[PATCH] tty: Permit some TIOCL_SETSEL modes without CAP_SYS_ADMIN

From: Günther Noack
Date: Mon Dec 16 2024 - 10:09:30 EST


With this, processes without CAP_SYS_ADMIN are able to use TIOCLINUX with
subcode TIOCL_SETSEL, in the selection modes TIOCL_SETPOINTER,
TIOCL_SELCLEAR and TIOCL_SELMOUSEREPORT.

TIOCL_SETSEL was previously changed to require CAP_SYS_ADMIN, as this IOCTL
let callers change the selection buffer and could be used to simulate
keypresses. These three TIOCL_SETSEL selection modes, however, are safe to
use, as they do not modify the selection buffer.

This fixes a mouse support regression that affected Emacs (invisible mouse
cursor).

Link: https://lore.kernel.org/r/ee3ec63269b43b34e1c90dd8c9743bf8@xxxxxxxxxx
Fixes: 8d1b43f6a6df ("tty: Restrict access to TIOCLINUX' copy-and-paste subcommands")
Signed-off-by: Günther Noack <gnoack@xxxxxxxxxx>
---
drivers/tty/vt/selection.c | 14 ++++++++++++++
drivers/tty/vt/vt.c | 3 +--
2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/vt/selection.c b/drivers/tty/vt/selection.c
index 564341f1a74f..0bd6544e30a6 100644
--- a/drivers/tty/vt/selection.c
+++ b/drivers/tty/vt/selection.c
@@ -192,6 +192,20 @@ int set_selection_user(const struct tiocl_selection __user *sel,
if (copy_from_user(&v, sel, sizeof(*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.
+ */
+ switch (v.sel_mode) {
+ case TIOCL_SELCLEAR:
+ case TIOCL_SELPOINTER:
+ case TIOCL_SELMOUSEREPORT:
+ break;
+ default:
+ if (!capable(CAP_SYS_ADMIN))
+ return -EPERM;
+ }
+
return set_selection_kernel(&v, tty);
}

diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index 96842ce817af..ed65b3b80fbd 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -3345,8 +3345,7 @@ int tioclinux(struct tty_struct *tty, unsigned long arg)

switch (type) {
case TIOCL_SETSEL:
- if (!capable(CAP_SYS_ADMIN))
- return -EPERM;
+ /* CAP_SYS_ADMIN check happens in set_selection_user(). */
return set_selection_user(param, tty);
case TIOCL_PASTESEL:
if (!capable(CAP_SYS_ADMIN))
--
2.47.1.613.gc27f4b7a9f-goog