CVE-2026-64191

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

i2c: stub: Reject I2C block transfers with invalid length

The I2C_SMBUS_I2C_BLOCK_DATA case in stub_xfer() uses data->block[0]

as the transfer length. The existing check only clamps it to avoid

overrunning the chip->words[256] register array, but does not validate

it against I2C_SMBUS_BLOCK_MAX (32), which is the limit of the union

i2c_smbus_data.block buffer (34 bytes total). The driver is a

development/test tool (CONFIG_I2C_STUB=m, not built by default)

that must be loaded with a chip_addr= parameter.

A local user with access to /dev/i2c-* can issue an I2C_SMBUS ioctl

with I2C_SMBUS_I2C_BLOCK_DATA and data->block[0] > 32, causing

stub_xfer() to read or write past the end of the union

i2c_smbus_data.block buffer:

BUG: KASAN: stack-out-of-bounds in stub_xfer (drivers/i2c/i2c-stub.c:223)

Read of size 1 at addr ffff88800abcfd92 by task exploit/81

Call Trace:

<TASK>

stub_xfer (drivers/i2c/i2c-stub.c:223)

__i2c_smbus_xfer (drivers/i2c/i2c-core-smbus.c:593)

i2c_smbus_xfer (drivers/i2c/i2c-core-smbus.c:536)

i2cdev_ioctl_smbus (drivers/i2c/i2c-dev.c:391)

i2cdev_ioctl (drivers/i2c/i2c-dev.c:478)

__x64_sys_ioctl (fs/ioctl.c:583)

do_syscall_64 (arch/x86/entry/syscall_64.c:94)

entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)

</TASK>

The bug exists because i2c-stub implements .smbus_xfer directly,

bypassing the I2C_SMBUS_BLOCK_MAX validation in

i2c_smbus_xfer_emulated(). The I2C_SMBUS_BLOCK_DATA case in the same

function correctly validates against I2C_SMBUS_BLOCK_MAX, but the

I2C_SMBUS_I2C_BLOCK_DATA case does not.

Fix by rejecting transfers with data->block[0] == 0 or

data->block[0] > I2C_SMBUS_BLOCK_MAX with -EINVAL, consistent with

both the I2C_SMBUS_BLOCK_DATA case in the same function and the

I2C_SMBUS_I2C_BLOCK_DATA validation in i2c_smbus_xfer_emulated().