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    Subject[PATCH 4.14 085/125] vt: keyboard, extend func_buf_lock to readers
    Date
    From: Jiri Slaby <jslaby@suse.cz>

    commit 82e61c3909db51d91b9d3e2071557b6435018b80 upstream.

    Both read-side users of func_table/func_buf need locking. Without that,
    one can easily confuse the code by repeatedly setting altering strings
    like:
    while (1)
    for (a = 0; a < 2; a++) {
    struct kbsentry kbs = {};
    strcpy((char *)kbs.kb_string, a ? ".\n" : "88888\n");
    ioctl(fd, KDSKBSENT, &kbs);
    }

    When that program runs, one can get unexpected output by holding F1
    (note the unxpected period on the last line):
    .
    88888
    .8888

    So protect all accesses to 'func_table' (and func_buf) by preexisting
    'func_buf_lock'.

    It is easy in 'k_fn' handler as 'puts_queue' is expected not to sleep.
    On the other hand, KDGKBSENT needs a local (atomic) copy of the string
    because copy_to_user can sleep. Use already allocated, but unused
    'kbs->kb_string' for that purpose.

    Note that the program above needs at least CAP_SYS_TTY_CONFIG.

    This depends on the previous patch and on the func_buf_lock lock added
    in commit 46ca3f735f34 (tty/vt: fix write/write race in ioctl(KDSKBSENT)
    handler) in 5.2.

    Likely fixes CVE-2020-25656.

    Cc: <stable@vger.kernel.org>
    Reported-by: Minh Yuan <yuanmingbuaa@gmail.com>
    Signed-off-by: Jiri Slaby <jslaby@suse.cz>
    Link: https://lore.kernel.org/r/20201019085517.10176-2-jslaby@suse.cz
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    drivers/tty/vt/keyboard.c | 17 +++++++++++++----
    1 file changed, 13 insertions(+), 4 deletions(-)

    --- a/drivers/tty/vt/keyboard.c
    +++ b/drivers/tty/vt/keyboard.c
    @@ -713,8 +713,13 @@ static void k_fn(struct vc_data *vc, uns
    return;

    if ((unsigned)value < ARRAY_SIZE(func_table)) {
    + unsigned long flags;
    +
    + spin_lock_irqsave(&func_buf_lock, flags);
    if (func_table[value])
    puts_queue(vc, func_table[value]);
    + spin_unlock_irqrestore(&func_buf_lock, flags);
    +
    } else
    pr_err("k_fn called with value=%d\n", value);
    }
    @@ -1959,7 +1964,7 @@ out:
    #undef s
    #undef v

    -/* FIXME: This one needs untangling and locking */
    +/* FIXME: This one needs untangling */
    int vt_do_kdgkb_ioctl(int cmd, struct kbsentry __user *user_kdgkb, int perm)
    {
    struct kbsentry *kbs;
    @@ -1991,10 +1996,14 @@ int vt_do_kdgkb_ioctl(int cmd, struct kb
    switch (cmd) {
    case KDGKBSENT: {
    /* size should have been a struct member */
    - unsigned char *from = func_table[i] ? : "";
    + ssize_t len = sizeof(user_kdgkb->kb_string);
    +
    + spin_lock_irqsave(&func_buf_lock, flags);
    + len = strlcpy(kbs->kb_string, func_table[i] ? : "", len);
    + spin_unlock_irqrestore(&func_buf_lock, flags);

    - ret = copy_to_user(user_kdgkb->kb_string, from,
    - strlen(from) + 1) ? -EFAULT : 0;
    + ret = copy_to_user(user_kdgkb->kb_string, kbs->kb_string,
    + len + 1) ? -EFAULT : 0;

    goto reterr;
    }

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