tty: Require CAP_SYS_ADMIN for all usages of TIOCL_SELMOUSEREPORT

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

tty: Require CAP_SYS_ADMIN for all usages of TIOCL_SELMOUSEREPORT

This requirement was overeagerly loosened in commit 2f83e38a095f

("tty: Permit some TIOCL_SETSEL modes without CAP_SYS_ADMIN"), but as

it turns out,

(1) the logic I implemented there was inconsistent (apologies!),

(2) TIOCL_SELMOUSEREPORT might actually be a small security risk

after all, and

(3) TIOCL_SELMOUSEREPORT is only meant to be used by the mouse

daemon (GPM or Consolation), which runs as CAP_SYS_ADMIN

already.

In more detail:

1. The previous patch has inconsistent logic:

In commit 2f83e38a095f ("tty: Permit some TIOCL_SETSEL modes

without CAP_SYS_ADMIN"), we checked for sel_mode ==

TIOCL_SELMOUSEREPORT, but overlooked that the lower four bits of

this "mode" parameter were actually used as an additional way to

pass an argument. So the patch did actually still require

CAP_SYS_ADMIN, if any of the mouse button bits are set, but did not

require it if none of the mouse buttons bits are set.

This logic is inconsistent and was not intentional. We should have

the same policies for using TIOCL_SELMOUSEREPORT independent of the

value of the "hidden" mouse button argument.

I sent a separate documentation patch to the man page list with

more details on TIOCL_SELMOUSEREPORT:

https://lore.kernel.org/all/[email protected]/

2. TIOCL_SELMOUSEREPORT is indeed a potential security risk which can

let an attacker simulate "keyboard" input to command line

applications on the same terminal, like TIOCSTI and some other

TIOCLINUX "selection mode" IOCTLs.

By enabling mouse reporting on a terminal and then injecting mouse

reports through TIOCL_SELMOUSEREPORT, an attacker can simulate

mouse movements on the same terminal, similar to the TIOCSTI

keystroke injection attacks that were previously possible with

TIOCSTI and other TIOCL_SETSEL selection modes.

Many programs (including libreadline/bash) are then prone to

misinterpret these mouse reports as normal keyboard input because

they do not expect input in the X11 mouse protocol form. The

attacker does not have complete control over the escape sequence,

but they can at least control the values of two consecutive bytes

in the binary mouse reporting escape sequence.

I went into more detail on that in the discussion at

https://lore.kernel.org/all/[email protected]/

It is not equally trivial to simulate arbitrary keystrokes as it

was with TIOCSTI (commit 83efeeeb3d04 ("tty: Allow TIOCSTI to be

disabled")), but the general mechanism is there, and together with

the small number of existing legit use cases (see below), it would

be better to revert back to requiring CAP_SYS_ADMIN for

TIOCL_SELMOUSEREPORT, as it was already the case before

commit 2f83e38a095f ("tty: Permit some TIOCL_SETSEL modes without

CAP_SYS_ADMIN").

3. TIOCL_SELMOUSEREPORT is only used by the mouse daemons (GPM or

Consolation), and they are the only legit use case:

To quote console_codes(4):

The mouse tracking facility is intended to return

xterm(1)-compatible mouse status reports. Because the console

driver has no way to know the device or type of the mouse, these

reports are returned in the console input stream only when the

virtual terminal driver receives a mouse update ioctl. These

ioctls must be generated by a mouse-aware user-mode application

such as the gpm(8) daemon.

Jared Finder has also confirmed in

https://lore.kernel.org/all/[email protected]/

that Emacs does not call TIOCL_SELMOUSEREPORT directly, and it

would be difficult to find good reasons for doing that, given that

it would interfere with the reports that GPM is sending.

More information on the interaction between GPM, terminals and th

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