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Message-ID: <CABV1ukEiHm2kHwLxFzAdSVix3Kuc1gvgHPZxU3OZ6M_E7DMgWQ@mail.gmail.com>
Date: Tue, 2 Aug 2011 19:44:23 -0700
From: "Andrew G. Morgan" <agm@...gle.com>
To: Mike Waychison <mikew@...gle.com>
Cc: "H. Peter Anvin" <hpa@...or.com>, Maximilian Attems <max@...o.at>,
Alan Cox <alan@...rguk.ukuu.org.uk>,
Eric Northup <digitaleric@...gle.com>,
Eric Paris <eparis@...isplace.org>, klibc@...or.com,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] kinit: Add drop_capabilities support.
On Tue, Aug 2, 2011 at 6:16 PM, Mike Waychison <mikew@...gle.com> wrote:
> This patch adds the ability to kinit to allow the dropping of POSIX
> capabilities.
>
> kinit is modified by this change, such that it understands the new
> kernel command line "drop_capabilities=" that specifies a comma
> separated list of capability names that should be dropped before
> switching over to the next init in the boot strap (typically on the root
> disk).
>
> When processing capabilities to drop, CAP_SETPCAP is special cased to be
> dropped last, so that the order that capabilities are given does not
> cause dropping of later enumerated capabilities to fail if it is listed
> early on.
Given your change to only touch the inheritable set, you do not need
to treat CAP_SETPCAP any differently from the other capabilities,
since dropping it from the inheritable and bounding set will not
affect the running process in any problematic way until it runs
exec().
>
> Dropping of capabilities happens in three parts. We explicitly drop the
> capability from init's inherited masks. We also drop the capability
> from the bounding set using PR_CAPBSET_DROP so that later setuid execs
> are bounded. Lastly, we drop the capabilities from the bset and
> inheritted masks exposed at /proc/sys/kernel/usermodehelper if available
s/tt/t/
Cheers
Andrew
> (introduced in Linux v3.0.0).
>
> In all paths, we treat errors as fatal, as we do not want to continue to
> boot if there was a problem dropping capabilities. We fail because the
> new drop_capabilities= option on the command line mandates enforcement
> of a security policy, and we should err on the side of caution if we
> ever fail to satisfy the administrator's intention.
>
> Signed-off-by: Mike Waychison <mikew@...gle.com>
> ---
> v2
> - Only drop from the inherited capabilities mask. Only this mask
> was required as per Andrew Morgan. This also allows us to
> continue having capabilities to call run-init later on, allowing
> the following:
> - Moved dropping of capabilities out of run-init and into kinit.
> This was required to ensure access to the proc filesystem, which
> is required for manipulating the masks the kernel uses when
> spawning new tasks on the kernel's behalf via the
> call_usermodehelper() routine.
> - Fixed paths to the usermodehelper settings files.
> - Fixed sscanf to allow for a newline character when reading the
> usermodehelper capability mask settings.
> - Fixed the return check of the write to the usermodehelper files.
> These proc files return 0 on successful write(2).
> - Added a format atttribute to the fail() wrapper for better
> compiler warnings.
> - Removed kernel version check. The boot will now always fail if we
> could not manipulate the usermodehelper settings.
> - Sent as a single patch as capset/capget are now in klibc master.
> ---
> usr/kinit/Kbuild | 1
> usr/kinit/capabilities.c | 237 ++++++++++++++++++++++++++++++++++++++++++++++
> usr/kinit/kinit.c | 2
> usr/kinit/kinit.h | 2
> 4 files changed, 242 insertions(+), 0 deletions(-)
> create mode 100644 usr/kinit/capabilities.c
>
> diff --git a/usr/kinit/Kbuild b/usr/kinit/Kbuild
> index ff1d449..8f6d08e 100644
> --- a/usr/kinit/Kbuild
> +++ b/usr/kinit/Kbuild
> @@ -10,6 +10,7 @@ kinit-y := lib.a
> kinit-y += kinit.o do_mounts.o ramdisk_load.o initrd.o
> kinit-y += getintfile.o readfile.o xpio.o
> kinit-y += do_mounts_md.o do_mounts_mtd.o nfsroot.o
> +kinit-y += capabilities.o
>
> kinit-y += ipconfig/
> kinit-y += nfsmount/
> diff --git a/usr/kinit/capabilities.c b/usr/kinit/capabilities.c
> new file mode 100644
> index 0000000..ad4d6ea
> --- /dev/null
> +++ b/usr/kinit/capabilities.c
> @@ -0,0 +1,237 @@
> +/*
> + * Copyright 2011 Google Inc. All Rights Reserved
> + * Author: mikew@...gle.com (Mike Waychison)
> + */
> +
> +/*
> + * We have to include the klibc types.h here to keep the kernel's
> + * types.h from being used.
> + */
> +#include <sys/types.h>
> +
> +#include <sys/capability.h>
> +#include <sys/prctl.h>
> +#include <sys/utsname.h>
> +#include <errno.h>
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <string.h>
> +#include <unistd.h>
> +
> +#include "kinit.h"
> +
> +#define ARRAY_SIZE(x) (sizeof(x) / sizeof(x[0]))
> +
> +#define MAKE_CAP(cap) [cap] = { .cap_name = #cap }
> +
> +struct capability {
> + const char *cap_name;
> +} capabilities[] = {
> + MAKE_CAP(CAP_CHOWN),
> + MAKE_CAP(CAP_DAC_OVERRIDE),
> + MAKE_CAP(CAP_DAC_READ_SEARCH),
> + MAKE_CAP(CAP_FOWNER),
> + MAKE_CAP(CAP_FSETID),
> + MAKE_CAP(CAP_KILL),
> + MAKE_CAP(CAP_SETGID),
> + MAKE_CAP(CAP_SETUID),
> + MAKE_CAP(CAP_SETPCAP),
> + MAKE_CAP(CAP_LINUX_IMMUTABLE),
> + MAKE_CAP(CAP_NET_BIND_SERVICE),
> + MAKE_CAP(CAP_NET_BROADCAST),
> + MAKE_CAP(CAP_NET_ADMIN),
> + MAKE_CAP(CAP_NET_RAW),
> + MAKE_CAP(CAP_IPC_LOCK),
> + MAKE_CAP(CAP_IPC_OWNER),
> + MAKE_CAP(CAP_SYS_MODULE),
> + MAKE_CAP(CAP_SYS_RAWIO),
> + MAKE_CAP(CAP_SYS_CHROOT),
> + MAKE_CAP(CAP_SYS_PTRACE),
> + MAKE_CAP(CAP_SYS_PACCT),
> + MAKE_CAP(CAP_SYS_ADMIN),
> + MAKE_CAP(CAP_SYS_BOOT),
> + MAKE_CAP(CAP_SYS_NICE),
> + MAKE_CAP(CAP_SYS_RESOURCE),
> + MAKE_CAP(CAP_SYS_TIME),
> + MAKE_CAP(CAP_SYS_TTY_CONFIG),
> + MAKE_CAP(CAP_MKNOD),
> + MAKE_CAP(CAP_LEASE),
> + MAKE_CAP(CAP_AUDIT_WRITE),
> + MAKE_CAP(CAP_AUDIT_CONTROL),
> + MAKE_CAP(CAP_SETFCAP),
> + MAKE_CAP(CAP_MAC_OVERRIDE),
> + MAKE_CAP(CAP_MAC_ADMIN),
> + MAKE_CAP(CAP_SYSLOG),
> +};
> +
> +static void fail(const char *fmt, ...) __attribute__((format(printf, 1, 2)));
> +static void fail(const char *fmt, ...)
> +{
> + va_list args;
> +
> + va_start(args, fmt);
> + vfprintf(stderr, fmt, args);
> + va_end(args);
> + exit(1);
> +}
> +
> +/*
> + * Find the capability ordinal by name, and return its ordinal.
> + * Returns -1 on failure.
> + */
> +static int find_capability(const char *s)
> +{
> + int i;
> +
> + for (i = 0; i < ARRAY_SIZE(capabilities); i++) {
> + if (capabilities[i].cap_name
> + && strcasecmp(s, capabilities[i].cap_name) == 0) {
> + return i;
> + }
> + }
> + return -1;
> +}
> +
> +static void do_capset(int cap_ordinal)
> +{
> + struct __user_cap_header_struct hdr;
> + struct __user_cap_data_struct caps[2];
> +
> + /* Get the current capability mask */
> + hdr.version = _LINUX_CAPABILITY_VERSION_3;
> + hdr.pid = getpid();
> + if (capget(&hdr, caps)) {
> + perror("capget()");
> + exit(1);
> + }
> +
> + /* Drop the bits */
> + if (cap_ordinal < 32)
> + caps[0].inheritable &= ~(1U << cap_ordinal);
> + else
> + caps[1].inheritable &= ~(1U << (cap_ordinal - 32));
> +
> + /* And drop the capability. */
> + hdr.version = _LINUX_CAPABILITY_VERSION_3;
> + hdr.pid = getpid();
> + if (capset(&hdr, caps))
> + fail("Couldn't drop the capability \"%s\"\n",
> + capabilities[cap_ordinal].cap_name);
> +}
> +
> +static void do_bset(int cap_ordinal)
> +{
> + int ret;
> +
> + ret = prctl(PR_CAPBSET_READ, cap_ordinal);
> + if (ret == 1) {
> + ret = prctl(PR_CAPBSET_DROP, cap_ordinal);
> + if (ret != 0)
> + fail("Error dropping capability %s from bset\n",
> + capabilities[cap_ordinal].cap_name);
> + } else if (ret < 0)
> + fail("Kernel doesn't recognize capability %d\n", cap_ordinal);
> +}
> +
> +static void do_usermodehelper_file(const char *filename, int cap_ordinal)
> +{
> + uint32_t lo32, hi32;
> + FILE *file;
> + static const size_t buf_size = 80;
> + char buf[buf_size];
> + char tail;
> + size_t bytes_read;
> + int ret;
> +
> + /* Try and open the file */
> + file = fopen(filename, "r+");
> + if (!file && errno == ENOENT)
> + fail("Could not disable usermode helpers capabilities as "
> + "%s is not available\n", filename);
> + if (!file)
> + fail("Failed to access file %s errno %d\n", filename, errno);
> +
> + /* Read and process the current bits */
> + bytes_read = fread(buf, 1, buf_size - 1, file);
> + if (bytes_read == 0)
> + fail("Trouble reading %s\n", filename);
> + buf[bytes_read] = '\0';
> + ret = sscanf(buf, "%u %u %c", &lo32, &hi32, &tail);
> + if (ret != 2)
> + fail("Failed to understand %s \"%s\"\n", filename, buf);
> +
> + /* Clear the bits in the local copy */
> + if (cap_ordinal < 32)
> + lo32 &= ~(1 << cap_ordinal);
> + else
> + hi32 &= ~(1 << (cap_ordinal - 32));
> +
> + /* Commit the new bit masks to the kernel */
> + sprintf(buf, "%u %u", lo32, hi32);
> + ret = fwrite(buf, 1, strlen(buf) + 1, file);
> + if (ret != 0)
> + fail("Failed to commit usermode helper bitmasks: %d\n", ret);
> +
> + /* Cleanup */
> + fclose(file);
> +}
> +
> +static void do_usermodehelper(int cap_ordinal)
> +{
> + static const char * const files[] = {
> + "/proc/sys/kernel/usermodehelper/bset",
> + "/proc/sys/kernel/usermodehelper/inheritable",
> + };
> + int i;
> +
> + for (i = 0; i < ARRAY_SIZE(files); i++)
> + do_usermodehelper_file(files[i], cap_ordinal);
> +}
> +
> +static void drop_capability(int cap_ordinal)
> +{
> + do_usermodehelper(cap_ordinal);
> + do_bset(cap_ordinal);
> + do_capset(cap_ordinal);
> +
> + printf("Dropped capability: %s\n", capabilities[cap_ordinal].cap_name);
> +}
> +
> +int drop_capabilities(const char *caps)
> +{
> + char *s, *saveptr = NULL;
> + char *token;
> + int drop_setpcap = 0;
> +
> + if (!caps)
> + return 0;
> +
> + /* Create a duplicate string that can be modified. */
> + s = strdup(caps);
> + if (!s)
> + fail("Failed to drop caps as requested. Exiting\n");
> +
> + token = strtok_r(s, ",", &saveptr);
> + while (token) {
> + int cap_ordinal = find_capability(token);
> +
> + if (cap_ordinal < 0)
> + fail("Could not understand capability name \"%s\" "
> + "on command line, failing init\n", token);
> +
> + /* We handle CAP_SETPCAP last because it is needed to
> + * drop all other caps. */
> + if (cap_ordinal == CAP_SETPCAP)
> + drop_setpcap = 1;
> + else
> + drop_capability(cap_ordinal);
> +
> + token = strtok_r(NULL, ",", &saveptr);
> + }
> +
> + if (drop_setpcap)
> + drop_capability(CAP_SETPCAP);
> +
> + free(s);
> + return 0;
> +}
> diff --git a/usr/kinit/kinit.c b/usr/kinit/kinit.c
> index 4a1f40b..098873b 100644
> --- a/usr/kinit/kinit.c
> +++ b/usr/kinit/kinit.c
> @@ -288,6 +288,8 @@ int main(int argc, char *argv[])
> check_path("/root");
> do_mounts(cmdc, cmdv);
>
> + drop_capabilities(get_arg(cmdc, cmdv, "drop_capabilities="));
> +
> if (mnt_procfs) {
> umount2("/proc", 0);
> mnt_procfs = 0;
> diff --git a/usr/kinit/kinit.h b/usr/kinit/kinit.h
> index c2e67b7..85960d8 100644
> --- a/usr/kinit/kinit.h
> +++ b/usr/kinit/kinit.h
> @@ -65,4 +65,6 @@ static inline void dump_args(int argc, char *argv[])
> }
> #endif
>
> +int drop_capabilities(const char *caps);
> +
> #endif /* KINIT_H */
>
>
--
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