tty: protect tty_write from odd low-level tty disciplines

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>

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 816e709afa5612941fa1241cf21c768710f51282..082da38762fc7c01e0d6a53810d3bc0160e86caf 100644 (file)
--- 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;