[PATCH v3 07/22] vfs: define a generic function to read a file from the kernel

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On Wed, Feb 3, 2016 at 11:06 AM, Mimi Zohar <zohar at linux.vnet.ibm.com> wrote:
> For a while it was looked down upon to directly read files from Linux.
> These days there exists a few mechanisms in the kernel that do just
> this though to load a file into a local buffer.  There are minor but
> important checks differences on each.  This patch set is the first
> attempt at resolving some of these differences.
>
> This patch introduces a common function for reading files from the kernel
> with the corresponding security post-read hook and function.
>
> Changelog v3:
> - additional bounds checking - Luis
> v2:
> - To simplify patch review, re-ordered patches
>
> Signed-off-by: Mimi Zohar <zohar at linux.vnet.ibm.com>
> Reviewed-by: Luis R. Rodriguez <mcgrof at suse.com>

Acked-by: Kees Cook <keescook at chromium.org>

-Kees

> ---
>  fs/exec.c                 | 53 +++++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/fs.h        |  1 +
>  include/linux/lsm_hooks.h |  9 ++++++++
>  include/linux/security.h  |  7 +++++++
>  security/security.c       |  7 +++++++
>  5 files changed, 77 insertions(+)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index b06623a..742de7a 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -56,6 +56,7 @@
>  #include <linux/pipe_fs_i.h>
>  #include <linux/oom.h>
>  #include <linux/compat.h>
> +#include <linux/vmalloc.h>
>
>  #include <asm/uaccess.h>
>  #include <asm/mmu_context.h>
> @@ -831,6 +832,58 @@ int kernel_read(struct file *file, loff_t offset,
>
>  EXPORT_SYMBOL(kernel_read);
>
> +int kernel_read_file(struct file *file, void **buf, loff_t *size,
> +                    loff_t max_size)
> +{
> +       loff_t i_size, pos;
> +       ssize_t bytes = 0;
> +       int ret;
> +
> +       if (!S_ISREG(file_inode(file)->i_mode) || max_size < 0)
> +               return -EINVAL;
> +
> +       i_size = i_size_read(file_inode(file));
> +       if (max_size > 0 && i_size > max_size)
> +               return -EFBIG;
> +       if (i_size <= 0)
> +               return -EINVAL;
> +
> +       *buf = vmalloc(i_size);
> +       if (!*buf)
> +               return -ENOMEM;
> +
> +       pos = 0;
> +       while (pos < i_size) {
> +               bytes = kernel_read(file, pos, (char *)(*buf) + pos,
> +                                   i_size - pos);
> +               if (bytes < 0) {
> +                       ret = bytes;
> +                       goto out;
> +               }
> +
> +               if (bytes == 0)
> +                       break;
> +               pos += bytes;
> +       }
> +
> +       if (pos != i_size) {
> +               ret = -EIO;
> +               goto out;
> +       }
> +
> +       ret = security_kernel_post_read_file(file, *buf, i_size);
> +       if (!ret)
> +               *size = pos;
> +
> +out:
> +       if (ret < 0) {
> +               vfree(*buf);
> +               *buf = NULL;
> +       }
> +       return ret;
> +}
> +EXPORT_SYMBOL_GPL(kernel_read_file);
> +
>  ssize_t read_code(struct file *file, unsigned long addr, loff_t pos, size_t len)
>  {
>         ssize_t res = vfs_read(file, (void __user *)addr, len, &pos);
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 3aa5142..93ca379 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -2527,6 +2527,7 @@ static inline void i_readcount_inc(struct inode *inode)
>  extern int do_pipe_flags(int *, int);
>
>  extern int kernel_read(struct file *, loff_t, char *, unsigned long);
> +extern int kernel_read_file(struct file *, void **, loff_t *, loff_t);
>  extern ssize_t kernel_write(struct file *, const char *, size_t, loff_t);
>  extern ssize_t __kernel_write(struct file *, const char *, size_t, loff_t *);
>  extern struct file * open_exec(const char *);
> diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
> index 71969de..f82631c 100644
> --- a/include/linux/lsm_hooks.h
> +++ b/include/linux/lsm_hooks.h
> @@ -561,6 +561,13 @@
>   *     the kernel module to load. If the module is being loaded from a blob,
>   *     this argument will be NULL.
>   *     Return 0 if permission is granted.
> + * @kernel_post_read_file:
> + *     Read a file specified by userspace.
> + *     @file contains the file structure pointing to the file being read
> + *     by the kernel.
> + *     @buf pointer to buffer containing the file contents.
> + *     @size length of the file contents.
> + *     Return 0 if permission is granted.
>   * @task_fix_setuid:
>   *     Update the module's state after setting one or more of the user
>   *     identity attributes of the current process.  The @flags parameter
> @@ -1457,6 +1464,7 @@ union security_list_options {
>         int (*kernel_fw_from_file)(struct file *file, char *buf, size_t size);
>         int (*kernel_module_request)(char *kmod_name);
>         int (*kernel_module_from_file)(struct file *file);
> +       int (*kernel_post_read_file)(struct file *file, char *buf, loff_t size);
>         int (*task_fix_setuid)(struct cred *new, const struct cred *old,
>                                 int flags);
>         int (*task_setpgid)(struct task_struct *p, pid_t pgid);
> @@ -1716,6 +1724,7 @@ struct security_hook_heads {
>         struct list_head kernel_act_as;
>         struct list_head kernel_create_files_as;
>         struct list_head kernel_fw_from_file;
> +       struct list_head kernel_post_read_file;
>         struct list_head kernel_module_request;
>         struct list_head kernel_module_from_file;
>         struct list_head task_fix_setuid;
> diff --git a/include/linux/security.h b/include/linux/security.h
> index 4824a4c..f30f564 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -301,6 +301,7 @@ int security_kernel_create_files_as(struct cred *new, struct inode *inode);
>  int security_kernel_fw_from_file(struct file *file, char *buf, size_t size);
>  int security_kernel_module_request(char *kmod_name);
>  int security_kernel_module_from_file(struct file *file);
> +int security_kernel_post_read_file(struct file *file, char *buf, loff_t size);
>  int security_task_fix_setuid(struct cred *new, const struct cred *old,
>                              int flags);
>  int security_task_setpgid(struct task_struct *p, pid_t pgid);
> @@ -866,6 +867,12 @@ static inline int security_kernel_module_from_file(struct file *file)
>         return 0;
>  }
>
> +static inline int security_kernel_post_read_file(struct file *file,
> +                                                char *buf, loff_t size)
> +{
> +       return 0;
> +}
> +
>  static inline int security_task_fix_setuid(struct cred *new,
>                                            const struct cred *old,
>                                            int flags)
> diff --git a/security/security.c b/security/security.c
> index e8ffd92..ae50730 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -910,6 +910,11 @@ int security_kernel_module_from_file(struct file *file)
>         return ima_module_check(file);
>  }
>
> +int security_kernel_post_read_file(struct file *file, char *buf, loff_t size)
> +{
> +       return call_int_hook(kernel_post_read_file, 0, file, buf, size);
> +}
> +
>  int security_task_fix_setuid(struct cred *new, const struct cred *old,
>                              int flags)
>  {
> @@ -1697,6 +1702,8 @@ struct security_hook_heads security_hook_heads = {
>                 LIST_HEAD_INIT(security_hook_heads.kernel_module_request),
>         .kernel_module_from_file =
>                 LIST_HEAD_INIT(security_hook_heads.kernel_module_from_file),
> +       .kernel_post_read_file =
> +               LIST_HEAD_INIT(security_hook_heads.kernel_post_read_file),
>         .task_fix_setuid =
>                 LIST_HEAD_INIT(security_hook_heads.task_fix_setuid),
>         .task_setpgid = LIST_HEAD_INIT(security_hook_heads.task_setpgid),
> --
> 2.1.0
>



-- 
Kees Cook
Chrome OS & Brillo Security



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