CVE-2018-12633 (published 2018-06-22T00:29Z)

An issue was discovered in the Linux kernel through 4.17.2. vbg_misc_device_ioctl() in drivers/virt/vboxguest/vboxguest_linux.c reads the same user data twice with copy_from_user. The header part of the user data is double-fetched, and a malicious user thread can tamper with the critical variables (hdr.size_in and hdr.size_out) in the header between the two fetches because of a race condition, leading to severe kernel errors, such as buffer over-accesses. This bug can cause a local denial of service and information leakage.

Severity: MEDIUM
CVSS v2 base score: 6.3 (AV:L/AC:M/Au:N/C:C/I:N/A:C)
CVSS v3 base score: 6.3, MEDIUM (CVSS:3.0/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:N/A:H)

Weakness information:
cwe_id: CWE-362
cwe_name: Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')
description: The product contains a code sequence that can run concurrently with other code, and the code sequence requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence that is operating concurrently.
extended_description: 
            This can have security implications when the expected synchronization is in security-critical code, such as recording whether a user is authenticated or modifying important state information that should not be influenced by an outsider.
            A race condition occurs within concurrent environments, and is effectively a property of a code sequence. Depending on the context, a code sequence may be in the form of a function call, a small number of instructions, a series of program invocations, etc.
            A race condition violates these properties, which are closely related:
               
                  Exclusivity - the code sequence is given exclusive access to the shared resource, i.e., no other code sequence can modify properties of the shared resource before the original sequence has completed execution.
                  Atomicity - the code sequence is behaviorally atomic, i.e., no other thread or process can concurrently execute the same sequence of instructions (or a subset) against the same resource.
               
            A race condition exists when an "interfering code sequence" can still access the shared resource, violating exclusivity. Programmers may assume that certain code sequences execute too quickly to be affected by an interfering code sequence; when they are not, this violates atomicity. For example, the single "x++" statement may appear atomic at the code layer, but it is actually non-atomic at the instruction layer, since it involves a read (the original value of x), followed by a computation (x+1), followed by a write (save the result to x).
            The interfering code sequence could be "trusted" or "untrusted." A trusted interfering code sequence occurs within the product; it cannot be modified by the attacker, and it can only be invoked indirectly. An untrusted interfering code sequence can be authored directly by the attacker, and typically it is external to the vulnerable product.
         
         

Fix information:
repo_url: https://github.com/torvalds/linux
commit_details: repo_url: https://github.com/torvalds/linux
author: Wenwen Wang
author_date: 2018-05-08 08:50:28-05:00
msg: virt: vbox: Only copy_from_user the request-header once

In vbg_misc_device_ioctl(), the header of the ioctl argument is copied from
the userspace pointer 'arg' and saved to the kernel object 'hdr'. Then the
'version', 'size_in', and 'size_out' fields of 'hdr' are verified.

Before this commit, after the checks a buffer for the entire request would
be allocated and then all data including the verified header would be
copied from the userspace 'arg' pointer again.

Given that the 'arg' pointer resides in userspace, a malicious userspace
process can race to change the data pointed to by 'arg' between the two
copies. By doing so, the user can bypass the verifications on the ioctl
argument.

This commit fixes this by using the already checked copy of the header
to fill the header part of the allocated buffer and only copying the
remainder of the data from userspace.

Signed-off-by: Wenwen Wang <wang6495@umn.edu>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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: vboxguest_linux.c
change_type: ModificationType.MODIFY
diff: @@ -121,7 +121,9 @@ static long vbg_misc_device_ioctl(struct file *filp, unsigned int req,
 	if (!buf)
 		return -ENOMEM;
 
-	if (copy_from_user(buf, (void *)arg, hdr.size_in)) {
+	*((struct vbg_ioctl_hdr *)buf) = hdr;
+	if (copy_from_user(buf + sizeof(hdr), (void *)arg + sizeof(hdr),
+			   hdr.size_in - sizeof(hdr))) {
 		ret = -EFAULT;
 		goto out;
 	}

programming_language: C