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    SubjectRe: [PATCH 4.4 22/32] net: hso: fix null-ptr-deref during tty device unregistration
    On Mon, Apr 26, 2021 at 09:29:20AM +0200, Greg Kroah-Hartman wrote:
    > From: Anirudh Rayabharam <mail@anirudhrb.com>
    >
    > commit 8a12f8836145ffe37e9c8733dce18c22fb668b66 upstream
    >
    > Multiple ttys try to claim the same the minor number causing a double
    > unregistration of the same device. The first unregistration succeeds
    > but the next one results in a null-ptr-deref.
    >
    > The get_free_serial_index() function returns an available minor number
    > but doesn't assign it immediately. The assignment is done by the caller
    > later. But before this assignment, calls to get_free_serial_index()
    > would return the same minor number.
    >
    > Fix this by modifying get_free_serial_index to assign the minor number
    > immediately after one is found to be and rename it to obtain_minor()
    > to better reflect what it does. Similary, rename set_serial_by_index()
    > to release_minor() and modify it to free up the minor number of the
    > given hso_serial. Every obtain_minor() should have corresponding
    > release_minor() call.
    >
    > Fixes: 72dc1c096c705 ("HSO: add option hso driver")
    > Reported-by: syzbot+c49fe6089f295a05e6f8@syzkaller.appspotmail.com
    > Tested-by: syzbot+c49fe6089f295a05e6f8@syzkaller.appspotmail.com
    > Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    > Signed-off-by: Anirudh Rayabharam <mail@anirudhrb.com>
    > Signed-off-by: David S. Miller <davem@davemloft.net>
    > [sudip: adjust context]
    > Signed-off-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
    > Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    > ---
    > drivers/net/usb/hso.c | 33 ++++++++++++---------------------
    > 1 file changed, 12 insertions(+), 21 deletions(-)

    We just got a regression report against this one. Perhaps better to hold
    off until that has been resolved.

    https://lore.kernel.org/r/20210425233509.9ce29da49037e1a421000bdd@aruba.it

    Johan

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