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Subject[PATCH 5.11 10/12] tty: protect tty_write from odd low-level tty disciplines
From: Linus Torvalds <>

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.

Fixes: 9bb48c82aced ("tty: implement write_iter")
Debugged-by: Al Viro <>
Signed-off-by: Linus Torvalds <>
Signed-off-by: Greg Kroah-Hartman <>

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)

+ 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)

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