[PATCH 2/2] selinux: add capability checks for TIOCSTI ioctl

[Abhinav Saxena via B4 Relay <devnull+xandfury.gmail.com@kernel.org>]

From: Abhinav Saxena <xandfury@gmail.com>

The TIOCSTI ioctl currently only checks the current process's
credentials, creating a TOCTOU vulnerability where an unprivileged
process can open a TTY fd and pass it to a privileged process via
SCM_RIGHTS.

Fix by requiring BOTH the file opener (file->f_cred) AND the current
process to have CAP_SYS_ADMIN. This prevents privilege escalation
while ensuring legitimate use cases continue to work.

Link: https://github.com/KSPP/linux/issues/156

Signed-off-by: Abhinav Saxena <xandfury@gmail.com>
---
 security/selinux/hooks.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 595ceb314aeb..a628551873ab 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -3847,6 +3847,12 @@ static int selinux_file_ioctl(struct file *file, unsigned int cmd,
 					    CAP_OPT_NONE, true);
 		break;
 
+	case TIOCSTI:
+		if (!file_ns_capable(file, &init_user_ns, CAP_SYS_ADMIN) ||
+		    !capable(CAP_SYS_ADMIN))
+			error = -EPERM;
+		break;
+
 	case FIOCLEX:
 	case FIONCLEX:
 		if (!selinux_policycap_ioctl_skip_cloexec())

-- 
2.43.0