Commit 70aca71f authored by Peter Hurley's avatar Peter Hurley Committed by Greg Kroah-Hartman
Browse files

n_tty: Fix unordered accesses to lockless read buffer 



Add commit_head buffer index, which the producer-side publishes
after input processing in non-canon mode. This ensures the consumer-side
observes correctly-ordered writes in non-canonical mode (ie., the buffer
data is written before the buffer index is advanced). Fix consumer-side
uses of read_cnt() to use commit_head instead.

Add required memory barriers to the tail index to guarantee
the consumer-side has completed the loads before the producer-side
begins writing new data. Open-code the producer-side receive_room()
into the i/o loop.

Remove no-longer-referenced receive_room().

Based on work by Christian Riesch <christian.riesch@omicron.at>

Cc: Christian Riesch <christian.riesch@omicron.at>
Signed-off-by: default avatarPeter Hurley <peter@hurleysoftware.com>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent 5e28cca1

drivers/tty/n_tty.c

+48 −53
Original line number Diff line number Diff line
@@ -90,6 +90,7 @@ 
struct n_tty_data {

	/* producer-published */

	size_t read_head;

	size_t commit_head;

	size_t canon_head;

	size_t echo_head;

	size_t echo_commit;
@@ -161,31 +162,6 @@ static inline int tty_put_user(struct tty_struct *tty, unsigned char x, 
	return put_user(x, ptr);

}



static int receive_room(struct tty_struct *tty)

{

	struct n_tty_data *ldata = tty->disc_data;

	int left;



	if (I_PARMRK(tty)) {

		/* Multiply read_cnt by 3, since each byte might take up to

		 * three times as many spaces when PARMRK is set (depending on

		 * its flags, e.g. parity error). */

		left = N_TTY_BUF_SIZE - read_cnt(ldata) * 3 - 1;

	} else

		left = N_TTY_BUF_SIZE - read_cnt(ldata) - 1;



	/*

	 * If we are doing input canonicalization, and there are no

	 * pending newlines, let characters through without limit, so

	 * that erase characters will be handled.  Other excess

	 * characters will be beeped.

	 */

	if (left <= 0)

		left = ldata->icanon && ldata->canon_head == ldata->read_tail;



	return left;

}



/**

 *	n_tty_kick_worker - start input worker (if required)

 *	@tty: terminal
@@ -224,7 +200,7 @@ static ssize_t chars_in_buffer(struct tty_struct *tty) 
	ssize_t n = 0;



	if (!ldata->icanon)

		n = read_cnt(ldata);

		n = ldata->commit_head - ldata->read_tail;

	else

		n = ldata->canon_head - ldata->read_tail;

	return n;
@@ -318,10 +294,6 @@ static void n_tty_check_unthrottle(struct tty_struct *tty) 
 *

 *	n_tty_receive_buf()/producer path:

 *		caller holds non-exclusive termios_rwsem

 *		modifies read_head

 *

 *	read_head is only considered 'published' if canonical mode is

 *	not active.

 */



static inline void put_tty_queue(unsigned char c, struct n_tty_data *ldata)
@@ -345,6 +317,7 @@ static void reset_buffer_flags(struct n_tty_data *ldata) 
{

	ldata->read_head = ldata->canon_head = ldata->read_tail = 0;

	ldata->echo_head = ldata->echo_tail = ldata->echo_commit = 0;

	ldata->commit_head = 0;

	ldata->echo_mark = 0;

	ldata->line_start = 0;
@@ -992,10 +965,6 @@ static inline void finish_erasing(struct n_tty_data *ldata) 
 *

 *	n_tty_receive_buf()/producer path:

 *		caller holds non-exclusive termios_rwsem

 *		modifies read_head

 *

 *	Modifying the read_head is not considered a publish in this context

 *	because canonical mode is active -- only canon_head publishes

 */



static void eraser(unsigned char c, struct tty_struct *tty)
@@ -1144,7 +1113,6 @@ static void isig(int sig, struct tty_struct *tty) 
 *

 *	n_tty_receive_buf()/producer path:

 *		caller holds non-exclusive termios_rwsem

 *		publishes read_head via put_tty_queue()

 *

 *	Note: may get exclusive termios_rwsem if flushing input buffer

 */
@@ -1214,7 +1182,6 @@ static void n_tty_receive_overrun(struct tty_struct *tty) 
 *

 *	n_tty_receive_buf()/producer path:

 *		caller holds non-exclusive termios_rwsem

 *		publishes read_head via put_tty_queue()

 */

static void n_tty_receive_parity_error(struct tty_struct *tty, unsigned char c)

{
@@ -1268,7 +1235,6 @@ n_tty_receive_signal_char(struct tty_struct *tty, int signal, unsigned char c) 
 *	n_tty_receive_buf()/producer path:

 *		caller holds non-exclusive termios_rwsem

 *		publishes canon_head if canonical mode is active

 *		otherwise, publishes read_head via put_tty_queue()

 *

 *	Returns 1 if LNEXT was received, else returns 0

 */
@@ -1381,7 +1347,7 @@ n_tty_receive_char_special(struct tty_struct *tty, unsigned char c) 
handle_newline:

			set_bit(ldata->read_head & (N_TTY_BUF_SIZE - 1), ldata->read_flags);

			put_tty_queue(c, ldata);

			ldata->canon_head = ldata->read_head;

			smp_store_release(&ldata->canon_head, ldata->read_head);

			kill_fasync(&tty->fasync, SIGIO, POLL_IN);

			if (waitqueue_active(&tty->read_wait))

				wake_up_interruptible_poll(&tty->read_wait, POLLIN);
@@ -1531,7 +1497,7 @@ n_tty_receive_char_lnext(struct tty_struct *tty, unsigned char c, char flag) 
 *

 *	n_tty_receive_buf()/producer path:

 *		claims non-exclusive termios_rwsem

 *		publishes read_head and canon_head

 *		publishes commit_head or canon_head

 */



static void
@@ -1542,16 +1508,14 @@ n_tty_receive_buf_real_raw(struct tty_struct *tty, const unsigned char *cp, 
	size_t n, head;



	head = ldata->read_head & (N_TTY_BUF_SIZE - 1);

	n = N_TTY_BUF_SIZE - max(read_cnt(ldata), head);

	n = min_t(size_t, count, n);

	n = min_t(size_t, count, N_TTY_BUF_SIZE - head);

	memcpy(read_buf_addr(ldata, head), cp, n);

	ldata->read_head += n;

	cp += n;

	count -= n;



	head = ldata->read_head & (N_TTY_BUF_SIZE - 1);

	n = N_TTY_BUF_SIZE - max(read_cnt(ldata), head);

	n = min_t(size_t, count, n);

	n = min_t(size_t, count, N_TTY_BUF_SIZE - head);

	memcpy(read_buf_addr(ldata, head), cp, n);

	ldata->read_head += n;

}
@@ -1681,8 +1645,13 @@ static void __receive_buf(struct tty_struct *tty, const unsigned char *cp, 
			tty->ops->flush_chars(tty);

	}



	if ((!ldata->icanon && (read_cnt(ldata) >= ldata->minimum_to_wake)) ||

		L_EXTPROC(tty)) {

	if (ldata->icanon && !L_EXTPROC(tty))

		return;



	/* publish read_head to consumer */

	smp_store_release(&ldata->commit_head, ldata->read_head);



	if ((read_cnt(ldata) >= ldata->minimum_to_wake) || L_EXTPROC(tty)) {

		kill_fasync(&tty->fasync, SIGIO, POLL_IN);

		if (waitqueue_active(&tty->read_wait))

			wake_up_interruptible_poll(&tty->read_wait, POLLIN);
@@ -1699,7 +1668,31 @@ n_tty_receive_buf_common(struct tty_struct *tty, const unsigned char *cp, 
	down_read(&tty->termios_rwsem);



	while (1) {

		room = receive_room(tty);

		/*

		 * When PARMRK is set, multiply read_cnt by 3, since each byte

		 * might take up to three times as many spaces (depending on

		 * its flags, e.g. parity error). [This calculation is wrong.]

		 *

		 * If we are doing input canonicalization, and there are no

		 * pending newlines, let characters through without limit, so

		 * that erase characters will be handled.  Other excess

		 * characters will be beeped.

		 *

		 * paired with store in *_copy_from_read_buf() -- guarantees

		 * the consumer has loaded the data in read_buf up to the new

		 * read_tail (so this producer will not overwrite unread data)

		 */

		size_t tail = smp_load_acquire(&ldata->read_tail);

		size_t head = ldata->read_head;



		if (I_PARMRK(tty))

			room = N_TTY_BUF_SIZE - (head - tail) * 3 - 1;

		else

			room = N_TTY_BUF_SIZE - (head - tail) - 1;



		if (room <= 0)

			room = ldata->icanon && ldata->canon_head == tail;



		n = min(count, room);

		if (!n) {

			if (flow && !room)
@@ -1769,6 +1762,7 @@ static void n_tty_set_termios(struct tty_struct *tty, struct ktermios *old) 
			ldata->canon_head = ldata->read_head;

			ldata->push = 1;

		}

		ldata->commit_head = ldata->read_head;

		ldata->erasing = 0;

		ldata->lnext = 0;

	}
@@ -1909,7 +1903,7 @@ static inline int input_available_p(struct tty_struct *tty, int poll) 
	if (ldata->icanon && !L_EXTPROC(tty))

		return ldata->canon_head != ldata->read_tail;

	else

		return read_cnt(ldata) >= amt;

		return ldata->commit_head - ldata->read_tail >= amt;

}



/**
@@ -1941,10 +1935,11 @@ static int copy_from_read_buf(struct tty_struct *tty, 
	int retval;

	size_t n;

	bool is_eof;

	size_t head = smp_load_acquire(&ldata->commit_head);

	size_t tail = ldata->read_tail & (N_TTY_BUF_SIZE - 1);



	retval = 0;

	n = min(read_cnt(ldata), N_TTY_BUF_SIZE - tail);

	n = min(head - ldata->read_tail, N_TTY_BUF_SIZE - tail);

	n = min(*nr, n);

	if (n) {

		retval = copy_to_user(*b, read_buf_addr(ldata, tail), n);
@@ -1952,9 +1947,10 @@ static int copy_from_read_buf(struct tty_struct *tty, 
		is_eof = n == 1 && read_buf(ldata, tail) == EOF_CHAR(tty);

		tty_audit_add_data(tty, read_buf_addr(ldata, tail), n,

				ldata->icanon);

		ldata->read_tail += n;

		smp_store_release(&ldata->read_tail, ldata->read_tail + n);

		/* Turn single EOF into zero-length read */

		if (L_EXTPROC(tty) && ldata->icanon && is_eof && !read_cnt(ldata))

		if (L_EXTPROC(tty) && ldata->icanon && is_eof &&

		    (head == ldata->read_tail))

			n = 0;

		*b += n;

		*nr -= n;
@@ -1997,7 +1993,7 @@ static int canon_copy_from_read_buf(struct tty_struct *tty, 
	bool eof_push = 0;



	/* N.B. avoid overrun if nr == 0 */

	n = min(*nr, read_cnt(ldata));

	n = min(*nr, smp_load_acquire(&ldata->canon_head) - ldata->read_tail);

	if (!n)

		return 0;
@@ -2047,8 +2043,7 @@ static int canon_copy_from_read_buf(struct tty_struct *tty, 


	if (found)

		clear_bit(eol, ldata->read_flags);

	smp_mb__after_atomic();

	ldata->read_tail += c;

	smp_store_release(&ldata->read_tail, ldata->read_tail + c);



	if (found) {

		if (!ldata->push)

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