The Linux kernel before 2.6.16.9 and the FreeBSD kernel, when running on AMD64 and other 7th and 8th generation AuthenticAMD processors, only save/restore the FOP, FIP, and FDP x87 registers in FXSAVE/FXRSTOR when an exception is pending, which allows one process to determine portions of the state of floating point instructions of other processes, which can be leveraged to obtain sensitive information such as cryptographic keys. NOTE: this is the documented behavior of AMD64 processors, but it is inconsistent with Intel processors in a security-relevant fashion that was not addressed by the kernels.
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