Re: [PATCH v2] efi/efi_test: lock down /dev/efi_test and require CAP_SYS_ADMIN
From: Laszlo Ersek <hidden>
Date: 2019-10-08 18:15:44
Also in:
linux-efi, lkml
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 <redacted> --- 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
quoted hunk ↗ jump to hunk
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 wediff --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",