Re: [PATCH] audit: Report suspicious O_CREAT usage

[Kees Cook <keescook@chromium.org>]

On Wed, Sep 25, 2019 at 02:02:33PM -0700, Kees Cook wrote:
> This renames the very specific audit_log_link_denied() to
> audit_log_path_denied() and adds the AUDIT_* type as an argument. This
> allows for the creation of the new AUDIT_ANOM_CREAT that can be used to
> report the fifo/regular file creation restrictions that were introduced
> in commit 30aba6656f61 ("namei: allow restricted O_CREAT of FIFOs and
> regular files"). Without this change, discovering that the restriction
> is enabled can be very challenging:
> https://lore.kernel.org/lkml/CA+jJMxvkqjXHy3DnV5MVhFTL2RUhg0WQ-XVFW3ngDQOdkFq0PA@mail.gmail.com
> 
> Reported-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
> This is not a complete fix because reporting was broken in commit
> 15564ff0a16e ("audit: make ANOM_LINK obey audit_enabled and
> audit_dummy_context")
> which specifically goes against the intention of these records: they
> should _always_ be reported. If auditing isn't enabled, they should be
> ratelimited.
> 
> Instead of using audit, should this just go back to using
> pr_ratelimited()?
> ---
>  fs/namei.c                 |  7 +++++--
>  include/linux/audit.h      |  5 +++--
>  include/uapi/linux/audit.h |  1 +
>  kernel/audit.c             | 11 ++++++-----
>  4 files changed, 15 insertions(+), 9 deletions(-)
> 
> diff --git a/fs/namei.c b/fs/namei.c
> index 671c3c1a3425..0e60f81e1d5a 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -925,7 +925,7 @@ static inline int may_follow_link(struct nameidata *nd)
>  		return -ECHILD;
>  
>  	audit_inode(nd->name, nd->stack[0].link.dentry, 0);
> -	audit_log_link_denied("follow_link");
> +	audit_log_path_denied(AUDIT_ANOM_LINK, "follow_link");
>  	return -EACCES;
>  }
>  
> @@ -993,7 +993,7 @@ static int may_linkat(struct path *link)
>  	if (safe_hardlink_source(inode) || inode_owner_or_capable(inode))
>  		return 0;
>  
> -	audit_log_link_denied("linkat");
> +	audit_log_path_denied(AUDIT_ANOM_LINK, "linkat");
>  	return -EPERM;
>  }
>  
> @@ -1031,6 +1031,9 @@ static int may_create_in_sticky(struct dentry * const dir,
>  	    (dir->d_inode->i_mode & 0020 &&
>  	     ((sysctl_protected_fifos >= 2 && S_ISFIFO(inode->i_mode)) ||
>  	      (sysctl_protected_regular >= 2 && S_ISREG(inode->i_mode))))) {
> +		audit_log_path_denied(AUDIT_ANOM_CREAT,
> +				      S_ISFIFO(inode->i_mode) ? "fifo"
> +							      : "regular");
>  		return -EACCES;
>  	}
>  	return 0;
> diff --git a/include/linux/audit.h b/include/linux/audit.h
> index aee3dc9eb378..b3715e2ee1c5 100644
> --- a/include/linux/audit.h
> +++ b/include/linux/audit.h
> @@ -156,7 +156,8 @@ extern void		    audit_log_d_path(struct audit_buffer *ab,
>  					     const struct path *path);
>  extern void		    audit_log_key(struct audit_buffer *ab,
>  					  char *key);
> -extern void		    audit_log_link_denied(const char *operation);
> +extern void		    audit_log_path_denied(int type,
> +						  const char *operation);
>  extern void		    audit_log_lost(const char *message);
>  
>  extern int audit_log_task_context(struct audit_buffer *ab);
> @@ -217,7 +218,7 @@ static inline void audit_log_d_path(struct audit_buffer *ab,
>  { }
>  static inline void audit_log_key(struct audit_buffer *ab, char *key)
>  { }
> -static inline void audit_log_link_denied(const char *string)
> +static inline void audit_log_path_denied(int type, const char *string);

Oops, typo above (should be no trailing ";"). Thanks 0day-bot! I didn't
build without CONFIG_AUDIT. :)

-Kees

>  { }
>  static inline int audit_log_task_context(struct audit_buffer *ab)
>  {
> diff --git a/include/uapi/linux/audit.h b/include/uapi/linux/audit.h
> index c89c6495983d..3ad935527177 100644
> --- a/include/uapi/linux/audit.h
> +++ b/include/uapi/linux/audit.h
> @@ -143,6 +143,7 @@
>  #define AUDIT_ANOM_PROMISCUOUS      1700 /* Device changed promiscuous mode */
>  #define AUDIT_ANOM_ABEND            1701 /* Process ended abnormally */
>  #define AUDIT_ANOM_LINK		    1702 /* Suspicious use of file links */
> +#define AUDIT_ANOM_CREAT	    1703 /* Suspicious file creation */
>  #define AUDIT_INTEGRITY_DATA	    1800 /* Data integrity verification */
>  #define AUDIT_INTEGRITY_METADATA    1801 /* Metadata integrity verification */
>  #define AUDIT_INTEGRITY_STATUS	    1802 /* Integrity enable status */
> diff --git a/kernel/audit.c b/kernel/audit.c
> index da8dc0db5bd3..ed7402ac81b6 100644
> --- a/kernel/audit.c
> +++ b/kernel/audit.c
> @@ -2155,18 +2155,19 @@ void audit_log_task_info(struct audit_buffer *ab)
>  EXPORT_SYMBOL(audit_log_task_info);
>  
>  /**
> - * audit_log_link_denied - report a link restriction denial
> - * @operation: specific link operation
> + * audit_log_path_denied - report a path restriction denial
> + * @type: audit message type (AUDIT_ANOM_LINK, AUDIT_ANOM_CREAT, etc)
> + * @operation: specific operation name
>   */
> -void audit_log_link_denied(const char *operation)
> +void audit_log_path_denied(int type, const char *operation)
>  {
>  	struct audit_buffer *ab;
>  
>  	if (!audit_enabled || audit_dummy_context())
>  		return;
>  
> -	/* Generate AUDIT_ANOM_LINK with subject, operation, outcome. */
> -	ab = audit_log_start(audit_context(), GFP_KERNEL, AUDIT_ANOM_LINK);
> +	/* Generate log with subject, operation, outcome. */
> +	ab = audit_log_start(audit_context(), GFP_KERNEL, type);
>  	if (!ab)
>  		return;
>  	audit_log_format(ab, "op=%s", operation);
> -- 
> 2.17.1
> 
> 
> -- 
> Kees Cook

-- 
Kees Cook