CVE-2017-7495 (published 2017-05-15T18:29Z)

fs/ext4/inode.c in the Linux kernel before 4.6.2, when ext4 data=ordered mode is used, mishandles a needs-flushing-before-commit list, which allows local users to obtain sensitive information from other users' files in opportunistic circumstances by waiting for a hardware reset, creating a new file, making write system calls, and reading this file.

Severity: LOW
CVSS v2 base score: 2.1 (AV:L/AC:L/Au:N/C:P/I:N/A:N)
CVSS v3 base score: 5.5, MEDIUM (CVSS:3.0/AV:L/AC:L/PR:L/UI:N/S:U/C:H/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: Jan Kara
author_date: 2016-04-24 00:56:03-04:00
msg: ext4: fix data exposure after a crash

Huang has reported that in his powerfail testing he is seeing stale
block contents in some of recently allocated blocks although he mounts
ext4 in data=ordered mode. After some investigation I have found out
that indeed when delayed allocation is used, we don't add inode to
transaction's list of inodes needing flushing before commit. Originally
we were doing that but commit f3b59291a69d removed the logic with a
flawed argument that it is not needed.

The problem is that although for delayed allocated blocks we write their
contents immediately after allocating them, there is no guarantee that
the IO scheduler or device doesn't reorder things and thus transaction
allocating blocks and attaching them to inode can reach stable storage
before actual block contents. Actually whenever we attach freshly
allocated blocks to inode using a written extent, we should add inode to
transaction's ordered inode list to make sure we properly wait for block
contents to be written before committing the transaction. So that is
what we do in this patch. This also handles other cases where stale data
exposure was possible - like filling hole via mmap in
data=ordered,nodelalloc mode.

The only exception to the above rule are extending direct IO writes where
blkdev_direct_IO() waits for IO to complete before increasing i_size and
thus stale data exposure is not possible. For now we don't complicate
the code with optimizing this special case since the overhead is pretty
low. In case this is observed to be a performance problem we can always
handle it using a special flag to ext4_map_blocks().

CC: stable@vger.kernel.org
Fixes: f3b59291a69d0b734be1fc8be489fef2dd846d3d
Reported-by: "HUANG Weller (CM/ESW12-CN)" <Weller.Huang@cn.bosch.com>
Tested-by: "HUANG Weller (CM/ESW12-CN)" <Weller.Huang@cn.bosch.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
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: inode.c
change_type: ModificationType.MODIFY
diff: @@ -684,6 +684,21 @@ int ext4_map_blocks(handle_t *handle, struct inode *inode,
 		ret = check_block_validity(inode, map);
 		if (ret != 0)
 			return ret;
+
+		/*
+		 * Inodes with freshly allocated blocks where contents will be
+		 * visible after transaction commit must be on transaction's
+		 * ordered data list.
+		 */
+		if (map->m_flags & EXT4_MAP_NEW &&
+		    !(map->m_flags & EXT4_MAP_UNWRITTEN) &&
+		    !(flags & EXT4_GET_BLOCKS_ZERO) &&
+		    !IS_NOQUOTA(inode) &&
+		    ext4_should_order_data(inode)) {
+			ret = ext4_jbd2_file_inode(handle, inode);
+			if (ret)
+				return ret;
+		}
 	}
 	return retval;
 }
@@ -1289,15 +1304,6 @@ static int ext4_write_end(struct file *file,
 	int i_size_changed = 0;
 
 	trace_ext4_write_end(inode, pos, len, copied);
-	if (ext4_test_inode_state(inode, EXT4_STATE_ORDERED_MODE)) {
-		ret = ext4_jbd2_file_inode(handle, inode);
-		if (ret) {
-			unlock_page(page);
-			put_page(page);
-			goto errout;
-		}
-	}
-
 	if (ext4_has_inline_data(inode)) {
 		ret = ext4_write_inline_data_end(inode, pos, len,
 						 copied, page);

programming_language: C