lkml.org 
[lkml]   [2022]   [Jan]   [26]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    Patch in this message
    /
    From
    Subject[PATCH v3] fs/exec: require argv[0] presence in do_execveat_common()
    Date
    In several other operating systems, it is a hard requirement that the
    second argument to execve(2) be the name of a program, thus prohibiting
    a scenario where argc < 1. POSIX 2017 also recommends this behaviour,
    but it is not an explicit requirement[0]:

    The argument arg0 should point to a filename string that is
    associated with the process being started by one of the exec
    functions.

    To ensure that execve(2) with argc < 1 is not a useful tool for
    shellcode to use, we can validate this in do_execveat_common() and
    fail for this scenario, effectively blocking successful exploitation
    of CVE-2021-4034 and similar bugs which depend on execve(2) working
    with argc < 1.

    We use -EINVAL for this case, mirroring recent changes to FreeBSD and
    OpenBSD. -EINVAL is also used by QNX for this, while Solaris uses
    -EFAULT.

    In earlier versions of the patch, it was proposed that we create a
    fake argv for applications to use when argc < 1, but it was concluded
    that it would be better to just fail the execve(2) in these cases, as
    launching a process with an empty or NULL argv[0] was likely to just
    cause more problems.

    Interestingly, Michael Kerrisk opened an issue about this in 2008[1],
    but there was no consensus to support fixing this issue then.
    Hopefully now that CVE-2021-4034 shows practical exploitative use[2]
    of this bug in a shellcode, we can reconsider.

    This issue is being tracked in the KSPP issue tracker[3].

    There are a few[4][5] minor edge cases (primarily in test suites) that
    are caught by this, but we plan to work with the projects to fix those
    edge cases.

    [0]: https://pubs.opengroup.org/onlinepubs/9699919799/functions/exec.html
    [1]: https://bugzilla.kernel.org/show_bug.cgi?id=8408
    [2]: https://www.qualys.com/2022/01/25/cve-2021-4034/pwnkit.txt
    [3]: https://github.com/KSPP/linux/issues/176
    [4]: https://codesearch.debian.net/search?q=execve%5C+*%5C%28%5B%5E%2C%5D%2B%2C+*NULL&literal=0
    [5]: https://codesearch.debian.net/search?q=execlp%3F%5Cs*%5C%28%5B%5E%2C%5D%2B%2C%5Cs*NULL&literal=0

    Changes from v2:
    - Switch to using -EINVAL as the error code for this.
    - Use pr_warn_once() to warn when an execve(2) is rejected due to NULL
    argv.

    Changes from v1:
    - Rework commit message significantly.
    - Make the argv[0] check explicit rather than hijacking the error-check
    for count().

    Reported-by: Michael Kerrisk <mtk.manpages@gmail.com>
    To: Andrew Morton <akpm@linux-foundation.org>
    Cc: Matthew Wilcox <willy@infradead.org>
    Cc: Christian Brauner <brauner@kernel.org>
    Cc: Rich Felker <dalias@libc.org>
    Cc: Eric Biederman <ebiederm@xmission.com>
    Cc: Alexander Viro <viro@zeniv.linux.org.uk>
    Cc: Kees Cook <keescook@chromium.org>
    Cc: linux-fsdevel@vger.kernel.org
    Cc: linux-mm@kvack.org
    Cc: stable@vger.kernel.org
    Signed-off-by: Ariadne Conill <ariadne@dereferenced.org>
    ---
    fs/exec.c | 4 ++++
    1 file changed, 4 insertions(+)

    diff --git a/fs/exec.c b/fs/exec.c
    index 79f2c9483302..982730cfe3b8 100644
    --- a/fs/exec.c
    +++ b/fs/exec.c
    @@ -1897,6 +1897,10 @@ static int do_execveat_common(int fd, struct filename *filename,
    }

    retval = count(argv, MAX_ARG_STRINGS);
    + if (retval == 0) {
    + pr_warn_once("Attempted to run process '%s' with NULL argv\n", bprm->filename);
    + retval = -EINVAL;
    + }
    if (retval < 0)
    goto out_free;
    bprm->argc = retval;
    --
    2.34.1
    \
     
     \ /
      Last update: 2022-01-27 01:08    [W:5.983 / U:0.180 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site