[RFC PATCH 1/5] vfs: define a generic function to read a file from the kernel
Kees Cook
keescook at chromium.org
Fri Jan 8 12:24:41 PST 2016
On Fri, Jan 8, 2016 at 11:22 AM, Mimi Zohar <zohar at linux.vnet.ibm.com> wrote:
> In order to measure and appraise files being read by the kernel,
> new module and kexec syscalls were defined which include a file
> descriptor. Other places in the kernel (eg. firmware, IMA,
> sound) also read files.
>
> This patch introduces a common function for reading files from
> the kernel with the corresponding security post-read hook and
> function.
>
> Changelog:
> - Add missing <linux/vmalloc.h>
>
> Signed-off-by: Mimi Zohar <zohar at linux.vnet.ibm.com>
> ---
> fs/exec.c | 56 +++++++++++++++++++++++++++++++++++++++++++++++
> include/linux/fs.h | 1 +
> include/linux/lsm_hooks.h | 11 ++++++++++
> include/linux/security.h | 9 ++++++++
> security/security.c | 16 ++++++++++++++
> 5 files changed, 93 insertions(+)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index b06623a..3c48a19 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 +832,61 @@ 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, int policy_id)
> +{
> + loff_t i_size, pos;
> + ssize_t bytes = 0;
> + int ret;
> +
> + if (!S_ISREG(file_inode(file)->i_mode))
> + 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);
This could get very large -- what risks do we have to system stability
here? Having userspace able to trigger such a massive allocation could
be a problem. The firmware loader was limited to MAX_INT...
-Kees
> + if (!*buf) {
> + ret = -ENOMEM;
> + goto out;
> + }
> +
> + pos = 0;
> + while (pos < i_size) {
> + bytes = kernel_read(file, pos, (char *)(*buf) + pos,
> + i_size - pos);
> + if (bytes < 0) {
> + ret = bytes;
> + goto out_free;
> + }
> +
> + if (bytes == 0)
> + break;
> + pos += bytes;
> + }
> +
> + if (pos != i_size) {
> + ret = -EBADF; /* firmware uses -EIO */
> + goto out_free;
> + }
> +
> + ret = security_kernel_post_read_file(file, *buf, i_size, policy_id);
> + if (!ret)
> + *size = pos;
> +
> +out_free:
> + if (ret < 0) {
> + vfree(*buf);
> + *buf = NULL;
> + }
> +out:
> + 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..9b1468c 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, int);
> 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..10baa8f 100644
> --- a/include/linux/lsm_hooks.h
> +++ b/include/linux/lsm_hooks.h
> @@ -561,6 +561,14 @@
> * 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.
> + * @policy_id contains the calling function identifier.
> + * 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 +1465,8 @@ 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 policy_id);
> 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 +1726,7 @@ 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..44d8832 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -301,6 +301,8 @@ 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 policy_id);
> 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 +868,13 @@ 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,
> + int policy_id)
> +{
> + 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..e979c2d 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -910,6 +910,20 @@ 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,
> + int policy_id)
> +{
> + int ret;
> +
> + ret = call_int_hook(kernel_post_read_file, 0, file, buf, size,
> + policy_id);
> + if (ret)
> + return ret;
> +
> + return ima_hash_and_process_file(file, buf, size, policy_id);
> +}
> +EXPORT_SYMBOL_GPL(security_kernel_post_read_file);
> +
> int security_task_fix_setuid(struct cred *new, const struct cred *old,
> int flags)
> {
> @@ -1697,6 +1711,8 @@ 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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