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    SubjectRe: [PATCH V34 14/29] ACPI: Limit access to custom_method when the kernel is locked down
    On Fri, Jun 21, 2019 at 05:03:43PM -0700, Matthew Garrett wrote:
    > From: Matthew Garrett <mjg59@srcf.ucam.org>
    >
    > custom_method effectively allows arbitrary access to system memory, making
    > it possible for an attacker to circumvent restrictions on module loading.
    > Disable it if the kernel is locked down.
    >
    > Signed-off-by: Matthew Garrett <mjg59@google.com>

    Reviewed-by: Kees Cook <keescook@chromium.org>

    -Kees

    > Signed-off-by: David Howells <dhowells@redhat.com>
    > cc: linux-acpi@vger.kernel.org
    > ---
    > drivers/acpi/custom_method.c | 6 ++++++
    > include/linux/security.h | 1 +
    > security/lockdown/lockdown.c | 1 +
    > 3 files changed, 8 insertions(+)
    >
    > diff --git a/drivers/acpi/custom_method.c b/drivers/acpi/custom_method.c
    > index aa972dc5cb7e..6e56f9f43492 100644
    > --- a/drivers/acpi/custom_method.c
    > +++ b/drivers/acpi/custom_method.c
    > @@ -8,6 +8,7 @@
    > #include <linux/uaccess.h>
    > #include <linux/debugfs.h>
    > #include <linux/acpi.h>
    > +#include <linux/security.h>
    >
    > #include "internal.h"
    >
    > @@ -28,6 +29,11 @@ static ssize_t cm_write(struct file *file, const char __user * user_buf,
    >
    > struct acpi_table_header table;
    > acpi_status status;
    > + int ret;
    > +
    > + ret = security_locked_down(LOCKDOWN_ACPI_TABLES);
    > + if (ret)
    > + return ret;
    >
    > if (!(*ppos)) {
    > /* parse the table header to get the table length */
    > diff --git a/include/linux/security.h b/include/linux/security.h
    > index 30bc6f058926..cc2b5ee4cadd 100644
    > --- a/include/linux/security.h
    > +++ b/include/linux/security.h
    > @@ -89,6 +89,7 @@ enum lockdown_reason {
    > LOCKDOWN_PCI_ACCESS,
    > LOCKDOWN_IOPORT,
    > LOCKDOWN_MSR,
    > + LOCKDOWN_ACPI_TABLES,
    > LOCKDOWN_INTEGRITY_MAX,
    > LOCKDOWN_CONFIDENTIALITY_MAX,
    > };
    > diff --git a/security/lockdown/lockdown.c b/security/lockdown/lockdown.c
    > index 297a065e6261..1725224f0024 100644
    > --- a/security/lockdown/lockdown.c
    > +++ b/security/lockdown/lockdown.c
    > @@ -25,6 +25,7 @@ static char *lockdown_reasons[LOCKDOWN_CONFIDENTIALITY_MAX+1] = {
    > [LOCKDOWN_PCI_ACCESS] = "direct PCI access",
    > [LOCKDOWN_IOPORT] = "raw io port access",
    > [LOCKDOWN_MSR] = "raw MSR access",
    > + [LOCKDOWN_ACPI_TABLES] = "modified ACPI tables",
    > [LOCKDOWN_INTEGRITY_MAX] = "integrity",
    > [LOCKDOWN_CONFIDENTIALITY_MAX] = "confidentiality",
    > };
    > --
    > 2.22.0.410.gd8fdbe21b5-goog
    >

    --
    Kees Cook

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