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/20250223091342.35523-2-gnoack3000@gmail.com/

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/20250221.0a947528d8f3@gnoack.org/

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/491f3df9de6593df8e70dbe77614b026@finder.org/ 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 the kernel with additional pointers is also available in this patch: https://lore.kernel.org/all/a773e48920aa104a65073671effbdee665c105fc.1603963...

For background on who else uses TIOCL_SELMOUSEREPORT: Debian Code search finds one page of results, the only two known callers are the two mouse daemons GPM and Consolation. (GPM does not show up in the search results because it uses literal numbers to refer to TIOCLINUX-related enums. I looked through GPM by hand instead. TIOCL_SELMOUSEREPORT is also not used from libgpm.) https://codesearch.debian.net/search?q=TIOCL_SELMOUSEREPORT

Cc: Jared Finder jared@finder.org Cc: Jann Horn jannh@google.com Cc: Hanno Böck hanno@hboeck.de Cc: Jiri Slaby jirislaby@kernel.org Cc: Kees Cook kees@kernel.org Cc: stable@vger.kernel.org Fixes: 2f83e38a095f ("tty: Permit some TIOCL_SETSEL modes without CAP_SYS_ADMIN") Signed-off-by: Günther Noack gnoack3000@gmail.com --- drivers/tty/vt/selection.c | 5 ++--- 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/drivers/tty/vt/selection.c b/drivers/tty/vt/selection.c index 0bd6544e30a6b..791e2f1f7c0b6 100644 --- a/drivers/tty/vt/selection.c +++ b/drivers/tty/vt/selection.c @@ -193,13 +193,12 @@ int set_selection_user(const struct tiocl_selection __user *sel, return -EFAULT;

/* - * TIOCL_SELCLEAR, TIOCL_SELPOINTER and TIOCL_SELMOUSEREPORT are OK to - * use without CAP_SYS_ADMIN as they do not modify the selection. + * TIOCL_SELCLEAR and TIOCL_SELPOINTER are OK to use without + * CAP_SYS_ADMIN as they do not modify the selection. */ switch (v.sel_mode) { case TIOCL_SELCLEAR: case TIOCL_SELPOINTER: - case TIOCL_SELMOUSEREPORT: break; default: if (!capable(CAP_SYS_ADMIN))

base-commit: 27102b38b8ca7ffb1622f27bcb41475d121fb67f