In the Linux kernel, the following vulnerability has been resolved:
arm64/fpsimd: signal: Allocate SSVE storage when restoring ZA
The code to restore a ZA context doesn't attempt to allocate the task's
sve_state before setting TIF_SME. Consequently, restoring a ZA context
can place a task into an invalid state where TIF_SME is set but the
task's sve_state is NULL.
In legitimate but uncommon cases where the ZA signal context was NOT
created by the kernel in the context of the same task (e.g. if the task
is saved/restored with something like CRIU), we have no guarantee that
sve_state had been allocated previously. In these cases, userspace can
enter streaming mode without trapping while sve_state is NULL, causing a
later NULL pointer dereference when the kernel attempts to store the
register state:
| # ./sigreturn-za
| Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000
| Mem abort info:
| ESR = 0x0000000096000046
| EC = 0x25: DABT (current EL), IL = 32 bits
| SET = 0, FnV = 0
| EA = 0, S1PTW = 0
| FSC = 0x06: level 2 translation fault
| Data abort info:
| ISV = 0, ISS = 0x00000046, ISS2 = 0x00000000
| CM = 0, WnR = 1, TnD = 0, TagAccess = 0
| GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
| user pgtable: 4k pages, 52-bit VAs, pgdp=0000000101f47c00
| [0000000000000000] pgd=08000001021d8403, p4d=0800000102274403, pud=0800000102275403, pmd=0000000000000000
| Internal error: Oops: 0000000096000046 [#1] SMP
| Modules linked in:
| CPU: 0 UID: 0 PID: 153 Comm: sigreturn-za Not tainted 6.19.0-rc1 #1 PREEMPT
| Hardware name: linux,dummy-virt (DT)
| pstate: 214000c9 (nzCv daIF +PAN -UAO -TCO +DIT -SSBS BTYPE=--)
| pc : sve_save_state+0x4/0xf0
| lr : fpsimd_save_user_state+0xb0/0x1c0
| sp : ffff80008070bcc0
| x29: ffff80008070bcc0 x28: fff00000c1ca4c40 x27: 63cfa172fb5cf658
| x26: fff00000c1ca5228 x25: 0000000000000000 x24: 0000000000000000
| x23: 0000000000000000 x22: fff00000c1ca4c40 x21: fff00000c1ca4c40
| x20: 0000000000000020 x19: fff00000ff6900f0 x18: 0000000000000000
| x17: fff05e8e0311f000 x16: 0000000000000000 x15: 028fca8f3bdaf21c
| x14: 0000000000000212 x13: fff00000c0209f10 x12: 0000000000000020
| x11: 0000000000200b20 x10: 0000000000000000 x9 : fff00000ff69dcc0
| x8 : 00000000000003f2 x7 : 0000000000000001 x6 : fff00000c1ca5b48
| x5 : fff05e8e0311f000 x4 : 0000000008000000 x3 : 0000000000000000
| x2 : 0000000000000001 x1 : fff00000c1ca5970 x0 : 0000000000000440
| Call trace:
| sve_save_state+0x4/0xf0 (P)
| fpsimd_thread_switch+0x48/0x198
| __switch_to+0x20/0x1c0
| __schedule+0x36c/0xce0
| schedule+0x34/0x11c
| exit_to_user_mode_loop+0x124/0x188
| el0_interrupt+0xc8/0xd8
| __el0_irq_handler_common+0x18/0x24
| el0t_64_irq_handler+0x10/0x1c
| el0t_64_irq+0x198/0x19c
| Code: 54000040 d51b4408 d65f03c0 d503245f (e5bb5800)
| ---[ end trace 0000000000000000 ]---
Fix this by having restore_za_context() ensure that the task's sve_state
is allocated, matching what we do when taking an SME trap. Any live
SVE/SSVE state (which is restored earlier from a separate signal
context) must be preserved, and hence this is not zeroed.
What the EG score is made of (scorer v4.0: the three questions the 0–10 score mixes, published apart · methodology)
The EG score (5.5): severity, escalated by confirmed exploitation. This CVE's 5.5 is its impact severity (EG Severity), not escalated.
- EG Severity
- 5.5 MEDIUMCVSS-B, impact only: no KEV listing, EPSS or EG-KEV tier raises it.GitHub Security Advisory CVSS
- EG Likelihood
- 0.2% within 30 daysSource: FIRST EPSS. FIRST's exploitation probability, not an EchelonGraph model.
- Confirmed exploitation
- None on record (CISA KEV, EG-KEV Tier 1)Reported apart from the likelihood: an observed fact, not a probability.
- EG Priority
- SSVC Track at medium mission impactTrack at low or medium mission impact; Track* at high (mission-essential systems).CISA BOD 26-04: 14 days if the affected system is publicly exposed; Fix on system upgrade if it is not.Each input and its source, and whom BOD 26-04 binds: the SSVC and BOD 26-04 blocks below.
SSVC (CISA's decision table, applied by EchelonGraph)Track at low or medium mission impact; Track* at high (mission-essential systems).
A fix is available. Apply it within your standard update timelines.
Exploitation none (no CISA value, KEV listing or exploit code on record) · Automatable no (CISA's BOD 26-04 default) · Technical impact total (CISA's BOD 26-04 default). EchelonGraph holds no SSVC assessment of this CVE from CISA: none of these inputs is from Vulnrichment. EchelonGraph applied CISA's decision table to them; CISA publishes SSVC inputs and the table, not a decision for each CVE. Mission impact is CISA's Mission & Well-being decision point, and only you can judge it. CISA's table makes it high when Mission Prevalence is Essential — the vulnerable component "directly provides capabilities that constitute at least one MEF for at least one entity" (MEF: mission essential function) — or when Public Well-Being Impact is Irreversible: "multiple fatalities are likely", the cyber-physical system "is likely lost or destroyed", "extreme or serious externalities" are imposed on other parties, or social systems such as elections or the financial grid "are destabilized and potentially collapse". CISA's decision table
BOD 26-04 (CISA's remediation timeline, Table 1)14 days if the affected system is publicly exposed; Fix on system upgrade if it is not.
The score's inputs: In CISA KEV no (CISA's KEV catalog) · Automatable no (CISA's BOD 26-04 default) · Technical impact total (CISA's BOD 26-04 default). Where an input reads “CISA's BOD 26-04 default”, no one has published it for this CVE, and CISA's implementation guidance says: “If “Automatable” and “Technical Impact” information items about a CVE ID are not available and the CVE ID is not in the KEV Catalog, CISA will treat its values as “no” and “total,” respectively, until CISA or a third party provides metadata about the CVE ID. CISA will always provide the Vulnrichment data for a CVE ID listed in the KEV Catalog.” Whether your system is publicly exposed is yours to answer. BOD 26-04 binds US Federal Civilian Executive Branch agencies; for anyone else it is CISA's published timeline, a reference rather than an obligation. The directive · how EchelonGraph applies it