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    Subject[PATCH 4.14 13/23] exec: Force single empty string when argv is empty
    Date
    From: Kees Cook <keescook@chromium.org>

    commit dcd46d897adb70d63e025f175a00a89797d31a43 upstream.

    Quoting[1] Ariadne Conill:

    "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[2]:

    The argument arg0 should point to a filename string that is
    associated with the process being started by one of the exec
    functions.
    ...
    Interestingly, Michael Kerrisk opened an issue about this in 2008[3],
    but there was no consensus to support fixing this issue then.
    Hopefully now that CVE-2021-4034 shows practical exploitative use[4]
    of this bug in a shellcode, we can reconsider.

    This issue is being tracked in the KSPP issue tracker[5]."

    While the initial code searches[6][7] turned up what appeared to be
    mostly corner case tests, trying to that just reject argv == NULL
    (or an immediately terminated pointer list) quickly started tripping[8]
    existing userspace programs.

    The next best approach is forcing a single empty string into argv and
    adjusting argc to match. The number of programs depending on argc == 0
    seems a smaller set than those calling execve with a NULL argv.

    Account for the additional stack space in bprm_stack_limits(). Inject an
    empty string when argc == 0 (and set argc = 1). Warn about the case so
    userspace has some notice about the change:

    process './argc0' launched './argc0' with NULL argv: empty string added

    Additionally WARN() and reject NULL argv usage for kernel threads.

    [1] https://lore.kernel.org/lkml/20220127000724.15106-1-ariadne@dereferenced.org/
    [2] https://pubs.opengroup.org/onlinepubs/9699919799/functions/exec.html
    [3] https://bugzilla.kernel.org/show_bug.cgi?id=8408
    [4] https://www.qualys.com/2022/01/25/cve-2021-4034/pwnkit.txt
    [5] https://github.com/KSPP/linux/issues/176
    [6] https://codesearch.debian.net/search?q=execve%5C+*%5C%28%5B%5E%2C%5D%2B%2C+*NULL&literal=0
    [7] https://codesearch.debian.net/search?q=execlp%3F%5Cs*%5C%28%5B%5E%2C%5D%2B%2C%5Cs*NULL&literal=0
    [8] https://lore.kernel.org/lkml/20220131144352.GE16385@xsang-OptiPlex-9020/

    Reported-by: Ariadne Conill <ariadne@dereferenced.org>
    Reported-by: Michael Kerrisk <mtk.manpages@gmail.com>
    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: linux-fsdevel@vger.kernel.org
    Cc: stable@vger.kernel.org
    Signed-off-by: Kees Cook <keescook@chromium.org>
    Acked-by: Christian Brauner <brauner@kernel.org>
    Acked-by: Ariadne Conill <ariadne@dereferenced.org>
    Acked-by: Andy Lutomirski <luto@kernel.org>
    Link: https://lore.kernel.org/r/20220201000947.2453721-1-keescook@chromium.org
    [vegard: fixed conflicts due to missing
    886d7de631da71e30909980fdbf318f7caade262^- and
    3950e975431bc914f7e81b8f2a2dbdf2064acb0f^- and
    655c16a8ce9c15842547f40ce23fd148aeccc074]
    Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    fs/exec.c | 17 +++++++++++++++++
    1 file changed, 17 insertions(+)

    This has been tested in both argc == 0 and argc >= 1 cases, but I would
    still appreciate a review given the differences with mainline. If it's
    considered too risky I'm also fine with dropping it -- just wanted to
    make sure this didn't fall through the cracks, as it does block a real
    (albeit old by now) exploit.

    --- a/fs/exec.c
    +++ b/fs/exec.c
    @@ -1788,6 +1788,9 @@ static int do_execveat_common(int fd, st
    goto out_unmark;

    bprm->argc = count(argv, MAX_ARG_STRINGS);
    + if (bprm->argc == 0)
    + pr_warn_once("process '%s' launched '%s' with NULL argv: empty string added\n",
    + current->comm, bprm->filename);
    if ((retval = bprm->argc) < 0)
    goto out;

    @@ -1812,6 +1815,20 @@ static int do_execveat_common(int fd, st
    if (retval < 0)
    goto out;

    + /*
    + * When argv is empty, add an empty string ("") as argv[0] to
    + * ensure confused userspace programs that start processing
    + * from argv[1] won't end up walking envp. See also
    + * bprm_stack_limits().
    + */
    + if (bprm->argc == 0) {
    + const char *argv[] = { "", NULL };
    + retval = copy_strings_kernel(1, argv, bprm);
    + if (retval < 0)
    + goto out;
    + bprm->argc = 1;
    + }
    +
    retval = exec_binprm(bprm);
    if (retval < 0)
    goto out;

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