n_tty: Access echo_* variables carefully.

syzbot is reporting stalls at __process_echoes() [1]. This is because
since ldata->echo_commit < ldata->echo_tail becomes true for some reason,
the discard loop is serving as almost infinite loop. This patch tries to
avoid falling into ldata->echo_commit < ldata->echo_tail situation by
making access to echo_* variables more carefully.

Since reset_buffer_flags() is called without output_lock held, it should
not touch echo_* variables. And omit a call to reset_buffer_flags() from
n_tty_open() by using vzalloc().

Since add_echo_byte() is called without output_lock held, it needs memory
barrier between storing into echo_buf[] and incrementing echo_head counter.
echo_buf() needs corresponding memory barrier before reading echo_buf[].
Lack of handling the possibility of not-yet-stored multi-byte operation
might be the reason of falling into ldata->echo_commit < ldata->echo_tail
situation, for if I do WARN_ON(ldata->echo_commit == tail + 1) prior to
echo_buf(ldata, tail + 1), the WARN_ON() fires.

Also, explicitly masking with buffer for the former "while" loop, and
use ldata->echo_commit > tail for the latter "while" loop.

[1] https://syzkaller.appspot.com/bug?id=17f23b094cd80df750e5b0f8982c521ee6bcbf40

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reported-by: syzbot <syzbot+108696293d7a21ab688f@syzkaller.appspotmail.com>
Cc: Peter Hurley <peter@hurleysoftware.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index b279f8730e04e76b70438c8549457c2b4d34114c..43174220170924e094567ad6271703bd881e2a6f 100644 (file)
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -143,6 +143,7 @@ static inline unsigned char *read_buf_addr(struct n_tty_data *ldata, size_t i)
 
 static inline unsigned char echo_buf(struct n_tty_data *ldata, size_t i)
 {
+       smp_rmb(); /* Matches smp_wmb() in add_echo_byte(). */
        return ldata->echo_buf[i & (N_TTY_BUF_SIZE - 1)];
 }
 
@@ -318,9 +319,7 @@ static inline void put_tty_queue(unsigned char c, struct n_tty_data *ldata)
 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;
 
        ldata->erasing = 0;
@@ -619,12 +618,19 @@ static size_t __process_echoes(struct tty_struct *tty)
        old_space = space = tty_write_room(tty);
 
        tail = ldata->echo_tail;
-       while (ldata->echo_commit != tail) {
+       while (MASK(ldata->echo_commit) != MASK(tail)) {
                c = echo_buf(ldata, tail);
                if (c == ECHO_OP_START) {
                        unsigned char op;
                        int no_space_left = 0;
 
+                       /*
+                        * Since add_echo_byte() is called without holding
+                        * output_lock, we might see only portion of multi-byte
+                        * operation.
+                        */
+                       if (MASK(ldata->echo_commit) == MASK(tail + 1))
+                               goto not_yet_stored;
                        /*
                         * If the buffer byte is the start of a multi-byte
                         * operation, get the next byte, which is either the
@@ -636,6 +642,8 @@ static size_t __process_echoes(struct tty_struct *tty)
                                unsigned int num_chars, num_bs;
 
                        case ECHO_OP_ERASE_TAB:
+                               if (MASK(ldata->echo_commit) == MASK(tail + 2))
+                                       goto not_yet_stored;
                                num_chars = echo_buf(ldata, tail + 2);
 
                                /*
@@ -730,7 +738,8 @@ static size_t __process_echoes(struct tty_struct *tty)
        /* If the echo buffer is nearly full (so that the possibility exists
         * of echo overrun before the next commit), then discard enough
         * data at the tail to prevent a subsequent overrun */
-       while (ldata->echo_commit - tail >= ECHO_DISCARD_WATERMARK) {
+       while (ldata->echo_commit > tail &&
+              ldata->echo_commit - tail >= ECHO_DISCARD_WATERMARK) {
                if (echo_buf(ldata, tail) == ECHO_OP_START) {
                        if (echo_buf(ldata, tail + 1) == ECHO_OP_ERASE_TAB)
                                tail += 3;
@@ -740,6 +749,7 @@ static size_t __process_echoes(struct tty_struct *tty)
                        tail++;
        }
 
+ not_yet_stored:
        ldata->echo_tail = tail;
        return old_space - space;
 }
@@ -750,6 +760,7 @@ static void commit_echoes(struct tty_struct *tty)
        size_t nr, old, echoed;
        size_t head;
 
+       mutex_lock(&ldata->output_lock);
        head = ldata->echo_head;
        ldata->echo_mark = head;
        old = ldata->echo_commit - ldata->echo_tail;
@@ -758,10 +769,12 @@ static void commit_echoes(struct tty_struct *tty)
         * is over the threshold (and try again each time another
         * block is accumulated) */
        nr = head - ldata->echo_tail;
-       if (nr < ECHO_COMMIT_WATERMARK || (nr % ECHO_BLOCK > old % ECHO_BLOCK))
+       if (nr < ECHO_COMMIT_WATERMARK ||
+           (nr % ECHO_BLOCK > old % ECHO_BLOCK)) {
+               mutex_unlock(&ldata->output_lock);
                return;
+       }
 
-       mutex_lock(&ldata->output_lock);
        ldata->echo_commit = head;
        echoed = __process_echoes(tty);
        mutex_unlock(&ldata->output_lock);
@@ -812,7 +825,9 @@ static void flush_echoes(struct tty_struct *tty)
 
 static inline void add_echo_byte(unsigned char c, struct n_tty_data *ldata)
 {
-       *echo_buf_addr(ldata, ldata->echo_head++) = c;
+       *echo_buf_addr(ldata, ldata->echo_head) = c;
+       smp_wmb(); /* Matches smp_rmb() in echo_buf(). */
+       ldata->echo_head++;
 }
 
 /**
@@ -1881,30 +1896,21 @@ static int n_tty_open(struct tty_struct *tty)
        struct n_tty_data *ldata;
 
        /* Currently a malloc failure here can panic */
-       ldata = vmalloc(sizeof(*ldata));
+       ldata = vzalloc(sizeof(*ldata));
        if (!ldata)
-               goto err;
+               return -ENOMEM;
 
        ldata->overrun_time = jiffies;
        mutex_init(&ldata->atomic_read_lock);
        mutex_init(&ldata->output_lock);
 
        tty->disc_data = ldata;
-       reset_buffer_flags(tty->disc_data);
-       ldata->column = 0;
-       ldata->canon_column = 0;
-       ldata->num_overrun = 0;
-       ldata->no_room = 0;
-       ldata->lnext = 0;
        tty->closing = 0;
        /* indicate buffer work may resume */
        clear_bit(TTY_LDISC_HALTED, &tty->flags);
        n_tty_set_termios(tty, NULL);
        tty_unthrottle(tty);
-
        return 0;
-err:
-       return -ENOMEM;
 }
 
 static inline int input_available_p(struct tty_struct *tty, int poll)