Speculative Return Stack Overflow (SRSO) — The Linux Kernel 6.14.4-gnu-4-vanilla documentation

# Speculative Return Stack Overflow (SRSO)

This is a mitigation for the speculative return stack overflow (SRSO) vulnerability found on AMD processors. The mechanism is by now the well known scenario of poisoning CPU functional units - the Branch Target Buffer (BTB) and Return Address Predictor (RAP) in this case - and then tricking the elevated privilege domain (the kernel) into leaking sensitive data.

AMD CPUs predict RET instructions using a Return Address Predictor (aka Return Address Stack/Return Stack Buffer). In some cases, a non-architectural CALL instruction (i.e., an instruction predicted to be a CALL but is not actually a CALL) can create an entry in the RAP which may be used to predict the target of a subsequent RET instruction.

The specific circumstances that lead to this varies by microarchitecture but the concern is that an attacker can mis-train the CPU BTB to predict non-architectural CALL instructions in kernel space and use this to control the speculative target of a subsequent kernel RET, potentially leading to information disclosure via a speculative side-channel.

The issue is tracked under CVE-2023-20569.

## Affected processors

AMD Zen, generations 1-4. That is, all families 0x17 and 0x19. Older processors have not been investigated.

## System information and options

First of all, it is required that the latest microcode be loaded for mitigations to be effective.

The sysfs file showing SRSO mitigation status is:

> /sys/devices/system/cpu/vulnerabilities/spec_rstack_overflow

The possible values in this file are:

> - ‘Not affected’:

>   The processor is not vulnerable

- ‘Vulnerable’:

  > The processor is vulnerable and no mitigations have been applied.

> - ‘Vulnerable: No microcode’:

>   The processor is vulnerable, no microcode extending IBPB functionality to address the vulnerability has been applied.

> - ‘Vulnerable: Safe RET, no microcode’:

>   The “Safe RET” mitigation (see below) has been applied to protect the kernel, but the IBPB-extending microcode has not been applied. User space tasks may still be vulnerable.

> - ‘Vulnerable: Microcode, no safe RET’:

>   Extended IBPB functionality microcode patch has been applied. It does not address User->Kernel and Guest->Host transitions protection but it does address User->User and VM->VM attack vectors.

>   Note that User->User mitigation is controlled by how the IBPB aspect in the Spectre v2 mitigation is selected:

>   > - conditional IBPB:

>   >   where each process can select whether it needs an IBPB issued around it PR_SPEC_DISABLE/_ENABLE etc, see Spectre Side Channels

>   > - strict:

>   >   i.e., always on - by supplying spectre_v2_user=on on the kernel command line

>   (spec_rstack_overflow=microcode)

> - ‘Mitigation: Safe RET’:

>   Combined microcode/software mitigation. It complements the extended IBPB microcode patch functionality by addressing User->Kernel and Guest->Host transitions protection.

>   Selected by default or by spec_rstack_overflow=safe-ret

> - ‘Mitigation: IBPB’:

>   Similar protection as “safe RET” above but employs an IBPB barrier on privilege domain crossings (User->Kernel, Guest->Host).

> > (spec_rstack_overflow=ibpb)

> - ‘Mitigation: IBPB on VMEXIT’:

>   Mitigation addressing the cloud provider scenario - the Guest->Host transitions only.

>   (spec_rstack_overflow=ibpb-vmexit)

In order to exploit vulnerability, an attacker needs to:

> - gain local access on the machine

> - break kASLR

> - find gadgets in the running kernel in order to use them in the exploit

> - potentially create and pin an additional workload on the sibling thread, depending on the microarchitecture (not necessary on fam 0x19)

> - run the exploit

Considering the performance implications of each mitigation type, the default one is ‘Mitigation: safe RET’ which should take care of most attack vectors, including the local User->Kernel one.

As always, the user is advised to keep her/his system up-to-date by applying software updates regularly.

The default setting will be reevaluated when needed and especially when new attack vectors appear.

As one can surmise, ‘Mitigation: safe RET’ does come at the cost of some performance depending on the workload. If one trusts her/his userspace and does not want to suffer the performance impact, one can always disable the mitigation with spec_rstack_overflow=off.

Similarly, ‘Mitigation: IBPB’ is another full mitigation type employing an indirect branch prediction barrier after having applied the required microcode patch for one’s system. This mitigation comes also at a performance cost.

## Mitigation: Safe RET

The mitigation works by ensuring all RET instructions speculate to a controlled location, similar to how speculation is controlled in the retpoline sequence. To accomplish this, the __x86_return_thunk forces the CPU to mispredict every function return using a ‘safe return’ sequence.

To ensure the safety of this mitigation, the kernel must ensure that the safe return sequence is itself free from attacker interference. In Zen3 and Zen4, this is accomplished by creating a BTB alias between the untraining function srso_alias_untrain_ret() and the safe return function srso_alias_safe_ret() which results in evicting a potentially poisoned BTB entry and using that safe one for all function returns.

In older Zen1 and Zen2, this is accomplished using a reinterpretation technique similar to Retbleed one: srso_untrain_ret() and srso_safe_ret().

## Checking the safe RET mitigation actually works

In case one wants to validate whether the SRSO safe RET mitigation works on a kernel, one could use two performance counters

- PMC_0xc8 - Count of RET/RET lw retired

- PMC_0xc9 - Count of RET/RET lw retired mispredicted

and compare the number of RETs retired properly vs those retired mispredicted, in kernel mode. Another way of specifying those events is:

    # perf list ex_ret_near_ret

    List of pre-defined events (to be used in -e or -M):

    core:
      ex_ret_near_ret
           [Retired Near Returns]
      ex_ret_near_ret_mispred
           [Retired Near Returns Mispredicted]

Either the command using the event mnemonics:

    # perf stat -e ex_ret_near_ret:k -e ex_ret_near_ret_mispred:k sleep 10s

or using the raw PMC numbers:

    # perf stat -e cpu/event=0xc8,umask=0/k -e cpu/event=0xc9,umask=0/k sleep 10s

should give the same amount. I.e., every RET retired should be mispredicted:

    [root@brent: ~/kernel/linux/tools/perf> ./perf stat -e cpu/event=0xc8,umask=0/k -e cpu/event=0xc9,umask=0/k sleep 10s

     Performance counter stats for 'sleep 10s':

               137,167      cpu/event=0xc8,umask=0/k
               137,173      cpu/event=0xc9,umask=0/k

          10.004110303 seconds time elapsed

           0.000000000 seconds user
           0.004462000 seconds sys

vs the case when the mitigation is disabled (spec_rstack_overflow=off) or not functioning properly, showing usually a lot smaller number of mispredicted retired RETs vs the overall count of retired RETs during a workload:

    [root@brent: ~/kernel/linux/tools/perf> ./perf stat -e cpu/event=0xc8,umask=0/k -e cpu/event=0xc9,umask=0/k sleep 10s

     Performance counter stats for 'sleep 10s':

               201,627      cpu/event=0xc8,umask=0/k
                 4,074      cpu/event=0xc9,umask=0/k

          10.003267252 seconds time elapsed

           0.002729000 seconds user
           0.000000000 seconds sys

Also, there is a selftest which performs the above, go to tools/testing/selftests/x86/ and do:

    make srso
    ./srso