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Subject[PATCH] vt: Handle recursion in vc_do_resize().
Date
syzbot is reporting OOB read bug in vc_do_resize() [1] caused by memcpy()
based on outdated old_{rows,row_size} values, for resize_screen() can
recurse into vc_do_resize() which changes vc->vc_{cols,rows} that outdates
old_{rows,row_size} values which were read before calling resize_screen().

Minimal fix might be to read vc->vc_{rows,size_row} after resize_screen().
A different fix might be to forbid recursive vc_do_resize() request.
I can't tell which fix is the better.

But since I guess that new_cols == vc->vc_cols && new_rows == vc->vc_rows
check could become true after returning from resize_screen(), and I assume
that not calling clear_selection() when resize_screen() will return error
is harmless, let's redo the check by moving resize_screen() earlier.

[1] https://syzkaller.appspot.com/bug?id=c70c88cfd16dcf6e1d3c7f0ab8648b3144b5b25e

Reported-by: syzbot <syzbot+c37a14770d51a085a520@syzkaller.appspotmail.com>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
---
drivers/tty/vt/vt.c | 24 +++++++++++++++++-------
1 file changed, 17 insertions(+), 7 deletions(-)

diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index 42d8c67..952a067 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -1217,7 +1217,24 @@ static int vc_do_resize(struct tty_struct *tty, struct vc_data *vc,

if (new_cols == vc->vc_cols && new_rows == vc->vc_rows)
return 0;
+ if (new_screen_size > KMALLOC_MAX_SIZE || !new_screen_size)
+ return -EINVAL;

+ /*
+ * Since fbcon_resize() from resize_screen() can recurse into
+ * this function via fb_set_var(), handle recursion now.
+ */
+ err = resize_screen(vc, new_cols, new_rows, user);
+ if (err)
+ return err;
+ /* Reload values in case recursion changed vc->vc_{cols,rows}. */
+ new_cols = (cols ? cols : vc->vc_cols);
+ new_rows = (lines ? lines : vc->vc_rows);
+ new_row_size = new_cols << 1;
+ new_screen_size = new_row_size * new_rows;
+
+ if (new_cols == vc->vc_cols && new_rows == vc->vc_rows)
+ return 0;
if (new_screen_size > KMALLOC_MAX_SIZE || !new_screen_size)
return -EINVAL;
newscreen = kzalloc(new_screen_size, GFP_USER);
@@ -1238,13 +1255,6 @@ static int vc_do_resize(struct tty_struct *tty, struct vc_data *vc,
old_rows = vc->vc_rows;
old_row_size = vc->vc_size_row;

- err = resize_screen(vc, new_cols, new_rows, user);
- if (err) {
- kfree(newscreen);
- vc_uniscr_free(new_uniscr);
- return err;
- }
-
vc->vc_rows = new_rows;
vc->vc_cols = new_cols;
vc->vc_size_row = new_row_size;
--
1.8.3.1
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