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SubjectRe: [PATCH v2] efi/efi_test: lock down /dev/efi_test and require CAP_SYS_ADMIN
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On 10/08/19 12:55, Javier Martinez Canillas wrote:
> The driver exposes EFI runtime services to user-space through an IOCTL
> interface, calling the EFI services function pointers directly without
> using the efivar API.
>
> Disallow access to the /dev/efi_test character device when the kernel is
> locked down to prevent arbitrary user-space to call EFI runtime services.
>
> Also require CAP_SYS_ADMIN to open the chardev to prevent unprivileged
> users to call the EFI runtime services, instead of just relying on the
> chardev file mode bits for this.
>
> The main user of this driver is the fwts [0] tool that already checks if
> the effective user ID is 0 and fails otherwise. So this change shouldn't
> cause any regression to this tool.
>
> [0]: https://wiki.ubuntu.com/FirmwareTestSuite/Reference/uefivarinfo
>
> Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
> Acked-by: Laszlo Ersek <lersek@redhat.com>
>
> ---
>
> Changes in v2:
> - Also disable /dev/efi_test access when the kernel is locked down as
> suggested by Matthew Garrett.

Right; if you remember the pre-patch discussion off-list, we kind of
expected that lockdown might affect this. :)

... And, I can see Matt's comment now, at
<https://bugzilla.redhat.com/show_bug.cgi?id=1759325#c1>. Thanks for that!

While this change decreases the usability of the module, I fully agree
it is justified for production use. While it's more convenient for me to
keep SB enabled in the test VM(s) in general, and just run the test
whenever I need it, security trumps convenience. I can disable SB when
necessary, or even dedicate separate VMs (with SB generally disabled) to
this kind of testing.

> - Add Acked-by tag from Laszlo Ersek.

My ACK stands -- I don't know enough to validate the
security_locked_down() call and its friends, but I'm OK with the intent.

Thanks all!
Laszlo

>
> drivers/firmware/efi/test/efi_test.c | 8 ++++++++
> include/linux/security.h | 1 +
> security/lockdown/lockdown.c | 1 +
> 3 files changed, 10 insertions(+)
>
> diff --git a/drivers/firmware/efi/test/efi_test.c b/drivers/firmware/efi/test/efi_test.c
> index 877745c3aaf..7baf48c01e7 100644
> --- a/drivers/firmware/efi/test/efi_test.c
> +++ b/drivers/firmware/efi/test/efi_test.c
> @@ -14,6 +14,7 @@
> #include <linux/init.h>
> #include <linux/proc_fs.h>
> #include <linux/efi.h>
> +#include <linux/security.h>
> #include <linux/slab.h>
> #include <linux/uaccess.h>
>
> @@ -717,6 +718,13 @@ static long efi_test_ioctl(struct file *file, unsigned int cmd,
>
> static int efi_test_open(struct inode *inode, struct file *file)
> {
> + int ret = security_locked_down(LOCKDOWN_EFI_TEST);
> +
> + if (ret)
> + return ret;
> +
> + if (!capable(CAP_SYS_ADMIN))
> + return -EACCES;
> /*
> * nothing special to do here
> * We do accept multiple open files at the same time as we
> diff --git a/include/linux/security.h b/include/linux/security.h
> index a8d59d612d2..9df7547afc0 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -105,6 +105,7 @@ enum lockdown_reason {
> LOCKDOWN_NONE,
> LOCKDOWN_MODULE_SIGNATURE,
> LOCKDOWN_DEV_MEM,
> + LOCKDOWN_EFI_TEST,
> LOCKDOWN_KEXEC,
> LOCKDOWN_HIBERNATION,
> LOCKDOWN_PCI_ACCESS,
> diff --git a/security/lockdown/lockdown.c b/security/lockdown/lockdown.c
> index 8a10b43daf7..40b790536de 100644
> --- a/security/lockdown/lockdown.c
> +++ b/security/lockdown/lockdown.c
> @@ -20,6 +20,7 @@ static const char *const lockdown_reasons[LOCKDOWN_CONFIDENTIALITY_MAX+1] = {
> [LOCKDOWN_NONE] = "none",
> [LOCKDOWN_MODULE_SIGNATURE] = "unsigned module loading",
> [LOCKDOWN_DEV_MEM] = "/dev/mem,kmem,port",
> + [LOCKDOWN_EFI_TEST] = "/dev/efi_test access",
> [LOCKDOWN_KEXEC] = "kexec of unsigned images",
> [LOCKDOWN_HIBERNATION] = "hibernation",
> [LOCKDOWN_PCI_ACCESS] = "direct PCI access",
>

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