QSB-089: Qrexec: Memory corruption in service request handling

system · May 11, 2023, 10:11pm

We have published Qubes Security Bulletin (QSB) 089: Qrexec: Memory corruption in service request handling. The text of this QSB and its accompanying cryptographic signatures are reproduced below. For an explanation of this announcement and instructions for authenticating this QSB, please see the end of this announcement.

Qubes Security Bulletin 089

             ---===[ Qubes Security Bulletin 089 ]===---

                             2023-05-11

       Qrexec: Memory corruption in service request handling

User action required
---------------------

Users must install the following specific packages in order to address
the issues discussed in this bulletin:

  For Qubes 4.1, in dom0:
  - qrexec packages, version 4.1.21

These packages will migrate from the security-testing repository to the
current (stable) repository over the next two weeks after being tested
by the community. [1] Once available, the packages are to be installed
via the Qubes Update tool or its command-line equivalents. [2]

Summary
--------

Due to a bug in qrexec [3], a malicious qube can cause memory corruption
in the qrexec-daemon. The Qubes Security Team is not aware of any way to
exploit this vulnerability in an attack (not even in a denial-of-service
attack that only crashes the process). However, we cannot completely
rule out such a possibility.

Impact
-------

While we consider the successful exploitation of this vulnerability to
be very unlikely, an attacker could theoretically use it to crash the
qrexec-daemon or execute arbitrary code in dom0.

Discussion
-----------

Qubes OS features a framework known as "qrexec," which allows different
qubes to communicate with each other in a controlled manner. [3][4]
These interactions are restricted by the system's RPC policies. [5] In
particular, qrexec can be used to allow less trusted qubes to
communicate with more trusted qubes, including dom0.

Incoming RPC calls are handled by the qrexec-daemon process. Qubes OS
4.1 introduced a new qrexec message type, `MSG_TRIGGER_SERVICE3`, which
allows much larger requests (theoretically up to 65000 bytes, compared
to 64 bytes in earlier versions). This message type uses a
dynamically-allocated buffer for the message body based on the request
length. The code used to validate the request length has an off-by-one
error that can cause memory corruption, as described below.

First, the incoming message is validated in the
`sanitize_message_from_agent()` function:

    1177 static void sanitize_message_from_agent(struct msg_header *untrusted_header)
    1178 {
    1179     switch (untrusted_header->type) {
    ...
    1191         case MSG_TRIGGER_SERVICE3:
    1192             if (protocol_version < QREXEC_PROTOCOL_V3) {
    1193                 LOG(ERROR, "agent sent (new) MSG_TRIGGER_SERVICE3 "
    1194                     "although it uses protocol %d", protocol_version);
    1195                 exit(1);
    1196             }
    1197             if (untrusted_header->len < sizeof(struct trigger_service_params3)) {
    1198                 LOG(ERROR, "agent sent invalid MSG_TRIGGER_SERVICE3 packet");
    1199                 exit(1);
    1200             }
    1201             if (untrusted_header->len - sizeof(struct trigger_service_params3)
    1202                     > MAX_SERVICE_NAME_LEN) {
    1203                 LOG(ERROR, "agent sent too large MSG_TRIGGER_SERVICE3 packet");
    1204                 exit(1);
    1205             }
    1206             break;
    ...

The second condition, on line 1197, verifies that the message sent by
the qrexec-agent (from a VM) is not shorter than the message header
defined in `struct trigger_service_params3`. However, it fails to
account for a mandatory NUL character at the end of the message payload
(the service name and its argument). Later on, the
`handle_message_from_agent()` function processes the message as follows:

    1222 static void handle_message_from_agent(void)
    1223 {
    1224     struct msg_header hdr, untrusted_hdr;
    1225     struct trigger_service_params untrusted_params, params;
    1226     struct trigger_service_params3 untrusted_params3, params3;
    1227     char *untrusted_service_name = NULL, *service_name = NULL;
    1228     size_t service_name_len;
    1229
    1230     if (libvchan_recv(vchan, &untrusted_hdr, sizeof(untrusted_hdr))
    1231             != sizeof(untrusted_hdr))
    1232         handle_vchan_error("recv hdr");
    1233     /* sanitize start */
    1234     sanitize_message_from_agent(&untrusted_hdr);
    1235     hdr = untrusted_hdr;
    1236     /* sanitize end */
    ...
    1241     switch (hdr.type) {
    ...
    1262         case MSG_TRIGGER_SERVICE3:
    1263             service_name_len = hdr.len - sizeof(untrusted_params3);
    1264             untrusted_service_name = malloc(service_name_len);
    1265             if (!untrusted_service_name)
    1266                 handle_vchan_error("malloc(service_name)");
    1267
    1268             if (libvchan_recv(vchan, &untrusted_params3, sizeof(untrusted_params3))
    1269                     != sizeof(untrusted_params3))
    1270                 handle_vchan_error("recv params3");
    1271             if (libvchan_recv(vchan, untrusted_service_name, service_name_len)
    1272                     != (int)service_name_len)
    1273                 handle_vchan_error("recv params3(service_name)");
    1274
    1275             /* sanitize start */
    1276             ENSURE_NULL_TERMINATED(untrusted_params3.target_domain);
    1277             ENSURE_NULL_TERMINATED(untrusted_params3.request_id.ident);
    1278             untrusted_service_name[service_name_len-1] = 0;
    1279             sanitize_name(untrusted_params3.target_domain, "@:");
    1280             sanitize_name(untrusted_params3.request_id.ident, " ");
    1281             sanitize_name(untrusted_service_name, "+");
    1282             params3 = untrusted_params3;
    1283             service_name = untrusted_service_name;
    1284             untrusted_service_name = NULL;
    1285             /* sanitize end */
    ...

The initial call to `sanitize_message_from_agent()` is visible in line
1234. Then, the function calculates the expected service name length in
line 1263, allocates memory for it in line 1264, and receives both the
rest of the header and its payload in lines 1268-1273. Since
`sanitize_message_from_agent()` allows the `hdr.len` to be equal to
`sizeof(untrusted_params3)`, `service_name_len` can be zero. This means
that adding the terminating NUL character in line 1278 can write outside
of the allocated buffer. Furthermore, in such a case, the
`untrusted_service_name` buffer is allocated with a size of zero, and
the `sanitize_name()` call in line 1281 can write beyond the buffer too.

The `sanitize_name()` function, listed below, replaces disallowed
characters with underscores (byte 0x5f) until it finds the terminating
NUL character:

    759 static void sanitize_name(char * untrusted_s_signed, char *extra_allowed_chars)
    760 {
    761     unsigned char * untrusted_s;
    762     for (untrusted_s=(unsigned char*)untrusted_s_signed; *untrusted_s; untrusted_s++) {
    763         if (*untrusted_s >= 'a' && *untrusted_s <= 'z')
    764             continue;
    765         if (*untrusted_s >= 'A' && *untrusted_s <= 'Z')
    766             continue;
    767         if (*untrusted_s >= '0' && *untrusted_s <= '9')
    768             continue;
    769         if (*untrusted_s == '_' ||
    770                *untrusted_s == '-' ||
    771                *untrusted_s == '.')
    772             continue;
    773         if (extra_allowed_chars && strchr(extra_allowed_chars, *untrusted_s))
    774             continue;
    775         *untrusted_s = '_';
    776     }
    777 }

This code runs in dom0. In Qubes OS 4.1, dom0 is based on Fedora 32
x86_64, which uses the GNU libc 2.31 library. In this implementation
[5], the `malloc()` call (line 1264) always allocates at least 32 bytes,
even if the requested size is 0. This means that the returned pointer is
not NULL in this case (which would be allowed by the specification for
zero-sized allocations), so the error handling in line 1266 does not
interrupt the processing. Consequently, the NUL character written in
line 1278 hits part of the malloc metadata, specifically the most
significant byte of the allocation size. Given that the size is small
(between the 32-byte `MIN_CHUNK_SIZE` and the 1024-byte
`MIN_LARGE_SIZE`), that byte is always zero already, so this write is
harmless.

The remaining concern is about the `sanitize_name()` function. For the
bug to be harmful, it must overwrite something beyond the allocated
buffer (at least 32 bytes). This means that there cannot be a zero byte
in this area. In our analysis, this is very unlikely to occur, for the
following reasons:

1. The `malloc()` call in `handle_message_from_agent()` is the one and
   only call that occurs outside of startup and connection setup. No
   other malloc call occurs in that process, not even an indirect one.
   The connection setup allocations can be redone when the qrexec-agent
   disconnects and connects again, but the points below also apply to
   this reconnection sequence. (Perhaps the only exception is handling
   an error message just before exit(1), but if the attacker hasn't
   corrupted malloc metadata up to this point, the attacker won't have a
   chance to do it after this point either.)

2. The qrexec-daemon handles only a single request at a time. At no
   point are multiple buffers for `service_name` allocated in the
   process at the same time. All remaining handling occurs in a separate
   process (which also handles only a single request).

3. Due to how GNU libc's malloc works, if there is a small chunk
   available, it will be used. It won't be split out of a bigger chunk.
   And, due to point 2 above, if there was a small chunk initially, it
   will get used again.

4. The allocations done during startup and reconnection leave a few
   small free chunks that are interleaved with allocated areas (the
   libvchan_t structure, something from libxengnttab, libxentoollog, and
   a few others). These memory chunks can't be merged due to their
   layout.

5. Every small allocation includes a NUL byte somewhere in the payload
   before being freed. This applies to both initial areas as well as to
   the buffers used for `service_name`.

Given all of these considerations, successfully exploiting this
vulnerability in an attack is very unlikely. However, due to the
complexity of the memory allocator, we cannot completely rule out such a
possibility.

Credits
--------

This issue was discovered by Demi Marie Obenour.

References
-----------

[1] https://www.qubes-os.org/doc/testing/
[2] https://www.qubes-os.org/doc/how-to-update/
[3] https://www.qubes-os.org/doc/qrexec/
[4] https://www.qubes-os.org/doc/qrexec-internals/
[5] https://sourceware.org/glibc/wiki/MallocInternals

--
The Qubes Security Team
https://www.qubes-os.org/security/

Source: https://github.com/QubesOS/qubes-secpack/blob/master/QSBs/qsb-089-2023.txt

Marek Marczykowski-Górecki’s PGP signature

Source: https://github.com/QubesOS/qubes-secpack/blob/master/QSBs/qsb-089-2023.txt.sig.marmarek

Simon Gaiser (aka HW42)’s PGP signature

Source: https://github.com/QubesOS/qubes-secpack/blob/master/QSBs/qsb-089-2023.txt.sig.simon

What is the purpose of this announcement?

The purpose of this announcement is to inform the Qubes community that a new Qubes Security Bulletin (QSB) has been published.

What is a Qubes Security Bulletin (QSB)?

A Qubes security bulletin (QSB) is a security announcement issued by the Qubes security team. A QSB typically provides a summary and impact analysis of one or more recently-discovered software vulnerabilities, including details about patching to address them. A list of all QSBs is available here.

Why should I care about QSBs?

QSBs tell you what actions you must take in order to protect yourself from recently-discovered security vulnerabilities. In most cases, security vulnerabilities are addressed by updating normally. However, in some cases, special user action is required. In all cases, the required actions are detailed in QSBs.

What are the PGP signatures that accompany QSBs?

A PGP signature is a cryptographic digital signature made in accordance with the OpenPGP standard. PGP signatures can be cryptographically verified with programs like GNU Privacy Guard (GPG). The Qubes security team cryptographically signs all QSBs so that Qubes users have a reliable way to check whether QSBs are genuine. The only way to be certain that a QSB is authentic is by verifying its PGP signatures.

Why should I care whether a QSB is authentic?

A forged QSB could deceive you into taking actions that adversely affect the security of your Qubes OS system, such as installing malware or making configuration changes that render your system vulnerable to attack. Falsified QSBs could sow fear, uncertainty, and doubt about the security of Qubes OS or the status of the Qubes OS Project.

How do I verify the PGP signatures on a QSB?

The following command-line instructions assume a Linux system with git and gpg installed. (See here for Windows and Mac options.)

Obtain the Qubes Master Signing Key (QMSK), e.g.:

$ gpg --fetch-keys https://keys.qubes-os.org/keys/qubes-master-signing-key.asc
gpg: directory '/home/user/.gnupg' created
gpg: keybox '/home/user/.gnupg/pubring.kbx' created
gpg: requesting key from 'https://keys.qubes-os.org/keys/qubes-master-signing-key.asc'
gpg: /home/user/.gnupg/trustdb.gpg: trustdb created
gpg: key DDFA1A3E36879494: public key "Qubes Master Signing Key" imported
gpg: Total number processed: 1
gpg:               imported: 1

(See here for more ways to obtain the QMSK.)

View the fingerprint of the PGP key you just imported. (Note: gpg> indicates a prompt inside of the GnuPG program. Type what appears after it when prompted.)

$ gpg --edit-key 0x427F11FD0FAA4B080123F01CDDFA1A3E36879494
gpg (GnuPG) 2.2.27; Copyright (C) 2021 Free Software Foundation, Inc.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
   
   
pub  rsa4096/DDFA1A3E36879494
     created: 2010-04-01  expires: never       usage: SC
     trust: unknown       validity: unknown
[ unknown] (1). Qubes Master Signing Key
   
gpg> fpr
pub   rsa4096/DDFA1A3E36879494 2010-04-01 Qubes Master Signing Key
 Primary key fingerprint: 427F 11FD 0FAA 4B08 0123  F01C DDFA 1A3E 3687 9494

Important: At this point, you still don’t know whether the key you just imported is the genuine QMSK or a forgery. In order for this entire procedure to provide meaningful security benefits, you must authenticate the QMSK out-of-band. Do not skip this step! The standard method is to obtain the QMSK fingerprint from multiple independent sources in several different ways and check to see whether they match the key you just imported. See here for more details and ideas for how to do that.

Tip: Record the genuine QMSK fingerprint in a safe place (or several) so that you don’t have to repeat this step in the future.

Once you are satisfied that you have the genuine QMSK, set its trust level to 5 (“ultimate”), then quit GnuPG with q.

gpg> trust
pub  rsa4096/DDFA1A3E36879494
     created: 2010-04-01  expires: never       usage: SC
     trust: unknown       validity: unknown
[ unknown] (1). Qubes Master Signing Key
   
Please decide how far you trust this user to correctly verify other users' keys
(by looking at passports, checking fingerprints from different sources, etc.)
   
  1 = I don't know or won't say
  2 = I do NOT trust
  3 = I trust marginally
  4 = I trust fully
  5 = I trust ultimately
  m = back to the main menu
   
Your decision? 5
Do you really want to set this key to ultimate trust? (y/N) y
   
pub  rsa4096/DDFA1A3E36879494
     created: 2010-04-01  expires: never       usage: SC
     trust: ultimate      validity: unknown
[ unknown] (1). Qubes Master Signing Key
Please note that the shown key validity is not necessarily correct
unless you restart the program.
   
gpg> q

Use Git to clone the qubes-secpack repo.

$ git clone https://github.com/QubesOS/qubes-secpack.git
Cloning into 'qubes-secpack'...
remote: Enumerating objects: 4065, done.
remote: Counting objects: 100% (1474/1474), done.
remote: Compressing objects: 100% (742/742), done.
remote: Total 4065 (delta 743), reused 1413 (delta 731), pack-reused 2591
Receiving objects: 100% (4065/4065), 1.64 MiB | 2.53 MiB/s, done.
Resolving deltas: 100% (1910/1910), done.

Import the included PGP keys. (See our PGP key policies for important information about these keys.)

$ gpg --import qubes-secpack/keys/*/*
gpg: key 063938BA42CFA724: public key "Marek Marczykowski-Górecki (Qubes OS signing key)" imported
gpg: qubes-secpack/keys/core-devs/retired: read error: Is a directory
gpg: no valid OpenPGP data found.
gpg: key 8C05216CE09C093C: 1 signature not checked due to a missing key
gpg: key 8C05216CE09C093C: public key "HW42 (Qubes Signing Key)" imported
gpg: key DA0434BC706E1FCF: public key "Simon Gaiser (Qubes OS signing key)" imported
gpg: key 8CE137352A019A17: 2 signatures not checked due to missing keys
gpg: key 8CE137352A019A17: public key "Andrew David Wong (Qubes Documentation Signing Key)" imported
gpg: key AAA743B42FBC07A9: public key "Brennan Novak (Qubes Website & Documentation Signing)" imported
gpg: key B6A0BB95CA74A5C3: public key "Joanna Rutkowska (Qubes Documentation Signing Key)" imported
gpg: key F32894BE9684938A: public key "Marek Marczykowski-Górecki (Qubes Documentation Signing Key)" imported
gpg: key 6E7A27B909DAFB92: public key "Hakisho Nukama (Qubes Documentation Signing Key)" imported
gpg: key 485C7504F27D0A72: 1 signature not checked due to a missing key
gpg: key 485C7504F27D0A72: public key "Sven Semmler (Qubes Documentation Signing Key)" imported
gpg: key BB52274595B71262: public key "unman (Qubes Documentation Signing Key)" imported
gpg: key DC2F3678D272F2A8: 1 signature not checked due to a missing key
gpg: key DC2F3678D272F2A8: public key "Wojtek Porczyk (Qubes OS documentation signing key)" imported
gpg: key FD64F4F9E9720C4D: 1 signature not checked due to a missing key
gpg: key FD64F4F9E9720C4D: public key "Zrubi (Qubes Documentation Signing Key)" imported
gpg: key DDFA1A3E36879494: "Qubes Master Signing Key" not changed
gpg: key 1848792F9E2795E9: public key "Qubes OS Release 4 Signing Key" imported
gpg: qubes-secpack/keys/release-keys/retired: read error: Is a directory
gpg: no valid OpenPGP data found.
gpg: key D655A4F21830E06A: public key "Marek Marczykowski-Górecki (Qubes security pack)" imported
gpg: key ACC2602F3F48CB21: public key "Qubes OS Security Team" imported
gpg: qubes-secpack/keys/security-team/retired: read error: Is a directory
gpg: no valid OpenPGP data found.
gpg: key 4AC18DE1112E1490: public key "Simon Gaiser (Qubes Security Pack signing key)" imported
gpg: Total number processed: 17
gpg:               imported: 16
gpg:              unchanged: 1
gpg: marginals needed: 3  completes needed: 1  trust model: pgp
gpg: depth: 0  valid:   1  signed:   6  trust: 0-, 0q, 0n, 0m, 0f, 1u
gpg: depth: 1  valid:   6  signed:   0  trust: 6-, 0q, 0n, 0m, 0f, 0u

Verify signed Git tags.

$ cd qubes-secpack/
$ git tag -v `git describe`
object 266e14a6fae57c9a91362c9ac784d3a891f4d351
type commit
tag marmarek_sec_266e14a6
tagger Marek Marczykowski-Górecki 1677757924 +0100
   
Tag for commit 266e14a6fae57c9a91362c9ac784d3a891f4d351
gpg: Signature made Thu 02 Mar 2023 03:52:04 AM PST
gpg:                using RSA key 2D1771FE4D767EDC76B089FAD655A4F21830E06A
gpg: Good signature from "Marek Marczykowski-Górecki (Qubes security pack)" [full]

The exact output will differ, but the final line should always start with gpg: Good signature from... followed by an appropriate key. The [full] indicates full trust, which this key inherits in virtue of being validly signed by the QMSK.

Verify PGP signatures, e.g.:

$ cd QSBs/
$ gpg --verify qsb-087-2022.txt.sig.marmarek qsb-087-2022.txt
gpg: Signature made Wed 23 Nov 2022 04:05:51 AM PST
gpg:                using RSA key 2D1771FE4D767EDC76B089FAD655A4F21830E06A
gpg: Good signature from "Marek Marczykowski-Górecki (Qubes security pack)" [full]
$ gpg --verify qsb-087-2022.txt.sig.simon qsb-087-2022.txt
gpg: Signature made Wed 23 Nov 2022 03:50:42 AM PST
gpg:                using RSA key EA18E7F040C41DDAEFE9AA0F4AC18DE1112E1490
gpg: Good signature from "Simon Gaiser (Qubes Security Pack signing key)" [full]
$ cd ../canaries/
$ gpg --verify canary-034-2023.txt.sig.marmarek canary-034-2023.txt
gpg: Signature made Thu 02 Mar 2023 03:51:48 AM PST
gpg:                using RSA key 2D1771FE4D767EDC76B089FAD655A4F21830E06A
gpg: Good signature from "Marek Marczykowski-Górecki (Qubes security pack)" [full]
$ gpg --verify canary-034-2023.txt.sig.simon canary-034-2023.txt
gpg: Signature made Thu 02 Mar 2023 01:47:52 AM PST
gpg:                using RSA key EA18E7F040C41DDAEFE9AA0F4AC18DE1112E1490
gpg: Good signature from "Simon Gaiser (Qubes Security Pack signing key)" [full]

Again, the exact output will differ, but the final line of output from each gpg --verify command should always start with gpg: Good signature from... followed by an appropriate key.

For this announcement (QSB-089), the commands are:

$ gpg --verify qsb-089-2023.txt.sig.marmarek qsb-089-2023.txt
$ gpg --verify qsb-089-2023.txt.sig.simon qsb-089-2023.txt

You can also verify the signatures directly from this announcement in addition to or instead of verifying the files from the qubes-secpack. Simply copy and paste the QSB-089 text into a plain text file and do the same for both signature files. Then, perform the same authentication steps as listed above, substituting the filenames above with the names of the files you just created.

This is a companion discussion topic for the original entry at https://www.qubes-os.org/news/2023/05/11/qsb-089/