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tty: protect tty_write from odd low-level tty disciplines

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commit 3342ff2698 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
9bb48c82ac ("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: 9bb48c82ac ("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>
This commit is contained in:
Linus Torvalds 2021-02-20 21:15:00 -08:00 committed by Greg Kroah-Hartman
parent 00805af45a
commit ffec7ee218
1 changed files with 4 additions and 1 deletions
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5
drivers/tty/tty_io.c
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@ -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;