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    SubjectRe: buffer overflow in /proc/sys/sunrpc/transports
    On Sat, Aug 30, 2008 at 9:06 PM, Cyrill Gorcunov <gorcunov@gmail.com> wrote:
    > [Vegard Nossum - Sat, Aug 30, 2008 at 08:44:22PM +0200]
    > | Hi,
    > |
    > | 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. So I have created a test program, see it at the bottom of
    > | this e-mail. Here is its output:
    > |
    > ...
    >
    > Indeed, maybe just add checking for user buffer length?
    > As proc_dodebug() in this file are doing. I don't think
    > the user would be happy with his stack burned :)
    >
    > Something like:
    > ---
    >
    > Index: linux-2.6.git/net/sunrpc/sysctl.c
    > ===================================================================
    > --- linux-2.6.git.orig/net/sunrpc/sysctl.c 2008-07-20 11:40:14.000000000 +0400
    > +++ linux-2.6.git/net/sunrpc/sysctl.c 2008-08-30 23:05:30.000000000 +0400
    > @@ -69,6 +69,8 @@ static int proc_do_xprt(ctl_table *table
    > return -EINVAL;
    > else {
    > len = svc_print_xprts(tmpbuf, sizeof(tmpbuf));
    > + if (*lenp < len)
    > + return -EFAULT;
    > if (!access_ok(VERIFY_WRITE, buffer, len))
    > return -EFAULT;
    >
    >

    BTW, look at this:

    $ od -A x -t x1z /proc/sys/sunrpc/transports
    000000 74 63 70 20 31 30 34 38 35 37 36 0a 75 64 70 20 >tcp 1048576.udp <
    000010 33 32 37 36 38 0a 00 00 00 00 00 00 00 00 00 00 >32768...........<
    000020 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 >................<
    *
    0003e0 00 00 00 00 00 00 00 00 00 00 >..........<
    0003ea

    ...and:

    $ strace -e trace=read cat /proc/sys/sunrpc/transports > /dev/null
    read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0\3\0\1\0\0\0@G\316E4\0\0\0"...,
    512) = 512
    read(3, "tcp 1048576\nudp 32768\n\0\0\0\0\0\0\0\0\0\0"..., 4096) = 4074
    read(3, "", 4096) = 0

    ...why does it have a huge return value? The output is only about 40
    bytes... why add all the \0? Would your patch also fix this?


    Vegard

    --
    "The animistic metaphor of the bug that maliciously sneaked in while
    the programmer was not looking is intellectually dishonest as it
    disguises that the error is the programmer's own creation."
    -- E. W. Dijkstra, EWD1036


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