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    Subjectbuffer overflow in /proc/sys/sunrpc/transports

    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:

    $ gcc -Wall -std=gnu99 proc-sys-sunrpc-transports.c && ./a.out
    read(0) returned 0, errno = 0
    read(1) returned -1, errno = 21
    read(2) returned -1, errno = 20
    read(3) returned -1, errno = 19
    read(4) returned -1, errno = 18
    read(5) returned -1, errno = 17
    read(6) returned -1, errno = 16
    read(7) returned -1, errno = 15

    ...etc. This program just reopens the file and tries to read a different
    number of characters each time. Strace can be used to verify that it really
    returns these errnos.

    With a small change to the program, we can also compare the buffer address
    passed to read() and see that the kernel wrote the full file into the buffer
    regardless of how many bytes we wanted to get back.

    I don't know if this could be used in some malicious way. Maybe if a setuid
    root program tried to open a user-supplied file (which could be this one in
    /proc), it could crash the program quite easily. But since there is no way
    to change the contents of the file... I don't know. Maybe a denial of
    service attack where some user can make a daemon open a custom file and
    crash? But it relies on the buffer being small enough, and the file is only
    38 bytes on my machine. Who would pass a buffer to read that was smaller
    than 38 bytes anyway? :-)

    This file was introduced in

    commit dc9a16e49dbba3dd042e6aec5d9a7929e099a89b
    Author: Tom Tucker <>
    Date: Sun Dec 30 21:08:31 2007 -0600

    svc: Add /proc/sys/sunrpc/transport files

    Add a file that when read lists the set of registered svc

    Signed-off-by: Tom Tucker <>
    Acked-by: Neil Brown <>
    Reviewed-by: Chuck Lever <>
    Reviewed-by: Greg Banks <>
    Signed-off-by: J. Bruce Fields <>

    ...adding everybody to Cc.




    #include <errno.h>
    #include <fcntl.h>
    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    #include <unistd.h>

    for (int i = 0; i < 20; ++i) {
    int fd = open("/proc/sys/sunrpc/transports", O_RDONLY);
    if (fd == -1)

    char buf[512];
    memset(buf, 0, sizeof(buf));

    int ret = read(fd, buf, i);
    printf("read(%d) returned %d, errno = %d\n", i, ret, errno);


    return EXIT_SUCCESS;

     \ /
      Last update: 2008-08-30 20:47    [W:0.023 / U:0.052 seconds]
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