[PATCH 5.11 10/12] tty: protect tty_write from odd low-level tty disciplines

Greg Kroah-Hartman
2021-02-22T12:15:03Z

From: Linus Torvalds <torvalds@linux-foundation.org>

commit 3342ff2698e9720f4040cc458a2744b2b32f5c3a upstream.

Al root-caused a new warning from syzbot to the ttyprintk tty driver
returning a write count larger than the data the tty layer actually gave
it. Which confused the tty write code mightily, and with the new
iov_iter based code, caused a WARNING in iov_iter_revert().

syzbot correctly bisected the source of the new warning to commit
9bb48c82aced ("tty: implement write_iter"), but the oddity goes back
much further, it just didn't get caught by anything before.

Reported-by: syzbot+3d2c27c2b7dc2a94814d@syzkaller.appspotmail.com
Fixes: 9bb48c82aced ("tty: implement write_iter")
Debugged-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/tty/tty_io.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)

--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -962,11 +962,14 @@ static inline ssize_t do_tty_write(
if (ret <= 0)
break;

+ written += ret;
+ if (ret > size)
+ break;
+
/* FIXME! Have Al check this! */
if (ret != size)
iov_iter_revert(from, size-ret);

- written += ret;
count -= ret;
if (!count)
break;