[PATCH 4.4 06/47] n_tty: Access echo_* variables carefully.
From: Greg Kroah-Hartman
Date: Tue Jul 10 2018 - 14:28:03 EST
4.4-stable review patch. If anyone has any objections, please let me know.
------------------
From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
commit ebec3f8f5271139df618ebdf8427e24ba102ba94 upstream.
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@xxxxxxxxxxxxxxxxxxx>
Reported-by: syzbot <syzbot+108696293d7a21ab688f@xxxxxxxxxxxxxxxxxxxxxxxxx>
Cc: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>
Cc: stable <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
drivers/tty/n_tty.c | 42 ++++++++++++++++++++++++------------------
1 file changed, 24 insertions(+), 18 deletions(-)
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -147,6 +147,7 @@ static inline unsigned char *read_buf_ad
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)];
}
@@ -324,9 +325,7 @@ static inline void put_tty_queue(unsigne
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;
@@ -647,13 +646,20 @@ static size_t __process_echoes(struct tt
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.
@@ -664,6 +670,8 @@ static size_t __process_echoes(struct tt
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);
/*
@@ -758,7 +766,8 @@ static size_t __process_echoes(struct tt
/* 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;
@@ -768,6 +777,7 @@ static size_t __process_echoes(struct tt
tail++;
}
+ not_yet_stored:
ldata->echo_tail = tail;
return old_space - space;
}
@@ -778,6 +788,7 @@ static void commit_echoes(struct tty_str
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;
@@ -786,10 +797,12 @@ static void commit_echoes(struct tty_str
* 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);
@@ -840,7 +853,9 @@ static void flush_echoes(struct tty_stru
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++;
}
/**
@@ -1920,31 +1935,22 @@ static int n_tty_open(struct tty_struct
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->minimum_to_wake = 1;
- 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)