Bug 1750410 (CVE-2019-15919) - CVE-2019-15919 kernel: use-after-free in SMB2_write function in fs/cifs/smb2pdu.c
Summary: CVE-2019-15919 kernel: use-after-free in SMB2_write function in fs/cifs/smb2p...
Keywords:
Status: CLOSED ERRATA
Alias: CVE-2019-15919
Product: Security Response
Classification: Other
Component: vulnerability
Version: unspecified
Hardware: All
OS: Linux
medium
medium
Target Milestone: ---
Assignee: Red Hat Product Security
QA Contact:
URL:
Whiteboard:
Depends On: 1750411 1782009 1782010
Blocks: 1750412
TreeView+ depends on / blocked
 
Reported: 2019-09-09 14:35 UTC by Marian Rehak
Modified: 2021-10-25 09:53 UTC (History)
41 users (show)

Fixed In Version:
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An flaw was discovered in the Linux kernel's CIFS client implementation. While issuing an SMB2_write, a value can be used after it was intended to be freed when CIFS function tracing is enabled. Even though the data is used after being freed, using it to for privilege escalation does not seem possible.
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Environment:
Last Closed: 2021-10-25 09:53:30 UTC
Embargoed:

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Description Marian Rehak 2019-09-09 14:35:09 UTC 
An issue was discovered in the Linux kernels CIFS client implementation.
While issuing an SMB2_write a value can be used after it was intended to be freed when CIFS function tracing is enabled.

While no privilege escalation is immediately obvious, Red Hat will not rule out that it may be possible.
Comment 1 Marian Rehak 2019-09-09 14:35:27 UTC 
Created kernel tracking bugs for this issue:

Affects: fedora-all [bug 1750411]
Comment 2 Marian Rehak 2019-09-09 14:36:04 UTC 
Upstream Issue:

https://github.com/torvalds/linux/commit/6a3eb3360667170988f8a6477f6686242061488a
Comment 3 Justin M. Forbes 2019-09-10 06:37:28 UTC 
This was fixed for Fedora with the 5.0.10 stable kernel updates.
Comment 7 Wade Mealing 2019-12-10 06:06:59 UTC 
So, taking a look at the context of this apparent UAF.

	   rc = cifs_send_recv(xid, io_parms->tcon->ses, &rqst,
			       &resp_buftype, flags, &rsp_iov);

	   cifs_small_buf_release(req); <-- FREE HERE.

	   rsp = (struct smb2_write_rsp *)rsp_iov.iov_base;

	   if (rc) {
		   trace_smb3_write_err(xid, req->PersistentFileId,
					io_parms->tcon->tid,
					io_parms->tcon->ses->Suid,
					io_parms->offset, io_parms->length, rc);
		   cifs_stats_fail_inc(io_parms->tcon, SMB2_WRITE_HE);
		   cifs_dbg(VFS, "Send error in write = %d\n", rc);
	   } else {
		   *nbytes = le32_to_cpu(rsp->DataLength);
		   trace_smb3_write_done(xid, req->PersistentFileId,
					io_parms->tcon->tid,
					io_parms->tcon->ses->Suid,
					io_parms->offset, *nbytes);
	   }

	   free_rsp_buf(resp_buftype, rsp);
	   return rc;
   }


So, we can see that its used in trace_smb3_write_err, which is part of the cifs tracing subsystem (not enabled by default) in the kernel. In this example all it does is print out the req->PersistentFileId.  If an atacker can groom it between free, this may be a null pointer deference, or an information leak.. But I don't see how it can be privesc.

The fix was simply to move the free code after the use below...
Comment 10 Eric Christensen 2019-12-11 19:46:13 UTC 
Mitigation:

As the CIFS module will be auto-loaded when required, its use can be disabled by preventing the module from loading with the following instructions:

  # echo "install cifs /bin/true" >> /etc/modprobe.d/disable-cifs.conf 
 
The system will need to be restarted if the CIFS modules are loaded. In most circumstances, the CIFS kernel modules will be unable to be unloaded while any network interfaces are active and the protocol is in use.

If you need further assistance, see KCS article https://access.redhat.com/solutions/41278 or contact Red Hat Global Support Services.
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