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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>
This commit is contained in:
Tetsuo Handa 2018-05-26 09:53:14 +09:00 committed by Greg Kroah-Hartman
parent 3d63b7e4ae
commit ebec3f8f52
1 changed files with 24 additions and 18 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

42
drivers/tty/n_tty.c
View File

@ -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)