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1.. _security-advisory-tfv-9:
2
3Advisory TFV-9 (CVE-2022-23960)
4============================================================
5
6+----------------+-------------------------------------------------------------+
7| Title          | Trusted Firmware-A exposure to speculative processor        |
8|                | vulnerabilities with branch prediction target reuse         |
9+================+=============================================================+
10| CVE ID         | `CVE-2022-23960`_                                           |
11+----------------+-------------------------------------------------------------+
12| Date           | 08 Mar 2022                                                 |
13+----------------+-------------------------------------------------------------+
14| Versions       | All, up to and including v2.6                               |
15| Affected       |                                                             |
16+----------------+-------------------------------------------------------------+
17| Configurations | All                                                         |
18| Affected       |                                                             |
19+----------------+-------------------------------------------------------------+
20| Impact         | Potential leakage of secure world data to normal world      |
21|                | if an attacker is able to find a TF-A exfiltration primitive|
22|                | that can be predicted as a valid branch target, and somehow |
23|                | induce misprediction onto that primitive. There are         |
24|                | currently no known exploits.                                |
25+----------------+-------------------------------------------------------------+
26| Fix Version    | `Gerrit topic #spectre_bhb`_                                |
27+----------------+-------------------------------------------------------------+
28| Credit         | Systems and Network Security Group at Vrije Universiteit    |
29|                | Amsterdam for CVE-2022-23960, Arm for patches               |
30+----------------+-------------------------------------------------------------+
31
32This security advisory describes the current understanding of the Trusted
33Firmware-A exposure to the new speculative processor vulnerability.
34To understand the background and wider impact of these vulnerabilities on Arm
35systems, please refer to the `Arm Processor Security Update`_. The whitepaper
36referred to below describes the Spectre attack and mitigation in more detail
37including implementation specific mitigation details for all impacted Arm CPUs.
38
39
40`CVE-2022-23960`_
41-----------------
42
43Where possible on vulnerable CPUs that implement FEAT_CSV2, Arm recommends
44inserting a loop workaround with implementation specific number of iterations
45that will discard the branch history on exception entry to a higher exception
46level for the given CPU. This is done as early as possible on entry into EL3,
47before any branch instruction is executed. This is sufficient to mitigate
48Spectre-BHB on behalf of all secure world code, assuming that no secure world
49code is under attacker control.
50
51The below table lists the CPUs that mitigate against this vulnerability in
52TF-A using the loop workaround(all cores that implement FEAT_CSV2 except the
53revisions of Cortex-A73 and Cortex-A75 that implements FEAT_CSV2).
54
55+----------------------+
56| Core                 |
57+----------------------+
58| Cortex-A72(from r1p0)|
59+----------------------+
60| Cortex-A76           |
61+----------------------+
62| Cortex-A76AE         |
63+----------------------+
64| Cortex-A77           |
65+----------------------+
66| Cortex-A78           |
67+----------------------+
68| Cortex-A78AE         |
69+----------------------+
70| Cortex-A78C          |
71+----------------------+
72| Cortex-X1            |
73+----------------------+
74| Cortex-X2            |
75+----------------------+
76| Cortex-X3            |
77+----------------------+
78| Cortex-A710          |
79+----------------------+
80| Cortex-A715          |
81+----------------------+
82| Neoverse-N1          |
83+----------------------+
84| Neoverse-N2          |
85+----------------------+
86| Neoverse-V1          |
87+----------------------+
88| Neoverse-V2          |
89+----------------------+
90
91For all other cores impacted by Spectre-BHB, some of which that do not implement
92FEAT_CSV2 and some that do e.g. Cortex-A73, the recommended mitigation is to
93flush all branch predictions via an implementation specific route.
94
95In case local workaround is not feasible, the Rich OS can invoke the SMC
96(``SMCCC_ARCH_WORKAROUND_3``) to apply the workaround. Refer to `SMCCC Calling
97Convention specification`_ for more details.
98
99`Gerrit topic #spectre_bhb`_ This patchset implements the Spectre-BHB loop
100workaround for CPUs mentioned in the above table. For CPUs supporting
101speculative barrier instruction, the loop workaround is optimised by using SB
102in place of the common DSB and ISB sequence. It also mitigates against
103this vulnerability for Cortex-A72 CPU versions that support the CSV2 feature
104(from r1p0). The patch stack also includes an implementation for a specified
105`CVE-2022-23960`_ workaround SMC(``SMCCC_ARCH_WORKAROUND_3``) for use by normal
106world privileged software. Details of ``SMCCC_ARCH_WORKAROUND_3`` can be found
107in the `SMCCC Calling Convention specification`_. The specification and
108implementation also enables the normal world to discover the presence of this
109firmware service. This patch also implements ``SMCCC_ARCH_WORKAROUND_3`` for
110Cortex-A57, Coxtex-A72, Cortex-A73 and Cortex-A75 using the existing workaround.
111for CVE-2017-5715. Cortex-A15 patch extends Spectre V2 mitigation to Spectre-BHB.
112
113The above workaround is enabled by default (on vulnerable CPUs only). Platforms
114can choose to disable them at compile time if they do not require them.
115
116For more information about non-Arm CPUs, please contact the CPU vendor.
117
118.. _Arm Processor Security Update: http://www.arm.com/security-update
119.. _CVE-2022-23960: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2022-23960
120.. _Gerrit topic #spectre_bhb: https://review.trustedfirmware.org/q/topic:"spectre_bhb"+(status:open%20OR%20status:merged)
121.. _CVE-2022-23960 mitigation specification: https://developer.arm.com/support/arm-security-updates/speculative-processor-vulnerability
122.. _SMCCC Calling Convention specification: https://developer.arm.com/documentation/den0028/latest
123