CVE-2026-80578: fbdev: core: Fix pointer desynchronization in fb_io_read()

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

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

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

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

fbdev: core: Fix pointer desynchronization in fb_io_read()

In fb_io_read(), if copy_to_user() performs a partial copy (e.g., due to
a faulty user buffer), the loop adjusts the chunk size 'c' and updates
the remaining 'count'. However, the hardware 'src' pointer has already
been eagerly advanced by the original chunk size.

If the loop is allowed to continue, the read will resume from an
incorrect, over-advanced offset. Since the remaining 'count' was only
decremented by the successful bytes, this desynchronization causes the
next iterations to execute more hardware reads than originally bounded,
eventually leading to out-of-bounds I/O reads.

Fix this by breaking out of the loop immediately upon a partial
copy_to_user(). A partial copy indicates a faulty user buffer, making
subsequent read attempts futile. Breaking out ensures we return the
number of successfully read bytes without risking out-of-bounds hardware
accesses in subsequent mismatched iterations.

The Linux kernel CVE team has assigned CVE-2026-80578 to this issue.


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

	Issue introduced in 6.5 with commit 6121cd9ef911432b14c2a17aefaf8cd2f3cfcdff and fixed in 6.6.153 with commit 42bc07b4e5a3c8a02a433388f562a8f46d093e11
	Issue introduced in 6.5 with commit 6121cd9ef911432b14c2a17aefaf8cd2f3cfcdff and fixed in 6.12.105 with commit 42a6d8126c194133eafab2b0fd5c8668ebfcba5b
	Issue introduced in 6.5 with commit 6121cd9ef911432b14c2a17aefaf8cd2f3cfcdff and fixed in 6.18.46 with commit 7ff87a01ae3a8cd0208f7499386998223a8b5dba
	Issue introduced in 6.5 with commit 6121cd9ef911432b14c2a17aefaf8cd2f3cfcdff and fixed in 7.1.10 with commit 7110b7b794a2aac2c5cf8eb06ebf2af724c74d50
	Issue introduced in 6.5 with commit 6121cd9ef911432b14c2a17aefaf8cd2f3cfcdff and fixed in 7.2 with commit 81cc73be40c6f028f1ee3f438ace46afe666dbae

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-2026-80578
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/video/fbdev/core/fb_io_fops.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/42bc07b4e5a3c8a02a433388f562a8f46d093e11
	https://git.kernel.org/stable/c/42a6d8126c194133eafab2b0fd5c8668ebfcba5b
	https://git.kernel.org/stable/c/7ff87a01ae3a8cd0208f7499386998223a8b5dba
	https://git.kernel.org/stable/c/7110b7b794a2aac2c5cf8eb06ebf2af724c74d50
	https://git.kernel.org/stable/c/81cc73be40c6f028f1ee3f438ace46afe666dbae