[PATCH 5.10 25/29] tty: protect tty_write from odd low-level tty disciplines

[Greg Kroah-Hartman]

From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>

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@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: 9bb48c82aced ("tty: implement write_iter")
Debugged-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 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
@@ -963,11 +963,14 @@ 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;