CVE-2025-39928

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

i2c: rtl9300: ensure data length is within supported range

Add an explicit check for the xfer length to 'rtl9300_i2c_config_xfer'

to ensure the data length isn't within the supported range. In

particular a data length of 0 is not supported by the hardware and

causes unintended or destructive behaviour.

This limitation becomes obvious when looking at the register

documentation [1]. 4 bits are reserved for DATA_WIDTH and the value

of these 4 bits is used as N + 1, allowing a data length range of

1 <= len <= 16.

Affected by this is the SMBus Quick Operation which works with a data

length of 0. Passing 0 as the length causes an underflow of the value

due to:

(len - 1) & 0xf

and effectively specifying a transfer length of 16 via the registers.

This causes a 16-byte write operation instead of a Quick Write. For

example, on SFP modules without write-protected EEPROM this soft-bricks

them by overwriting some initial bytes.

For completeness, also add a quirk for the zero length.

[1] https://svanheule.net/realtek/longan/register/i2c_mst1_ctrl2