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    Subject[PATCH 3.4 168/172] cdc-acm: prevent infinite loop when parsing CDC headers.
    Date
    From: Quentin Casasnovas <quentin.casasnovas@oracle.com>

    3.4.108-rc1 review patch. If anyone has any objections, please let me know.

    ------------------


    commit 0d3bba0287d4e284c3ec7d3397e81eec920d5e7e upstream.

    Phil and I found out a problem with commit:

    7e860a6e7aa6 ("cdc-acm: add sanity checks")

    It added some sanity checks to ignore potential garbage in CDC headers but
    also introduced a potential infinite loop. This can happen at the first
    loop iteration (elength = 0 in that case) if the description isn't a
    DT_CS_INTERFACE or later if 'buffer[0]' is zero.

    It should also be noted that the wrong length was being added to 'buffer'
    in case 'buffer[1]' was not a DT_CS_INTERFACE descriptor, since elength was
    assigned after that check in the loop.

    A specially crafted USB device could be used to trigger this infinite loop.

    Fixes: 7e860a6e7aa6 ("cdc-acm: add sanity checks")
    Signed-off-by: Phil Turnbull <phil.turnbull@oracle.com>
    Signed-off-by: Quentin Casasnovas <quentin.casasnovas@oracle.com>
    CC: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
    CC: Oliver Neukum <oneukum@suse.de>
    CC: Adam Lee <adam8157@gmail.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    Signed-off-by: Zefan Li <lizefan@huawei.com>
    ---
    drivers/usb/class/cdc-acm.c | 7 ++++++-
    1 file changed, 6 insertions(+), 1 deletion(-)

    diff --git a/drivers/usb/class/cdc-acm.c b/drivers/usb/class/cdc-acm.c
    index 57877a9..25bc1d6 100644
    --- a/drivers/usb/class/cdc-acm.c
    +++ b/drivers/usb/class/cdc-acm.c
    @@ -1044,11 +1044,16 @@ static int acm_probe(struct usb_interface *intf,
    }

    while (buflen > 0) {
    + elength = buffer[0];
    + if (!elength) {
    + dev_err(&intf->dev, "skipping garbage byte\n");
    + elength = 1;
    + goto next_desc;
    + }
    if (buffer[1] != USB_DT_CS_INTERFACE) {
    dev_err(&intf->dev, "skipping garbage\n");
    goto next_desc;
    }
    - elength = buffer[0];

    switch (buffer[2]) {
    case USB_CDC_UNION_TYPE: /* we've found it */
    --
    1.9.1


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