buf, size, max_size, pos, id);
>  	fput(file);
>  	return ret;
>  }
> +
> +int kernel_read_file_from_path_initns(const char *path, void **buf,
> +				      loff_t *size, loff_t max_size,
> +				      enum kernel_read_file_id id)
> +{
> +	return kernel_pread_file_from_path_initns(path, buf, size, max_size, 0, id);
> +}
>  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,
> -			     enum kernel_read_file_id id)
> +int kernel_pread_file_from_fd(int fd, void **buf, loff_t *size,
> +			      loff_t max_size, loff_t pos,
> +			      enum kernel_read_file_id id)
>  {
>  	struct fd f = fdget(fd);
>  	int ret = -EBADF;
> @@ -1052,11 +1093,17 @@ 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_pread_file(f.file, buf, size, max_size, pos, id);
>  out:
>  	fdput(f);
>  	return ret;
>  }
> +
> +int kernel_read_file_from_fd(int fd, void **buf, loff_t *size, loff_t max_size,
> +			     enum kernel_read_file_id id)
> +{
> +	return kernel_pread_file_from_fd(fd, buf, size, max_size, 0, id);
> +}
>  EXPORT_SYMBOL_GPL(kernel_read_file_from_fd);

For each of these execution path, the mapping to LSM hooks is:

- all path must call security_kernel_read_file(file, id) before reading
  (this appears to be fine as-is in your series).

- anything doing a "full" read needs to call
  security_kernel_post_read_file() with the file and full buffer, size,
  etc (so all the kernel_read_file*() paths). I imagine instead of
  adding 3 copy/pasted versions of this, it may be possible to refactor
  the helpers into a single core "full" caller that takes struct file,
  or doing some logic in kernel_pread_file() that notices it has the
  entire file in the buffer and doing the call then.
  As an example of what I mean about doing the call, here's how I might
  imagine it for one of the paths if it took struct file:

int kernel_read_file_from_file(struct file *file, void **buf, loff_t *size,
			       loff_t max_size, enum kernel_read_file_id id)
{
	int ret;

	ret = kernel_pread_file_from_file(file, buf, size, max_size, 0, id);
	if (ret)
		return ret;
	return security_kernel_post_read_file(file, buf, *size, id);
}

>  
>  #if defined(CONFIG_HAVE_AOUT) || defined(CONFIG_BINFMT_FLAT) || \
> diff --git a/include/linux/kernel_read_file.h b/include/linux/kernel_read_file.h
> index 53f5ca41519a..f061ccb8d0b4 100644
> --- a/include/linux/kernel_read_file.h
> +++ b/include/linux/kernel_read_file.h
> @@ -8,6 +8,7 @@
>  #define __kernel_read_file_id(id) \
>  	id(UNKNOWN, unknown)		\
>  	id(FIRMWARE, firmware)		\
> +	id(FIRMWARE_PARTIAL_READ, firmware)	\
>  	id(FIRMWARE_PREALLOC_BUFFER, firmware)	\
>  	id(FIRMWARE_EFI_EMBEDDED, firmware)	\

And again, sorry that this was in here as a misleading example.

>  	id(MODULE, kernel-module)		\
> @@ -36,15 +37,31 @@ static inline const char *kernel_read_file_id_str(enum kernel_read_file_id id)
>  	return kernel_read_file_str[id];
>  }
>  
> +int kernel_pread_file(struct file *file,
> +		      void **buf, loff_t *size, loff_t pos,
> +		      loff_t max_size,
> +		      enum kernel_read_file_id id);
>  int kernel_read_file(struct file *file,
>  		     void **buf, loff_t *size, loff_t max_size,
>  		     enum kernel_read_file_id id);
> +int kernel_pread_file_from_path(const char *path,
> +				void **buf, loff_t *size, loff_t pos,
> +				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,
>  			       enum kernel_read_file_id id);
> +int kernel_pread_file_from_path_initns(const char *path,
> +				       void **buf, loff_t *size, loff_t pos,
> +				       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,
>  				      enum kernel_read_file_id id);
> +int kernel_pread_file_from_fd(int fd,
> +			      void **buf, loff_t *size, loff_t pos,
> +			      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,
>  			     enum kernel_read_file_id id);

I remain concerned that adding these helpers will lead a poor
interaction with LSMs, but I guess I get to hold my tongue. :)

-- 
Kees Cook