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

spi: Fix deadlock when adding SPI controllers on SPI buses

Currently we have a global spi_add_lock which we take when adding new
devices so that we can check that we're not trying to reuse a chip
select that's already controlled. This means that if the SPI device is
itself a SPI controller and triggers the instantiation of further SPI
devices we trigger a deadlock as we try to register and instantiate
those devices while in the process of doing so for the parent controller
and hence already holding the global spi_add_lock. Since we only care
about concurrency within a single SPI bus move the lock to be per
controller, avoiding the deadlock.

This can be easily triggered in the case of spi-mux.

References

• https://nvd.nist.gov/vuln/detail/CVE-2021-47469
• https://git.kernel.org/stable/c/6098475d4cb48d821bdf453c61118c56e26294f0
• https://git.kernel.org/stable/c/722ef19a161ce3fffb3d1b01ce2301c306639bdd
• https://git.kernel.org/stable/c/aa3f3d7bef59583f2d3234173105a27ff61ef8fe
• https://git.kernel.org/stable/c/c8dce228db6f81dbc897a018dfc5c418e917cf64