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    Subject[PATCH V31 03/25] Restrict /dev/{mem,kmem,port} when the kernel is locked down
    From
    From: Matthew Garrett <mjg59@srcf.ucam.org>

    Allowing users to read and write to core kernel memory makes it possible
    for the kernel to be subverted, avoiding module loading restrictions, and
    also to steal cryptographic information.

    Disallow /dev/mem and /dev/kmem from being opened this when the kernel has
    been locked down to prevent this.

    Also disallow /dev/port from being opened to prevent raw ioport access and
    thus DMA from being used to accomplish the same thing.

    Signed-off-by: David Howells <dhowells@redhat.com>
    Signed-off-by: Matthew Garrett <mjg59@google.com>
    Cc: x86@kernel.org
    ---
    drivers/char/mem.c | 2 ++
    1 file changed, 2 insertions(+)

    diff --git a/drivers/char/mem.c b/drivers/char/mem.c
    index b08dc50f9f26..67b85939b1bd 100644
    --- a/drivers/char/mem.c
    +++ b/drivers/char/mem.c
    @@ -786,6 +786,8 @@ static loff_t memory_lseek(struct file *file, loff_t offset, int orig)

    static int open_port(struct inode *inode, struct file *filp)
    {
    + if (kernel_is_locked_down("/dev/mem,kmem,port", LOCKDOWN_INTEGRITY))
    + return -EPERM;
    return capable(CAP_SYS_RAWIO) ? 0 : -EPERM;
    }

    --
    2.21.0.392.gf8f6787159e-goog
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