CVE-2026-23102

In the Linux kernel, the following vulnerability has been resolved:

arm64/fpsimd: signal: Fix restoration of SVE context

When SME is supported, Restoring SVE signal context can go wrong in a few ways, including placing the task into an invalid state where the kernel may read from out-of-bounds memory (and may potentially take a fatal fault) and/or may kill the task with a SIGKILL.

(1) Restoring a context with SVE_SIG_FLAG_SM set can place the task into an invalid state where SVCR.SM is set (and sve_state is non-NULL) but TIF_SME is clear, consequently resuting in out-of-bounds memory reads and/or killing the task with SIGKILL.

This can only occur in unusual (but legitimate) cases where the SVE
signal context has either been modified by userspace or was saved in
the context of another task (e.g. as with CRIU), as otherwise the
presence of an SVE signal context with SVE_SIG_FLAG_SM implies that
TIF_SME is already set.

While in this state, task_fpsimd_load() will NOT configure SMCR_ELx
(leaving some arbitrary value configured in hardware) before
restoring SVCR and attempting to restore the streaming mode SVE
registers from memory via sve_load_state(). As the value of
SMCR_ELx.LEN may be larger than the tasks streaming SVE vector
length, this may read memory outside of the tasks allocated
sve_state, reading unrelated data and/or triggering a fault.

While this can result in secrets being loaded into streaming SVE
registers, these values are never exposed. As TIF_SME is clear,
fpsimd_bind_task_to_cpu() will configure CPACR_ELx.SMEN to trap EL0
accesses to streaming mode SVE registers, so these cannot be
accessed directly at EL0. As fpsimd_save_user_state() verifies the
live vector length before saving (S)SVE state to memory, no secret
values can be saved back to memory (and hence cannot be observed via
ptrace, signals, etc).

When the live vector length doesnt match the expected vector length
for the task, fpsimd_save_user_state() will send a fatal SIGKILL
signal to the task. Hence the task may be killed after executing
userspace for some period of time.

(2) Restoring a context with SVE_SIG_FLAG_SM clear does not clear the tasks SVCR.SM. If SVCR.SM was set prior to restoring the context, then the task will be left in streaming mode unexpectedly, and some register state will be combined inconsistently, though the task will be left in legitimate state from the kernels PoV.

This can only occur in unusual (but legitimate) cases where ptrace
has been used to set SVCR.SM after entry to the sigreturn syscall,
as syscall entry clears SVCR.SM.

In these cases, the the provided SVE register data will be loaded
into the tasks sve_state using the non-streaming SVE vector length
and the FPSIMD registers will be merged into this using the
streaming SVE vector length.

Fix (1) by setting TIF_SME when setting SVCR.SM. This also requires ensuring that the tasks sme_state has been allocated, but as this could contain live ZA state, it should not be zeroed. Fix (2) by clearing SVCR.SM when restoring a SVE signal context with SVE_SIG_FLAG_SM clear.

For consistency, Ive pulled the manipulation of SVCR, TIF_SVE, TIF_SME, and fp_type earlier, immediately after the allocation of sve_state/sme_state, before the restore of the actual register state. This makes it easier to ensure that these are always modified consistently, even if a fault is taken while reading the register data from the signal context. I do not expect any software to depend on the exact state restored when a fault is taken while reading the context.

Weakness

The product reads data past the end, or before the beginning, of the intended buffer.

Affected Software

Potential Mitigations

• Assume all input is malicious. Use an “accept known good” input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.
• When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, “boat” may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as “red” or “blue.”
• Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code’s environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
• To reduce the likelihood of introducing an out-of-bounds read, ensure that you validate and ensure correct calculations for any length argument, buffer size calculation, or offset. Be especially careful of relying on a sentinel (i.e. special character such as NUL) in untrusted inputs.

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/540.html

References

• https://git.kernel.org/stable/c/7b5a52cf252a0d2e89787b645290ad288878f332
• https://git.kernel.org/stable/c/9bc3adba8c35119be80ab20217027720446742f2
• https://git.kernel.org/stable/c/ce820dd4e6e2d711242dc4331713b9bb4fe06d09
• https://git.kernel.org/stable/c/d2907cbe9ea0a54cbe078076f9d089240ee1e2d9