kernel/drivers/tty/n_tty.c, branch linux-4.1.y

tty: make n_tty_read() always abort if hangup is in progress

2018-05-23T01:36:32Z

[ Upstream commit 28b0f8a6962a24ed21737578f3b1b07424635c9e ] A tty is hung up by __tty_hangup() setting file->f_op to hung_up_tty_fops, which is skipped on ttys whose write operation isn't tty_write(). This means that, for example, /dev/console whose write op is redirected_tty_write() is never actually marked hung up. Because n_tty_read() uses the hung up status to decide whether to abort the waiting readers, the lack of hung-up marking can lead to the following scenario. 1. A session contains two processes. The leader and its child. The child ignores SIGHUP. 2. The leader exits and starts disassociating from the controlling terminal (/dev/console). 3. __tty_hangup() skips setting f_op to hung_up_tty_fops. 4. SIGHUP is delivered and ignored. 5. tty_ldisc_hangup() is invoked. It wakes up the waits which should clear the read lockers of tty->ldisc_sem. 6. The reader wakes up but because tty_hung_up_p() is false, it doesn't abort and goes back to sleep while read-holding tty->ldisc_sem. 7. The leader progresses to tty_ldisc_lock() in tty_ldisc_hangup() and is now stuck in D sleep indefinitely waiting for tty->ldisc_sem. The following is Alan's explanation on why some ttys aren't hung up. http://lkml.kernel.org/r/20171101170908.6ad08580@alans-desktop 1. It broke the serial consoles because they would hang up and close down the hardware. With tty_port that *should* be fixable properly for any cases remaining. 2. The console layer was (and still is) completely broken and doens't refcount properly. So if you turn on console hangups it breaks (as indeed does freeing consoles and half a dozen other things). As neither can be fixed quickly, this patch works around the problem by introducing a new flag, TTY_HUPPING, which is used solely to tell n_tty_read() that hang-up is in progress for the console and the readers should be aborted regardless of the hung-up status of the device. The following is a sample hung task warning caused by this issue. INFO: task agetty:2662 blocked for more than 120 seconds. Not tainted 4.11.3-dbg-tty-lockup-02478-gfd6c7ee-dirty #28 "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. 0 2662 1 0x00000086 Call Trace: __schedule+0x267/0x890 schedule+0x36/0x80 schedule_timeout+0x23c/0x2e0 ldsem_down_write+0xce/0x1f6 tty_ldisc_lock+0x16/0x30 tty_ldisc_hangup+0xb3/0x1b0 __tty_hangup+0x300/0x410 disassociate_ctty+0x6c/0x290 do_exit+0x7ef/0xb00 do_group_exit+0x3f/0xa0 get_signal+0x1b3/0x5d0 do_signal+0x28/0x660 exit_to_usermode_loop+0x46/0x86 do_syscall_64+0x9c/0xb0 entry_SYSCALL64_slow_path+0x25/0x25 The following is the repro. Run "$PROG /dev/console". The parent process hangs in D state. #include #include #include #include #include #include #include #include #include #include #include #include int main(int argc, char **argv) { struct sigaction sact = { .sa_handler = SIG_IGN }; struct timespec ts1s = { .tv_sec = 1 }; pid_t pid; int fd; if (argc < 2) { fprintf(stderr, "test-hung-tty /dev/$TTY\n"); return 1; } /* fork a child to ensure that it isn't already the session leader */ pid = fork(); if (pid < 0) { perror("fork"); return 1; } if (pid > 0) { /* top parent, wait for everyone */ while (waitpid(-1, NULL, 0) >= 0) ; if (errno != ECHILD) perror("waitpid"); return 0; } /* new session, start a new session and set the controlling tty */ if (setsid() < 0) { perror("setsid"); return 1; } fd = open(argv[1], O_RDWR); if (fd < 0) { perror("open"); return 1; } if (ioctl(fd, TIOCSCTTY, 1) < 0) { perror("ioctl"); return 1; } /* fork a child, sleep a bit and exit */ pid = fork(); if (pid < 0) { perror("fork"); return 1; } if (pid > 0) { nanosleep(&ts1s, NULL); printf("Session leader exiting\n"); exit(0); } /* * The child ignores SIGHUP and keeps reading from the controlling * tty. Because SIGHUP is ignored, the child doesn't get killed on * parent exit and the bug in n_tty makes the read(2) block the * parent's control terminal hangup attempt. The parent ends up in * D sleep until the child is explicitly killed. */ sigaction(SIGHUP, &sact, NULL); printf("Child reading tty\n"); while (1) { char buf[1024]; if (read(fd, buf, sizeof(buf)) < 0) { perror("read"); return 1; } } return 0; } Signed-off-by: Tejun Heo Cc: Alan Cox Cc: stable@vger.kernel.org Signed-off-by: Greg Kroah-Hartman Signed-off-by: Sasha Levin

n_tty: fix EXTPROC vs ICANON interaction with TIOCINQ (aka FIONREAD)

2018-03-01T00:32:19Z

[ Upstream commit 966031f340185eddd05affcf72b740549f056348 ] We added support for EXTPROC back in 2010 in commit 26df6d13406d ("tty: Add EXTPROC support for LINEMODE") and the intent was to allow it to override some (all?) ICANON behavior. Quoting from that original commit message: There is a new bit in the termios local flag word, EXTPROC. When this bit is set, several aspects of the terminal driver are disabled. Input line editing, character echo, and mapping of signals are all disabled. This allows the telnetd to turn off these functions when in linemode, but still keep track of what state the user wants the terminal to be in. but the problem turns out that "several aspects of the terminal driver are disabled" is a bit ambiguous, and you can really confuse the n_tty layer by setting EXTPROC and then causing some of the ICANON invariants to no longer be maintained. This fixes at least one such case (TIOCINQ) becoming unhappy because of the confusion over whether ICANON really means ICANON when EXTPROC is set. This basically makes TIOCINQ match the case of read: if EXTPROC is set, we ignore ICANON. Also, make sure to reset the ICANON state ie EXTPROC changes, not just if ICANON changes. Fixes: 26df6d13406d ("tty: Add EXTPROC support for LINEMODE") Reported-by: Tetsuo Handa Reported-by: syzkaller Cc: Jiri Slaby Signed-off-by: Linus Torvalds Signed-off-by: Greg Kroah-Hartman Signed-off-by: Sasha Levin

Fix OpenSSH pty regression on close

2016-06-03T23:15:40Z

[ Upstream commit 0f40fbbcc34e093255a2b2d70b6b0fb48c3f39aa ] OpenSSH expects the (non-blocking) read() of pty master to return EAGAIN only if it has received all of the slave-side output after it has received SIGCHLD. This used to work on pre-3.12 kernels. This fix effectively forces non-blocking read() and poll() to block for parallel i/o to complete for all ttys. It also unwinds these changes: 1) f8747d4a466ab2cafe56112c51b3379f9fdb7a12 tty: Fix pty master read() after slave closes 2) 52bce7f8d4fc633c9a9d0646eef58ba6ae9a3b73 pty, n_tty: Simplify input processing on final close 3) 1a48632ffed61352a7810ce089dc5a8bcd505a60 pty: Fix input race when closing Inspired by analysis and patch from Marc Aurele La France Reported-by: Volth Reported-by: Marc Aurele La France BugLink: https://bugzilla.mindrot.org/show_bug.cgi?id=52 BugLink: https://bugzilla.mindrot.org/show_bug.cgi?id=2492 Signed-off-by: Brian Bloniarz Reviewed-by: Peter Hurley Cc: stable Signed-off-by: Greg Kroah-Hartman Signed-off-by: Sasha Levin

n_tty: Fix unsafe reference to "other" ldisc

2016-02-15T20:45:24Z

[ Upstream commit 6d27a63caad3f13e96cf065d2d96828c2006be6b ] Although n_tty_check_unthrottle() has a valid ldisc reference (since the tty core gets the ldisc ref in tty_read() before calling the line discipline read() method), it does not have a valid ldisc reference to the "other" pty of a pty pair. Since getting an ldisc reference for tty->link essentially open-codes tty_wakeup(), just replace with the equivalent tty_wakeup(). Cc: Signed-off-by: Peter Hurley Signed-off-by: Greg Kroah-Hartman Signed-off-by: Sasha Levin

tty: audit: Fix audit source

2015-12-09T19:03:27Z

commit 6b2a3d628aa752f0ab825fc6d4d07b09e274d1c1 upstream. The data to audit/record is in the 'from' buffer (ie., the input read buffer). Fixes: 72586c6061ab ("n_tty: Fix auditing support for cannonical mode") Cc: Miloslav Trmač Signed-off-by: Peter Hurley Acked-by: Laura Abbott Signed-off-by: Greg Kroah-Hartman Signed-off-by: Greg Kroah-Hartman

tty: fix stall caused by missing memory barrier in drivers/tty/n_tty.c

2015-10-22T21:43:26Z

commit e81107d4c6bd098878af9796b24edc8d4a9524fd upstream. My colleague ran into a program stall on a x86_64 server, where n_tty_read() was waiting for data even if there was data in the buffer in the pty. kernel stack for the stuck process looks like below. #0 [ffff88303d107b58] __schedule at ffffffff815c4b20 #1 [ffff88303d107bd0] schedule at ffffffff815c513e #2 [ffff88303d107bf0] schedule_timeout at ffffffff815c7818 #3 [ffff88303d107ca0] wait_woken at ffffffff81096bd2 #4 [ffff88303d107ce0] n_tty_read at ffffffff8136fa23 #5 [ffff88303d107dd0] tty_read at ffffffff81368013 #6 [ffff88303d107e20] __vfs_read at ffffffff811a3704 #7 [ffff88303d107ec0] vfs_read at ffffffff811a3a57 #8 [ffff88303d107f00] sys_read at ffffffff811a4306 #9 [ffff88303d107f50] entry_SYSCALL_64_fastpath at ffffffff815c86d7 There seems to be two problems causing this issue. First, in drivers/tty/n_tty.c, __receive_buf() stores the data and updates ldata->commit_head using smp_store_release() and then checks the wait queue using waitqueue_active(). However, since there is no memory barrier, __receive_buf() could return without calling wake_up_interactive_poll(), and at the same time, n_tty_read() could start to wait in wait_woken() as in the following chart. __receive_buf() n_tty_read() ------------------------------------------------------------------------ if (waitqueue_active(&tty->read_wait)) /* Memory operations issued after the RELEASE may be completed before the RELEASE operation has completed */ add_wait_queue(&tty->read_wait, &wait); ... if (!input_available_p(tty, 0)) { smp_store_release(&ldata->commit_head, ldata->read_head); ... timeout = wait_woken(&wait, TASK_INTERRUPTIBLE, timeout); ------------------------------------------------------------------------ The second problem is that n_tty_read() also lacks a memory barrier call and could also cause __receive_buf() to return without calling wake_up_interactive_poll(), and n_tty_read() to wait in wait_woken() as in the chart below. __receive_buf() n_tty_read() ------------------------------------------------------------------------ spin_lock_irqsave(&q->lock, flags); /* from add_wait_queue() */ ... if (!input_available_p(tty, 0)) { /* Memory operations issued after the RELEASE may be completed before the RELEASE operation has completed */ smp_store_release(&ldata->commit_head, ldata->read_head); if (waitqueue_active(&tty->read_wait)) __add_wait_queue(q, wait); spin_unlock_irqrestore(&q->lock,flags); /* from add_wait_queue() */ ... timeout = wait_woken(&wait, TASK_INTERRUPTIBLE, timeout); ------------------------------------------------------------------------ There are also other places in drivers/tty/n_tty.c which have similar calls to waitqueue_active(), so instead of adding many memory barrier calls, this patch simply removes the call to waitqueue_active(), leaving just wake_up*() behind. This fixes both problems because, even though the memory access before or after the spinlocks in both wake_up*() and add_wait_queue() can sneak into the critical section, it cannot go past it and the critical section assures that they will be serialized (please see "INTER-CPU ACQUIRING BARRIER EFFECTS" in Documentation/memory-barriers.txt for a better explanation). Moreover, the resulting code is much simpler. Latency measurement using a ping-pong test over a pty doesn't show any visible performance drop. Signed-off-by: Kosuke Tatsukawa Signed-off-by: Greg Kroah-Hartman

n_tty: signal and flush atomically

2015-08-10T19:21:57Z

commit 3b19e032295647b7be2aa3be62510db4aaeda759 upstream. When handling signalling char, claim the termios write lock before signalling waiting readers and writers to prevent further i/o before flushing the echo and output buffers. This prevents a userspace signal handler which may output from racing the terminal flush. Reference: Bugzilla #99351 ("Output truncated in ssh session after...") Fixes: commit d2b6f44779d3 ("n_tty: Fix signal handling flushes") Reported-by: Filipe Brandenburger Signed-off-by: Peter Hurley Signed-off-by: Greg Kroah-Hartman

n_tty: Fix auditing support for cannonical mode

2015-05-31T22:11:04Z

Commit 32f13521ca68bc624ff6effc77f308a52b038bf0 ("n_tty: Line copy to user buffer in canonical mode") changed cannonical mode copying to use copy_to_user but missed adding the call to the audit framework. Add in the appropriate functions to get audit support. Fixes: 32f13521ca68 ("n_tty: Line copy to user buffer in canonical mode") Reported-by: Miloslav Trmač Signed-off-by: Laura Abbott Reviewed-by: Peter Hurley Cc: stable Signed-off-by: Greg Kroah-Hartman

n_tty: Fix calculation of size in canon_copy_from_read_buf

2015-05-24T19:43:50Z

There was a hardcoded value of 4096 which should have been N_TTY_BUF_SIZE. This caused reads from tty to fail with EFAULT when they shouldn't have done if N_TTY_BUF_SIZE was declared to be something other than 4096. Signed-off-by: Mark Tomlinson Reviewed-by: Peter Hurley Signed-off-by: Greg Kroah-Hartman

pty: Fix input race when closing

2015-05-10T17:26:37Z

A read() from a pty master may mistakenly indicate EOF (errno == -EIO) after the pty slave has closed, even though input data remains to be read. For example, pty slave | input worker | pty master | | | | n_tty_read() pty_write() | | input avail? no add data | | sleep schedule worker --->| | . |---> flush_to_ldisc() | . pty_close() | fill read buffer | . wait for worker | wakeup reader --->| . | read buffer full? |---> input avail ? yes |<--- yes - exit worker | copy 4096 bytes to user TTY_OTHER_CLOSED <---| |<--- kick worker | | **** New read() before worker starts **** | | n_tty_read() | | input avail? no | | TTY_OTHER_CLOSED? yes | | return -EIO Several conditions are required to trigger this race: 1. the ldisc read buffer must become full so the input worker exits 2. the read() count parameter must be >= 4096 so the ldisc read buffer is empty 3. the subsequent read() occurs before the kicked worker has processed more input However, the underlying cause of the race is that data is pipelined, while tty state is not; ie., data already written by the pty slave end is not yet visible to the pty master end, but state changes by the pty slave end are visible to the pty master end immediately. Pipeline the TTY_OTHER_CLOSED state through input worker to the reader. 1. Introduce TTY_OTHER_DONE which is set by the input worker when TTY_OTHER_CLOSED is set and either the input buffers are flushed or input processing has completed. Readers/polls are woken when TTY_OTHER_DONE is set. 2. Reader/poll checks TTY_OTHER_DONE instead of TTY_OTHER_CLOSED. 3. A new input worker is started from pty_close() after setting TTY_OTHER_CLOSED, which ensures the TTY_OTHER_DONE state will be set if the last input worker is already finished (or just about to exit). Remove tty_flush_to_ldisc(); no in-tree callers. Fixes: 52bce7f8d4fc ("pty, n_tty: Simplify input processing on final close") Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=96311 BugLink: http://bugs.launchpad.net/bugs/1429756 Cc: # 3.19+ Reported-by: Andy Whitcroft Reported-by: H.J. Lu Signed-off-by: Peter Hurley Signed-off-by: Greg Kroah-Hartman