Zephyr: I2S unconfigure path lets a user thread reach kernel memory

Any thread running under CONFIG_USERSPACE that has access to an I2S device object can trigger this with a single i2s_configure() call. Most in-tree drivers special-case frame_clk_freq == 0 and never store the caller's structure, but drivers/i2s/i2s_litex.c does not: i2s_litex_configure() only logs a warning for a mismatched frame clock, then memcpy()s the caller's config into stream->cfg and marks the stream I2S_STATE_READY. A subsequent i2s_trigger() with I2S_TRIGGER_START enables the FIFO interrupt, and the interrupt handlers i2s_litex_isr_rx() / i2s_litex_isr_tx() then call k_mem_slab_alloc() and k_mem_slab_free() on the attacker-supplied pointer in supervisor context.

The result is an arbitrary-pointer read/write primitive executed by the kernel on behalf of an unprivileged thread — k_mem_slab_alloc() takes a spinlock at slab->lock and reads and writes slab->free_list and the link word inside the returned "block" — which defeats the CONFIG_USERSPACE isolation boundary entirely. The same missing validation is a hazard for out-of-tree I2S drivers that retain the configuration on the unconfigure path. Builds without CONFIG_USERSPACE are unaffected, since the verifier is not used.

The fix zeroes config.mem_slab and config.block_size on the unconfigure path so the driver can never receive an unvalidated pointer, while still allowing a stream to be unconfigured by a caller that does not own a memory slab, as the API documents.