CVE-2021-47226

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

x86/fpu: Invalidate FPU state after a failed XRSTOR from a user buffer

Both Intel and AMD consider it to be architecturally valid for XRSTOR to fail with #PF but nonetheless change the register state. The actual conditions under which this might occur are unclear [1], but it seems plausible that this might be triggered if one sibling thread unmaps a page and invalidates the shared TLB while another sibling thread is executing XRSTOR on the page in question.

__fpu__restore_sig() can execute XRSTOR while the hardware registers are preserved on behalf of a different victim task (using the fpu_fpregs_owner_ctx mechanism), and, in theory, XRSTOR could fail but modify the registers.

If this happens, then there is a window in which __fpu__restore_sig() could schedule out and the victim task could schedule back in without reloading its own FPU registers. This would result in part of the FPU state that __fpu__restore_sig() was attempting to load leaking into the victim tasks user-visible state.

Invalidate preserved FPU registers on XRSTOR failure to prevent this situation from corrupting any state.

[1] Frequent readers of the errata lists might imagine complex microarchitectural conditions.

Weakness

The product behaves differently or sends different responses under different circumstances in a way that is observable to an unauthorized actor, which exposes security-relevant information about the state of the product, such as whether a particular operation was successful or not.

Affected Software

Potential Mitigations

• Compartmentalize the system to have “safe” areas where trust boundaries can be unambiguously drawn. Do not allow sensitive data to go outside of the trust boundary and always be careful when interfacing with a compartment outside of the safe area.
• Ensure that appropriate compartmentalization is built into the system design, and the compartmentalization allows for and reinforces privilege separation functionality. Architects and designers should rely on the principle of least privilege to decide the appropriate time to use privileges and the time to drop privileges.
• Ensure that error messages only contain minimal details that are useful to the intended audience and no one else. The messages need to strike the balance between being too cryptic (which can confuse users) or being too detailed (which may reveal more than intended). The messages should not reveal the methods that were used to determine the error. Attackers can use detailed information to refine or optimize their original attack, thereby increasing their chances of success.
• If errors must be captured in some detail, record them in log messages, but consider what could occur if the log messages can be viewed by attackers. Highly sensitive information such as passwords should never be saved to log files.
• Avoid inconsistent messaging that might accidentally tip off an attacker about internal state, such as whether a user account exists or not.

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/189.html

References

• https://git.kernel.org/stable/c/002665dcba4bbec8c82f0aeb4bd3f44334ed2c14
• https://git.kernel.org/stable/c/a7748e021b9fb7739e3cb88449296539de0b6817
• https://git.kernel.org/stable/c/d8778e393afa421f1f117471144f8ce6deb6953a
• https://git.kernel.org/stable/c/002665dcba4bbec8c82f0aeb4bd3f44334ed2c14
• https://git.kernel.org/stable/c/a7748e021b9fb7739e3cb88449296539de0b6817
• https://git.kernel.org/stable/c/d8778e393afa421f1f117471144f8ce6deb6953a