rseq: Fix segfault on registration when rseq_cs is non-zero

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: rseq: Fix segfault on registration when rseq_cs is non-zero The rseq_cs field is documented as being set to 0 by user-space prior to registration, however this is not currently enforced by the kernel. This can result in a segfault on return to user-space if the value stored in the rseq_cs field doesn't point to a valid struct rseq_cs. The correct solution to this would be to fail the rseq registration when the rseq_cs field is non-zero. However, some older versions of glibc will reuse the rseq area of previous threads without clearing the rseq_cs field and will also terminate the process if the rseq registration fails in a secondary thread. This wasn't caught in testing because in this case the leftover rseq_cs does point to a valid struct rseq_cs. What we can do is clear the rseq_cs field on registration when it's non-zero which will prevent segfaults on registration and won't break the glibc versions that reuse rseq areas on thread creation.

References

• https://git.kernel.org/stable/c/48900d839a3454050fd5822e34be8d54c4ec9b86
• https://git.kernel.org/stable/c/3e4028ef31b69286c9d4878cee0330235f53f218
• https://git.kernel.org/stable/c/b2b05d0dc2f4f0646922068af435aed5763d16ba
• https://git.kernel.org/stable/c/eaf112069a904b6207b4106ff083e0208232a2eb
• https://git.kernel.org/stable/c/f004f58d18a2d3dc761cf973ad27b4a5997bd876
• https://git.kernel.org/stable/c/2df285dab00fa03a3ef939b6cb0d0d0aeb0791db
• https://git.kernel.org/stable/c/fd881d0a085fc54354414aed990ccf05f282ba53