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    Subject[PATCH] sunrpc - fixup userspace buffer possible overrun v2
    Vegard Nossum reported
    ----------------------
    > I noticed that something weird is going on with /proc/sys/sunrpc/transports.
    > This file is generated in net/sunrpc/sysctl.c, function proc_do_xprt(). When
    > I "cat" this file, I get the expected output:
    > $ cat /proc/sys/sunrpc/transports
    > tcp 1048576
    > udp 32768

    > But I think that it does not check the length of the buffer supplied by
    > userspace to read(). With my original program, I found that the stack was
    > being overwritten by the characters above, even when the length given to
    > read() was just 1.

    David Wagner added (among other things) that copy_to_user could be
    probably used here.

    The conclusion is that proc_do_xprt doesn't check for userside buffer
    size indeed so fix. Also set lenp to number of bytes were really written.

    Reported-by: Vegard Nossum <vegard.nossum@gmail.com>
    Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
    CC: David Wagner <daw@cs.berkeley.edu>
    ---

    Please review.

    Index: linux-2.6.git/net/sunrpc/sysctl.c
    ===================================================================
    --- linux-2.6.git.orig/net/sunrpc/sysctl.c 2008-08-31 13:43:46.000000000 +0400
    +++ linux-2.6.git/net/sunrpc/sysctl.c 2008-08-31 13:58:14.000000000 +0400
    @@ -60,23 +60,26 @@ static int proc_do_xprt(ctl_table *table
    void __user *buffer, size_t *lenp, loff_t *ppos)
    {
    char tmpbuf[256];
    - int len;
    + size_t len;
    +
    if ((*ppos && !write) || !*lenp) {
    *lenp = 0;
    return 0;
    }
    +
    if (write)
    return -EINVAL;
    else {
    len = svc_print_xprts(tmpbuf, sizeof(tmpbuf));
    - if (!access_ok(VERIFY_WRITE, buffer, len))
    - return -EFAULT;
    -
    - if (__copy_to_user(buffer, tmpbuf, len))
    + if (len > *lenp)
    + len = *lenp;
    + if (copy_to_user(buffer, tmpbuf, len))
    return -EFAULT;
    }
    - *lenp -= len;
    +
    + *lenp = len;
    *ppos += len;
    +
    return 0;
    }


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