CVE-2020-8428 (published 2020-01-29T00:15Z)

fs/namei.c in the Linux kernel before 5.5 has a may_create_in_sticky use-after-free, which allows local users to cause a denial of service (OOPS) or possibly obtain sensitive information from kernel memory, aka CID-d0cb50185ae9. One attack vector may be an open system call for a UNIX domain socket, if the socket is being moved to a new parent directory and its old parent directory is being removed.

Severity: LOW
CVSS v2 base score: 3.6 (AV:L/AC:L/Au:N/C:P/I:N/A:P)
CVSS v3 base score: 7.1, HIGH (CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:H)

Weakness information:
cwe_id: CWE-416
cwe_name: Use After Free
description: The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory "belongs" to the code that operates on the new pointer.
extended_description: 
             
             
             
			 
             
             
         
         

Fix information:
repo_url: https://github.com/torvalds/linux
commit_details: repo_url: https://github.com/torvalds/linux
author: Al Viro
author_date: 2020-01-26 09:29:34-05:00
msg: do_last(): fetch directory ->i_mode and ->i_uid before it's too late

may_create_in_sticky() call is done when we already have dropped the
reference to dir.

Fixes: 30aba6656f61e (namei: allow restricted O_CREAT of FIFOs and regular files)
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
repository: repo_url: https://github.com/torvalds/linux
repo_name: torvalds/linux
description: Linux kernel source tree
repo_language: C
owner: torvalds
file_changes: filename: namei.c
change_type: ModificationType.MODIFY
diff: @@ -1001,7 +1001,8 @@ static int may_linkat(struct path *link)
  * may_create_in_sticky - Check whether an O_CREAT open in a sticky directory
  *			  should be allowed, or not, on files that already
  *			  exist.
- * @dir: the sticky parent directory
+ * @dir_mode: mode bits of directory
+ * @dir_uid: owner of directory
  * @inode: the inode of the file to open
  *
  * Block an O_CREAT open of a FIFO (or a regular file) when:
@@ -1017,18 +1018,18 @@ static int may_linkat(struct path *link)
  *
  * Returns 0 if the open is allowed, -ve on error.
  */
-static int may_create_in_sticky(struct dentry * const dir,
+static int may_create_in_sticky(umode_t dir_mode, kuid_t dir_uid,
 				struct inode * const inode)
 {
 	if ((!sysctl_protected_fifos && S_ISFIFO(inode->i_mode)) ||
 	    (!sysctl_protected_regular && S_ISREG(inode->i_mode)) ||
-	    likely(!(dir->d_inode->i_mode & S_ISVTX)) ||
-	    uid_eq(inode->i_uid, dir->d_inode->i_uid) ||
+	    likely(!(dir_mode & S_ISVTX)) ||
+	    uid_eq(inode->i_uid, dir_uid) ||
 	    uid_eq(current_fsuid(), inode->i_uid))
 		return 0;
 
-	if (likely(dir->d_inode->i_mode & 0002) ||
-	    (dir->d_inode->i_mode & 0020 &&
+	if (likely(dir_mode & 0002) ||
+	    (dir_mode & 0020 &&
 	     ((sysctl_protected_fifos >= 2 && S_ISFIFO(inode->i_mode)) ||
 	      (sysctl_protected_regular >= 2 && S_ISREG(inode->i_mode))))) {
 		const char *operation = S_ISFIFO(inode->i_mode) ?
@@ -3201,6 +3202,8 @@ static int do_last(struct nameidata *nd,
 		   struct file *file, const struct open_flags *op)
 {
 	struct dentry *dir = nd->path.dentry;
+	kuid_t dir_uid = dir->d_inode->i_uid;
+	umode_t dir_mode = dir->d_inode->i_mode;
 	int open_flag = op->open_flag;
 	bool will_truncate = (open_flag & O_TRUNC) != 0;
 	bool got_write = false;
@@ -3331,7 +3334,7 @@ static int do_last(struct nameidata *nd,
 		error = -EISDIR;
 		if (d_is_dir(nd->path.dentry))
 			goto out;
-		error = may_create_in_sticky(dir,
+		error = may_create_in_sticky(dir_mode, dir_uid,
 					     d_backing_inode(nd->path.dentry));
 		if (unlikely(error))
 			goto out;

programming_language: C