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>

1 files changed, 24 insertions, 18 deletions

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index b279f8730e04..431742201709 100644
--- 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;
@@ -619,13 +618,20 @@ 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
                          * op code or a control character value.
@@ -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)