[PATCH] audit: Report suspicious O_CREAT usage

[Kees Cook]

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@xxxxxxxxxxxxxx

Reported-by: Jérémie Galarneau <jeremie.galarneau@xxxxxxxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
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);
 { }
 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