3.18-stable review patch. If anyone has any objections, please let me know. ------------------ From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> commit 3d63b7e4ae0dc5e02d28ddd2fa1f945defc68d81 upstream. syzbot is reporting stalls at n_tty_receive_char_special() [1]. This is because comparison is not working as expected since ldata->read_head can change at any moment. Mitigate this by explicitly masking with buffer size when checking condition for "while" loops. [1] https://syzkaller.appspot.com/bug?id=3d7481a346958d9469bebbeb0537d5f056bdd6e8 Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> Reported-by: syzbot <syzbot+18df353d7540aa6b5467@xxxxxxxxxxxxxxxxxxxxxxxxx> Fixes: bc5a5e3f45d04784 ("n_tty: Don't wrap input buffer indices at buffer size") Cc: stable <stable@xxxxxxxxxxxxxxx> Cc: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- drivers/tty/n_tty.c | 13 ++++++++----- 1 file changed, 8 insertions(+), 5 deletions(-) --- a/drivers/tty/n_tty.c +++ b/drivers/tty/n_tty.c @@ -127,6 +127,8 @@ struct n_tty_data { struct mutex output_lock; }; +#define MASK(x) ((x) & (N_TTY_BUF_SIZE - 1)) + static inline size_t read_cnt(struct n_tty_data *ldata) { return ldata->read_head - ldata->read_tail; @@ -1032,14 +1034,15 @@ static void eraser(unsigned char c, stru } seen_alnums = 0; - while (ldata->read_head != ldata->canon_head) { + while (MASK(ldata->read_head) != MASK(ldata->canon_head)) { head = ldata->read_head; /* erase a single possibly multibyte character */ do { head--; c = read_buf(ldata, head); - } while (is_continuation(c, tty) && head != ldata->canon_head); + } while (is_continuation(c, tty) && + MASK(head) != MASK(ldata->canon_head)); /* do not partially erase */ if (is_continuation(c, tty)) @@ -1081,7 +1084,7 @@ static void eraser(unsigned char c, stru * This info is used to go back the correct * number of columns. */ - while (tail != ldata->canon_head) { + while (MASK(tail) != MASK(ldata->canon_head)) { tail--; c = read_buf(ldata, tail); if (c == '\t') { @@ -1341,7 +1344,7 @@ n_tty_receive_char_special(struct tty_st finish_erasing(ldata); echo_char(c, tty); echo_char_raw('\n', ldata); - while (tail != ldata->read_head) { + while (MASK(tail) != MASK(ldata->read_head)) { echo_char(read_buf(ldata, tail), tty); tail++; } @@ -2505,7 +2508,7 @@ static unsigned long inq_canon(struct n_ tail = ldata->read_tail; nr = head - tail; /* Skip EOF-chars.. */ - while (head != tail) { + while (MASK(head) != MASK(tail)) { if (test_bit(tail & (N_TTY_BUF_SIZE - 1), ldata->read_flags) && read_buf(ldata, tail) == __DISABLED_CHAR) nr--;
