driver core: add TAINT_FORCED_BIND for when userspace manually messes with devices and drivers

The ability to add and remove devices from a driver through the sysfs
"bind" and "unbind" files was created all those decades ago as a way
that kernel developers can iterate faster, and provide a debugging way
for users to attempt to add a new device to a driver without having to
rebuild their kernel.

This api over the years has been abused and recently come under a major
fuzzing "attack" through tools like syzbot which decided that it would
attempt to just randomly bind any driver to any type of device, causing
loads of unneeded errors and pointless kernel patches to be generated by
unsuspecting new developers.

Handle all of this by adding a new taint flag, TAINT_FORCED_BIND, which
will be set on the driver if the bind/unbind sysfs files are ever
written to.  This lets kernel developers "know" that a user is
attempting to do something that is not normal, and as such, if the
kernel breaks they get to keep the shiny pieces laying around on the
floor.

The flag is 'Y' which was unused, and can remembered as the user is
"yeeting" the device being operated on here (thrown with force without
regard for the thing being thrown).

Note, the taint flag gets set _BEFORE_ the bind/unbind callback happens,
as many times crashes/oops/warnings/failures happen within the callback,
and the taint flag needs to be there to show what was being attempted.
If it were to be set after the callback happens, the oops report would
not properly reflect what foolishness was being attempted.

Fuzzing tools like syzbot, that doesn't have hand-crafted rules to keep
the tool from hitting bind/unbind, should be run with panic_on_taint
enabled so that they fall over and don't continue on, thinking that they
actually found a real issue.

Userspace operations that rely on the bind/unbind files to work around
the lack of will to upgrade a kernel image to a newer version with
proper support for new devices, or the lack of will to submit valid
device ids to driver authors, will still work properly, but now the
kernel will be flagged in a way that will show that perhaps those users
should reconsider their behavior and work to have the drivers properly
support these devices in a "native" manner.

Finally, the bind/unbind files can find real use-after-free issues with
some drivers by forcing the process to happen virtually without having
to rely on manual removal processes.  Those real bugs should still be
worked on, but by adding this taint flag, developers can more easily
determine bug reports that are actually worth looking at.

Reviewed-by: Johan Hovold <johan@kernel.org>
Tested-by: Johan Hovold <johan@kernel.org>
Reviewed-by: Aaron Tomlin <atomlin@atomlin.com>
Reviewed-by: Bradley Morgan <brads@mainlining.org>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patch.msgid.link/20260914-bind_taint-v4-3-eadf8a090903@linuxfoundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 8 insertions, 0 deletions

diff --git a/tools/debugging/kernel-chktaint b/tools/debugging/kernel-chktaint
index e1571c04afb5..d8628be37214 100755
--- a/tools/debugging/kernel-chktaint
+++ b/tools/debugging/kernel-chktaint
@@ -211,6 +211,14 @@ else
 	addout "J"
 	echo " * fwctl's mutating debug interface was used (#19)"
 fi
+
+T=`expr $T / 2`
+if [ `expr $T % 2` -eq 0 ]; then
+	addout " "
+else
+	addout "Y"
+	echo " * device was manually bound or unbound from a driver (#20)"
+fi
 echo "Raw taint value as int/string: $taint/'$out'"
 
 # report on any tainted loadable modules