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    Subject[PATCH 3.12 38/76] proc: prevent accessing /proc/<PID>/environ until it's ready
    Date
    From: Mathias Krause <minipli@googlemail.com>

    3.12-stable review patch. If anyone has any objections, please let me know.

    ===============

    commit 8148a73c9901a8794a50f950083c00ccf97d43b3 upstream.

    If /proc/<PID>/environ gets read before the envp[] array is fully set up
    in create_{aout,elf,elf_fdpic,flat}_tables(), we might end up trying to
    read more bytes than are actually written, as env_start will already be
    set but env_end will still be zero, making the range calculation
    underflow, allowing to read beyond the end of what has been written.

    Fix this as it is done for /proc/<PID>/cmdline by testing env_end for
    zero. It is, apparently, intentionally set last in create_*_tables().

    This bug was found by the PaX size_overflow plugin that detected the
    arithmetic underflow of 'this_len = env_end - (env_start + src)' when
    env_end is still zero.

    The expected consequence is that userland trying to access
    /proc/<PID>/environ of a not yet fully set up process may get
    inconsistent data as we're in the middle of copying in the environment
    variables.

    Fixes: https://forums.grsecurity.net/viewtopic.php?f=3&t=4363
    Fixes: https://bugzilla.kernel.org/show_bug.cgi?id=116461
    Signed-off-by: Mathias Krause <minipli@googlemail.com>
    Cc: Emese Revfy <re.emese@gmail.com>
    Cc: Pax Team <pageexec@freemail.hu>
    Cc: Al Viro <viro@zeniv.linux.org.uk>
    Cc: Mateusz Guzik <mguzik@redhat.com>
    Cc: Alexey Dobriyan <adobriyan@gmail.com>
    Cc: Cyrill Gorcunov <gorcunov@openvz.org>
    Cc: Jarod Wilson <jarod@redhat.com>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Jiri Slaby <jslaby@suse.cz>
    ---
    fs/proc/base.c | 3 ++-
    1 file changed, 2 insertions(+), 1 deletion(-)

    diff --git a/fs/proc/base.c b/fs/proc/base.c
    index 293c987a5dab..582d34aaa56e 100644
    --- a/fs/proc/base.c
    +++ b/fs/proc/base.c
    @@ -844,7 +844,8 @@ static ssize_t environ_read(struct file *file, char __user *buf,
    int ret = 0;
    struct mm_struct *mm = file->private_data;

    - if (!mm)
    + /* Ensure the process spawned far enough to have an environment. */
    + if (!mm || !mm->env_end)
    return 0;

    page = (char *)__get_free_page(GFP_TEMPORARY);
    --
    2.8.2
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