CVE-2025-39802

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

lib/crypto: arm/poly1305: Fix register corruption in no-SIMD contexts

Restore the SIMD usability check that was removed by commit 773426f4771b ("crypto: arm/poly1305 - Add block-only interface").

This safety check is cheap and is well worth eliminating a footgun. While the Poly1305 functions should not be called when SIMD registers are unusable, if they are anyway, they should just do the right thing instead of corrupting random tasks' registers and/or computing incorrect MACs. Fixing this is also needed for poly1305_kunit to pass.

Just use may_use_simd() instead of the original crypto_simd_usable(), since poly1305_kunit won't rely on crypto_simd_disabled_for_test.

This flaw does not affect Red Hat Enterprise Linux as the vulnerable code was introduced in an upstream kernel release not shipped in Red Hat products. Additionally, the issue is restricted to 32-bit ARM implementations of the Poly1305 cryptographic algorithm, which is not an active architecture tier for Red Hat enterprise kernels. Consequently, Red Hat supported environments are not exposed to register corruption or authentication tag miscalculation risks.