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    Subject[PATCH V38 05/29] 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>
    Reviewed-by: Kees Cook <keescook@chromium.org>
    Cc: x86@kernel.org
    ---
    drivers/char/mem.c | 7 +++++--
    include/linux/security.h | 1 +
    security/lockdown/lockdown.c | 1 +
    3 files changed, 7 insertions(+), 2 deletions(-)

    diff --git a/drivers/char/mem.c b/drivers/char/mem.c
    index b08dc50f9f26..d0148aee1aab 100644
    --- a/drivers/char/mem.c
    +++ b/drivers/char/mem.c
    @@ -29,8 +29,8 @@
    #include <linux/export.h>
    #include <linux/io.h>
    #include <linux/uio.h>
    -
    #include <linux/uaccess.h>
    +#include <linux/security.h>

    #ifdef CONFIG_IA64
    # include <linux/efi.h>
    @@ -786,7 +786,10 @@ static loff_t memory_lseek(struct file *file, loff_t offset, int orig)

    static int open_port(struct inode *inode, struct file *filp)
    {
    - return capable(CAP_SYS_RAWIO) ? 0 : -EPERM;
    + if (!capable(CAP_SYS_RAWIO))
    + return -EPERM;
    +
    + return security_locked_down(LOCKDOWN_DEV_MEM);
    }

    #define zero_lseek null_lseek
    diff --git a/include/linux/security.h b/include/linux/security.h
    index 8e70063074a1..9458152601b5 100644
    --- a/include/linux/security.h
    +++ b/include/linux/security.h
    @@ -104,6 +104,7 @@ enum lsm_event {
    enum lockdown_reason {
    LOCKDOWN_NONE,
    LOCKDOWN_MODULE_SIGNATURE,
    + LOCKDOWN_DEV_MEM,
    LOCKDOWN_INTEGRITY_MAX,
    LOCKDOWN_CONFIDENTIALITY_MAX,
    };
    diff --git a/security/lockdown/lockdown.c b/security/lockdown/lockdown.c
    index 2c53fd9f5c9b..d2ef29d9f0b2 100644
    --- a/security/lockdown/lockdown.c
    +++ b/security/lockdown/lockdown.c
    @@ -19,6 +19,7 @@ static enum lockdown_reason kernel_locked_down;
    static char *lockdown_reasons[LOCKDOWN_CONFIDENTIALITY_MAX+1] = {
    [LOCKDOWN_NONE] = "none",
    [LOCKDOWN_MODULE_SIGNATURE] = "unsigned module loading",
    + [LOCKDOWN_DEV_MEM] = "/dev/mem,kmem,port",
    [LOCKDOWN_INTEGRITY_MAX] = "integrity",
    [LOCKDOWN_CONFIDENTIALITY_MAX] = "confidentiality",
    };
    --
    2.22.0.770.g0f2c4a37fd-goog
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