CVE-2026-64191: i2c: stub: Reject I2C block transfers with invalid length

CVE-2026-64191: i2c: stub: Reject I2C block transfers with invalid length

[Greg Kroah-Hartman]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

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().

The Linux kernel CVE team has assigned CVE-2026-64191 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 2.6.33 with commit 4710317891e4824ce1510a6b5066abbd3e917750 and fixed in 5.10.260 with commit 7e9072dbd5f2f17934751873450d2c22080ead80
	Issue introduced in 2.6.33 with commit 4710317891e4824ce1510a6b5066abbd3e917750 and fixed in 5.15.211 with commit 21e87f336ac6303fed54a69b1d0d79a23b25c8d0
	Issue introduced in 2.6.33 with commit 4710317891e4824ce1510a6b5066abbd3e917750 and fixed in 6.1.177 with commit 3fd225f3e4cd67ec8ddab1afed9da03c7c43537c
	Issue introduced in 2.6.33 with commit 4710317891e4824ce1510a6b5066abbd3e917750 and fixed in 6.6.144 with commit 1c4ffe6b4f04365485ed58d64c9bb86b46fc9037
	Issue introduced in 2.6.33 with commit 4710317891e4824ce1510a6b5066abbd3e917750 and fixed in 6.12.95 with commit 4bd8635f28c135a08aac6badcd7d9b5cdb34335f
	Issue introduced in 2.6.33 with commit 4710317891e4824ce1510a6b5066abbd3e917750 and fixed in 6.18.37 with commit 5f4d2bd028ebb6e4c09a9d64842546022321d4a7
	Issue introduced in 2.6.33 with commit 4710317891e4824ce1510a6b5066abbd3e917750 and fixed in 7.0.14 with commit 0526931b16e5a118d367b7bfce7d797e63f7ac69
	Issue introduced in 2.6.33 with commit 4710317891e4824ce1510a6b5066abbd3e917750 and fixed in 7.1 with commit 6036b5067a8199ba7a2dc7b377d4b9dd276d5f9e

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-64191
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	drivers/i2c/i2c-stub.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/7e9072dbd5f2f17934751873450d2c22080ead80
	https://git.kernel.org/stable/c/21e87f336ac6303fed54a69b1d0d79a23b25c8d0
	https://git.kernel.org/stable/c/3fd225f3e4cd67ec8ddab1afed9da03c7c43537c
	https://git.kernel.org/stable/c/1c4ffe6b4f04365485ed58d64c9bb86b46fc9037
	https://git.kernel.org/stable/c/4bd8635f28c135a08aac6badcd7d9b5cdb34335f
	https://git.kernel.org/stable/c/5f4d2bd028ebb6e4c09a9d64842546022321d4a7
	https://git.kernel.org/stable/c/0526931b16e5a118d367b7bfce7d797e63f7ac69
	https://git.kernel.org/stable/c/6036b5067a8199ba7a2dc7b377d4b9dd276d5f9e