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    Subject[PATCH 05/24] Restrict /dev/mem and /dev/kmem when the kernel is locked down
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    From: Matthew Garrett <matthew.garrett@nebula.com>

    Allowing users to write to address space makes it possible for the kernel to
    be subverted, avoiding module loading restrictions. Prevent this when the
    kernel has been locked down.

    Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>
    Signed-off-by: David Howells <dhowells@redhat.com>
    ---

    drivers/char/mem.c | 6 ++++++
    1 file changed, 6 insertions(+)

    diff --git a/drivers/char/mem.c b/drivers/char/mem.c
    index 6d9cc2d39d22..f8144049bda3 100644
    --- a/drivers/char/mem.c
    +++ b/drivers/char/mem.c
    @@ -163,6 +163,9 @@ static ssize_t write_mem(struct file *file, const char __user *buf,
    if (p != *ppos)
    return -EFBIG;

    + if (kernel_is_locked_down())
    + return -EPERM;
    +
    if (!valid_phys_addr_range(p, count))
    return -EFAULT;

    @@ -513,6 +516,9 @@ static ssize_t write_kmem(struct file *file, const char __user *buf,
    char *kbuf; /* k-addr because vwrite() takes vmlist_lock rwlock */
    int err = 0;

    + if (kernel_is_locked_down())
    + return -EPERM;
    +
    if (p < (unsigned long) high_memory) {
    unsigned long to_write = min_t(unsigned long, count,
    (unsigned long)high_memory - p);
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