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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id ij19-20020a170902ab5300b0017f7628cbddsm2682023plb.30.2022.10.14.23.45.18 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 14 Oct 2022 23:45:19 -0700 (PDT) From: Kees Cook To: Greg Kroah-Hartman Cc: Kees Cook , Jiri Slaby , Simon Brand , linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org Subject: [PATCH v2 2/2] tty: Allow TIOCSTI to be disabled Date: Fri, 14 Oct 2022 23:45:15 -0700 Message-Id: <20221015064517.1554119-2-keescook@chromium.org> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20221015064222.gonna.435-kees@kernel.org> References: <20221015064222.gonna.435-kees@kernel.org> MIME-Version: 1.0 X-Developer-Signature: v=1; a=openpgp-sha256; l=3580; h=from:subject; bh=kjqVbumXKmpYQu5g+0jngSz3qjLAeN7VzRJkUWWFCAo=; b=owEBbQKS/ZANAwAKAYly9N/cbcAmAcsmYgBjSlb7Te+AzPWEJwitcFI9OGMy97KUI9l2gFWD3tbS B0aN6f2JAjMEAAEKAB0WIQSlw/aPIp3WD3I+bhOJcvTf3G3AJgUCY0pW+wAKCRCJcvTf3G3AJqm+D/ kBsaao5QXORYsXZ6tMoSk1qbF2cqCrh4sImxv9FzAPjhnD1er1Ff438V7Q0KYFh/q83pWtJRotFGBT QU8ZtVKPrtdOlQ+pn7+NbyXorwitwyyC9BCNdrU5ucbAo8xVCWgevYXYTpLrLG9nTOgYphzO2peCIS d7GU50JzErxaXhemK9uQdK2QS8H0XfcUV3rLbcyMD9JSMvI9fIv32/Tn8aU4bXR3GKbFAtByiEll6g b0w59cyuup1kESLx4UNX9EFvJQxap3zx1YZbWIu2YUFUC5wGFSeSmYBdGrrMb7sCeCNFueOtlQncBG MnkneyO334jImK70V/H4Ct2t4iEYSfik+z0HBC1qZCqwyuIKZsP8+e/K6E+e62rgzLL0bjuHlyBPhU cgdRAkIqLCZvFsS2uLklsE+28n+m9xBIDMEJ1cvDAHUvgsT/4k9l69yhubmwERLTUSuA6sVP0T7gXV m1rBbwxEG98C4/UYRH11ToRjUS+EwWYpakcqeR3auZgTfNxekmf+AmNH8QILMrNKNp72dFFywBXcVA J8HCKgLlguJLC/nQfI7CEv181xlDUJlQFiBRT1DPZZ62zEEFSAT2b/4qKJYd2RaWdcpiSuXz4Snnel kVmm2rHyemTv0O1Xl3Bu44zVQWAu2ENBcfOBPWpr3BX6IHT3hht7Zj6o3Q1Q== X-Developer-Key: i=keescook@chromium.org; a=openpgp; fpr=A5C3F68F229DD60F723E6E138972F4DFDC6DC026 X-Spam-Status: No, score=-2.1 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,RCVD_IN_DNSWL_NONE, SPF_HELO_NONE,SPF_PASS autolearn=unavailable autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org X-getmail-retrieved-from-mailbox: =?utf-8?q?INBOX?= X-GMAIL-THRID: =?utf-8?q?1746727118680787521?= X-GMAIL-MSGID: =?utf-8?q?1746738922428396301?= TIOCSTI continues its long history of being used in privilege escalation attacks[1]. Prior attempts to provide a mechanism to disable this have devolved into discussions around creating full-blown LSMs to provide arbitrary ioctl filtering, which is hugely over-engineered -- only TIOCSTI is being used this way. 3 years ago OpenBSD entirely removed TIOCSTI[2], Android has had it filtered for longer[3], and the tools that had historically used TIOCSTI either do not need it, are not commonly built with it, or have had its use removed. Provide a simple CONFIG and global sysctl to disable this for the system builders who have wanted this functionality for literally decades now, much like the ldisc_autoload CONFIG and sysctl. [1] https://lore.kernel.org/linux-hardening/Y0m9l52AKmw6Yxi1@hostpad [2] https://undeadly.org/cgi?action=article;sid=20170701132619 [3] https://lore.kernel.org/lkml/CAFJ0LnFGRuEEn1tCLhoki8ZyWrKfktbF+rwwN7WzyC_kBFoQVA@mail.gmail.com/ Cc: Greg Kroah-Hartman Cc: Jiri Slaby Cc: Simon Brand Signed-off-by: Kees Cook --- drivers/tty/Kconfig | 19 +++++++++++++++++++ drivers/tty/tty_io.c | 13 +++++++++++++ 2 files changed, 32 insertions(+) diff --git a/drivers/tty/Kconfig b/drivers/tty/Kconfig index cc30ff93e2e4..d35fc068da74 100644 --- a/drivers/tty/Kconfig +++ b/drivers/tty/Kconfig @@ -149,6 +149,25 @@ config LEGACY_PTY_COUNT When not in use, each legacy PTY occupies 12 bytes on 32-bit architectures and 24 bytes on 64-bit architectures. +config LEGACY_TIOCSTI + bool "Allow legacy TIOCSTI usage" + default y + help + Historically the kernel has allowed TIOCSTI, which will push + characters into a controlling TTY. This continues to be used + as a malicious privilege escalation mechanism, and provides no + meaningful real-world utility any more. Its use is considered + a dangerous legacy operation, and can be disabled on most + systems. + + Say 'Y here only if you have confirmed that your system's + userspace depends on this functionality to continue operating + normally. + + This functionality can be changed at runtime with the + dev.tty.legacy_tiocsti sysctl. This configuration option sets + the default value of the sysctl. + config LDISC_AUTOLOAD bool "Automatically load TTY Line Disciplines" default y diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c index b397b223eada..fa36dac7559e 100644 --- a/drivers/tty/tty_io.c +++ b/drivers/tty/tty_io.c @@ -2275,11 +2275,15 @@ static int tty_fasync(int fd, struct file *filp, int on) * * Called functions take tty_ldiscs_lock * * current->signal->tty check is safe without locks */ +static int tty_legacy_tiocsti __read_mostly = IS_BUILTIN(CONFIG_LEGACY_TIOCSTI); static int tiocsti(struct tty_struct *tty, char __user *p) { char ch, mbz = 0; struct tty_ldisc *ld; + if (!tty_legacy_tiocsti) + return -EIO; + if ((current->signal->tty != tty) && !capable(CAP_SYS_ADMIN)) return -EPERM; if (get_user(ch, p)) @@ -3582,6 +3586,15 @@ void console_sysfs_notify(void) } static struct ctl_table tty_table[] = { + { + .procname = "legacy_tiocsti", + .data = &tty_legacy_tiocsti, + .maxlen = sizeof(tty_legacy_tiocsti), + .mode = 0644, + .proc_handler = proc_dointvec, + .extra1 = SYSCTL_ZERO, + .extra2 = SYSCTL_ONE, + }, { .procname = "ldisc_autoload", .data = &tty_ldisc_autoload,