CVE-2026-72142: i2c: imx: fix locked bus on SMBus block-read of 0 (atomic)

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

i2c: imx: fix locked bus on SMBus block-read of 0 (atomic)

SMBus 3.1 6.5.7 allows a Block Read byte count of 0, but the atomic (polling) path rejects it as -EPROTO. Worse, it returns without a NACK+STOP: the next receive cycle has already started, so the target keeps holding SDA and the bus stays stuck until a power cycle for this i2c controller.

Reading I2DR to obtain the count likewise arms the next byte on the count > I2CSMBUSBLOCKMAX path, which also returned -EPROTO directly and left the bus held.

Handle both: NACK the in-flight dummy byte (TXAK) and extend msgs->len so the existing last-byte handling emits STOP; the dummy byte is discarded. A count of 0 is a valid empty block read; a count above I2CSMBUSBLOCKMAX is still reported as -EPROTO, but only after the bus has been released.

The interrupt-driven path has the same flaw from a later commit and is fixed separately, as it carries a different Fixes: tag and stable range.