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author | Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> | 2018-05-26 03:53:14 +0300 |
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committer | Greg Kroah-Hartman <gregkh@linuxfoundation.org> | 2018-06-28 15:30:16 +0300 |
commit | ebec3f8f5271139df618ebdf8427e24ba102ba94 (patch) | |
tree | 6badd6fd0faa57bf1947bd98af158590ef6857c9 /drivers/tty/n_tty.c | |
parent | 3d63b7e4ae0dc5e02d28ddd2fa1f945defc68d81 (diff) | |
download | linux-ebec3f8f5271139df618ebdf8427e24ba102ba94.tar.xz |
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>
Diffstat (limited to 'drivers/tty/n_tty.c')
-rw-r--r-- | drivers/tty/n_tty.c | 42 |
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) |