CVE-2014-2038 (published 2014-02-28T06:18Z)

The nfs_can_extend_write function in fs/nfs/write.c in the Linux kernel before 3.13.3 relies on a write delegation to extend a write operation without a certain up-to-date verification, which allows local users to obtain sensitive information from kernel memory in opportunistic circumstances by writing to a file in an NFS filesystem and then reading the same file.

Severity: LOW
CVSS v2 base score: 2.1 (AV:L/AC:L/Au:N/C:P/I:N/A:N)

Weakness information:
cwe_id: CWE-200
cwe_name: Exposure of Sensitive Information to an Unauthorized Actor
description: The product exposes sensitive information to an actor that is not explicitly authorized to have access to that information.
extended_description: 
	   There are many different kinds of mistakes that introduce information exposures. The severity of the error can range widely, depending on the context in which the product operates, the type of sensitive information that is revealed, and the benefits it may provide to an attacker.  Some kinds of sensitive information include:
       
         private, personal information, such as personal messages, financial data, health records, geographic location, or contact details
         system status and environment, such as the operating system and installed packages
		 business secrets and intellectual property
		 network status and configuration
		 the product's own code or internal state
         metadata, e.g. logging of connections or message headers
		 indirect information, such as a discrepancy between two internal operations that can be observed by an outsider
       
	    Information might be sensitive to different parties, each of which may have their own expectations for whether the information should be protected.  These parties include:
               
                  the product's own users
                  people or organizations whose information is created or used by the product, even if they are not direct product users
		  the product's administrators, including the admins of the system(s) and/or networks on which the product operates
                  the developer
               
	    Information exposures can occur in different ways:
               
                  the code explicitly inserts sensitive information into resources or messages that are intentionally made accessible to unauthorized actors, but should not contain the information - i.e., the information should have been "scrubbed" or "sanitized"
                  a different weakness or mistake indirectly inserts the sensitive information into resources, such as a web script error revealing the full system path of the program.
                  the code manages resources that intentionally contain sensitive information, but the resources are unintentionally made accessible to unauthorized actors. In this case, the information exposure is resultant - i.e., a different weakness enabled the access to the information in the first place.
               
	    It is common practice to describe any loss of confidentiality as an "information exposure," but this can lead to overuse of CWE-200 in CWE mapping. From the CWE perspective, loss of confidentiality is a technical impact that can arise from dozens of different weaknesses, such as insecure file permissions or out-of-bounds read.  CWE-200 and its lower-level descendants are intended to cover the mistakes that occur in behaviors that explicitly manage, store, transfer, or cleanse sensitive information.
         
         

Fix information:
repo_url: https://github.com/torvalds/linux
commit_details: repo_url: https://github.com/torvalds/linux
author: Scott Mayhew
author_date: 2014-01-17 15:12:05-05:00
msg: nfs: always make sure page is up-to-date before extending a write to cover the entire page

We should always make sure the cached page is up-to-date when we're
determining whether we can extend a write to cover the full page -- even
if we've received a write delegation from the server.

Commit c7559663 added logic to skip this check if we have a write
delegation, which can lead to data corruption such as the following
scenario if client B receives a write delegation from the NFS server:

Client A:
    # echo 123456789 > /mnt/file

Client B:
    # echo abcdefghi >> /mnt/file
    # cat /mnt/file
    0�D0�abcdefghi

Just because we hold a write delegation doesn't mean that we've read in
the entire page contents.

Cc: <stable@vger.kernel.org> # v3.11+
Signed-off-by: Scott Mayhew <smayhew@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
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: write.c
change_type: ModificationType.MODIFY
diff: @@ -922,19 +922,20 @@ static bool nfs_write_pageuptodate(struct page *page, struct inode *inode)
  * extend the write to cover the entire page in order to avoid fragmentation
  * inefficiencies.
  *
- * If the file is opened for synchronous writes or if we have a write delegation
- * from the server then we can just skip the rest of the checks.
+ * If the file is opened for synchronous writes then we can just skip the rest
+ * of the checks.
  */
 static int nfs_can_extend_write(struct file *file, struct page *page, struct inode *inode)
 {
 	if (file->f_flags & O_DSYNC)
 		return 0;
+	if (!nfs_write_pageuptodate(page, inode))
+		return 0;
 	if (NFS_PROTO(inode)->have_delegation(inode, FMODE_WRITE))
 		return 1;
-	if (nfs_write_pageuptodate(page, inode) && (inode->i_flock == NULL ||
-			(inode->i_flock->fl_start == 0 &&
+	if (inode->i_flock == NULL || (inode->i_flock->fl_start == 0 &&
 			inode->i_flock->fl_end == OFFSET_MAX &&
-			inode->i_flock->fl_type != F_RDLCK)))
+			inode->i_flock->fl_type != F_RDLCK))
 		return 1;
 	return 0;
 }

programming_language: C