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Message-ID: <CAGTjWtCYb0NOBFsMxWKWEwN-pdOD1fqimHco3nk_N-tKQvxdyQ@mail.gmail.com>
Date: Fri, 29 Jul 2011 13:46:57 -0700
From: Mike Waychison <mikew@...gle.com>
To: Maximilian Attems <max@...o.at>
Cc: "Andrew G. Morgan" <agm@...gle.com>,
Eric Northup <digitaleric@...gle.com>,
Alan Cox <alan@...rguk.ukuu.org.uk>,
"H. Peter Anvin" <hpa@...or.com>,
Eric Paris <eparis@...isplace.org>, klibc@...or.com,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 2/2] run-init: Add drop_capabilities support.
On Fri, Jul 29, 2011 at 1:45 PM, Maximilian Attems <max@...o.at> wrote:
> On Tue, 19 Jul 2011, Mike Waychison wrote:
>
>> This patch adds the ability to run-init to allow the dropping of
>> POSIX capabilities.
>>
>> This works by adding a "-d" flag to run-init, which takes a comma
>> separated list of capability names that should be dropped right before
>> exec'ing the real init binary.
>>
>> kinit is also modified by this change, such that it understands the same
>> argument when prepended with "drop_capabilities=" on the kernel command
>> line.
>>
>> 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.
>>
>> Dropping of capabilities happens in three parts. We explicitly drop the
>> capability from init's inherited, permitted and effective masks. We
>> also drop the capability from the bounding set using PR_CAPBSET_DROP.
>> Lastly, if available, we drop the capabilities from the bset and
>> inheritted masks exposed at /proc/sys/kernel/usermodehelper if available
>> (introduced in v3.0.0).
>
> hmm as 3.0 is out, I don't think we need more backward compatibility.
> do you have a strong arg for it?
> especially since this is an *optional* calling arg I really don't see
> the need of that backward crap.
I'd like to keep it for the time being. I'm still building both 2.6.34
and 2.6.39 kernels at the moment, though I can maintain these last few
compatibility bits in-house if that makes it easier for you.
>
>> In all paths, we treat errors as fatal, as we do not want to continue to
>> boot if there was a problem dropping capabilities. The only exception
>> to this rule is the handling of /proc/sys/kernel/usermodehelper, where
>> we print out a warning if we notice that the kernel is new enough to
>> support this interface, but could not find the proc file (as it may or
>> may not be available after the pivot, depending on early portions of the
>> boot strap process).
>>
>> Signed-off-by: Mike Waychison <mikew@...gle.com>
>> ---
>> usr/kinit/kinit.c | 4 -
>> usr/kinit/run-init/Kbuild | 2
>> usr/kinit/run-init/capabilities.c | 278 +++++++++++++++++++++++++++++++++++++
>> usr/kinit/run-init/capabilities.h | 6 +
>> usr/kinit/run-init/run-init.c | 11 +
>> usr/kinit/run-init/run-init.h | 3
>> usr/kinit/run-init/runinitlib.c | 11 +
>> 7 files changed, 307 insertions(+), 8 deletions(-)
>> create mode 100644 usr/kinit/run-init/capabilities.c
>> create mode 100644 usr/kinit/run-init/capabilities.h
>>
>> diff --git a/usr/kinit/kinit.c b/usr/kinit/kinit.c
>> index 4a1f40b..ae50ed6 100644
>> --- a/usr/kinit/kinit.c
>> +++ b/usr/kinit/kinit.c
>> @@ -307,7 +307,9 @@ int main(int argc, char *argv[])
>>
>> init_argv[0] = strrchr(init_path, '/') + 1;
>>
>> - errmsg = run_init("/root", "/dev/console", init_path, init_argv);
>> + errmsg = run_init("/root", "/dev/console",
>> + get_arg(cmdc, cmdv, "drop_capabilities="),
>> + init_path, init_argv);
>>
>> /* If run_init returned, something went bad */
>> fprintf(stderr, "%s: %s: %s\n", progname, errmsg, strerror(errno));
>> diff --git a/usr/kinit/run-init/Kbuild b/usr/kinit/run-init/Kbuild
>> index bf6e140..6451dd4 100644
>> --- a/usr/kinit/run-init/Kbuild
>> +++ b/usr/kinit/run-init/Kbuild
>> @@ -6,7 +6,7 @@ static-y := static/run-init
>> shared-y := shared/run-init
>>
>> # common .o files
>> -objs := run-init.o runinitlib.o
>> +objs := run-init.o runinitlib.o capabilities.o
>>
>> # TODO - do we want a stripped version
>> # TODO - do we want the static.g + shared.g directories?
>> diff --git a/usr/kinit/run-init/capabilities.c b/usr/kinit/run-init/capabilities.c
>> new file mode 100644
>> index 0000000..d262c01
>> --- /dev/null
>> +++ b/usr/kinit/run-init/capabilities.c
>> @@ -0,0 +1,278 @@
>> +/*
>> + * 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 <linux/version.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 "capabilities.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, ...)
>> +{
>> + va_list args;
>> +
>> + va_start(args, fmt);
>> + vfprintf(stderr, fmt, args);
>> + va_end(args);
>> + exit(1);
>> +}
>> +
>> +/*
>> + * Returns the currently running kernel version X.Y.Z in a format
>> + * compatible with the KERNEL_VERSION macro.
>> + */
>> +static unsigned kernel_version(void)
>> +{
>> + struct utsname utsname;
>> + int ret;
>> + unsigned char version, patchlevel, sublevel;
>> +
>> + ret = uname(&utsname);
>> + if (ret != 0)
>> + fail("uname returned %d\n", ret);
>> +
>> + ret = sscanf(utsname.release, "%hhu.%hhu.%hhu",
>> + &version, &patchlevel, &sublevel);
>> + if (ret != 3) {
>> + /* Try two level name? */
>> + sublevel = 0;
>> + ret = sscanf(utsname.release, "%hhu.%hhu",
>> + &version, &patchlevel);
>> + if (ret != 2)
>> + fail("Couldn't parse kernel version \"%s\"\n",
>> + utsname.release);
>> + }
>> +
>> + return KERNEL_VERSION(version, patchlevel, sublevel);
>> +}
>> +
>> +/*
>> + * 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].effective &= ~(1U << cap_ordinal);
>> + caps[0].permitted &= ~(1U << cap_ordinal);
>> + caps[0].inheritable &= ~(1U << cap_ordinal);
>> + } else {
>> + caps[1].effective &= ~(1U << (cap_ordinal - 32));
>> + caps[1].permitted &= ~(1U << (cap_ordinal - 32));
>> + 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) {
>> + /* Check if this kernel even supports this interface. */
>> + if (kernel_version() >= KERNEL_VERSION(3, 0, 0)) {
>> + static int printed_once;
>> + if (!printed_once++)
>> + fprintf(stderr, "WARNING: Could not disable "
>> + "capabilities for usermode helpers!\n");
>> + }
>> + return;
>> + }
>> + 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\n", filename);
>> +
>> + /* 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 != strlen(buf) + 1)
>> + 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/bset",
>> + "/proc/sys/kernel/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 do_capabilities(const char *drop_capabilities)
>> +{
>> + char *s, *saveptr = NULL;
>> + char *token;
>> + int drop_setpcap = 0;
>> +
>> + if (!drop_capabilities)
>> + return 0;
>> +
>> + /* Create a duplicate string that can be modified. */
>> + s = strdup(drop_capabilities);
>> + 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/run-init/capabilities.h b/usr/kinit/run-init/capabilities.h
>> new file mode 100644
>> index 0000000..bf51eec
>> --- /dev/null
>> +++ b/usr/kinit/run-init/capabilities.h
>> @@ -0,0 +1,6 @@
>> +#ifndef CAPABILITIES_H
>> +#define CAPABILITIES_H
>> +
>> +int do_capabilities(const char *drop_capabilities);
>> +
>> +#endif /* CAPABILITIES_H */
>> diff --git a/usr/kinit/run-init/run-init.c b/usr/kinit/run-init/run-init.c
>> index 0f150dd..cc602ef 100644
>> --- a/usr/kinit/run-init/run-init.c
>> +++ b/usr/kinit/run-init/run-init.c
>> @@ -35,6 +35,7 @@
>> * - Remounts /real-root onto the root filesystem;
>> * - Chroots;
>> * - Opens /dev/console;
>> + * - Drops capabilities
>> * - Spawns the specified init program (with arguments.)
>> */
>>
>> @@ -50,7 +51,8 @@ static const char *program;
>> static void __attribute__ ((noreturn)) usage(void)
>> {
>> fprintf(stderr,
>> - "Usage: exec %s [-c consoledev] /real-root /sbin/init [args]\n",
>> + "Usage: exec %s [-c consoledev] [-d <CAP_NAME,...>] "
>> + "/real-root /sbin/init [args]\n",
>> program);
>> exit(1);
>> }
>> @@ -62,6 +64,7 @@ int main(int argc, char *argv[])
>> const char *realroot;
>> const char *init;
>> const char *error;
>> + const char *drop_capabilities = NULL;
>> char **initargs;
>>
>> /* Variables... */
>> @@ -70,9 +73,11 @@ int main(int argc, char *argv[])
>> /* Parse the command line */
>> program = argv[0];
>>
>> - while ((o = getopt(argc, argv, "c:")) != -1) {
>> + while ((o = getopt(argc, argv, "c:d:")) != -1) {
>> if (o == 'c') {
>> console = optarg;
>> + } else if (o == 'd') {
>> + drop_capabilities = optarg;
>> } else {
>> usage();
>> }
>> @@ -85,7 +90,7 @@ int main(int argc, char *argv[])
>> init = argv[optind + 1];
>> initargs = argv + optind + 1;
>>
>> - error = run_init(realroot, console, init, initargs);
>> + error = run_init(realroot, console, drop_capabilities, init, initargs);
>>
>> /* If run_init returns, something went wrong */
>> fprintf(stderr, "%s: %s: %s\n", program, error, strerror(errno));
>> diff --git a/usr/kinit/run-init/run-init.h b/usr/kinit/run-init/run-init.h
>> index a95328e..30f78bf 100644
>> --- a/usr/kinit/run-init/run-init.h
>> +++ b/usr/kinit/run-init/run-init.h
>> @@ -29,6 +29,7 @@
>> #define RUN_INIT_H
>>
>> const char *run_init(const char *realroot, const char *console,
>> - const char *init, char **initargs);
>> + const char *drop_capabilities, const char *init,
>> + char **initargs);
>>
>> #endif
>> diff --git a/usr/kinit/run-init/runinitlib.c b/usr/kinit/run-init/runinitlib.c
>> index 8f1562f..a5cb10c 100644
>> --- a/usr/kinit/run-init/runinitlib.c
>> +++ b/usr/kinit/run-init/runinitlib.c
>> @@ -26,7 +26,7 @@
>> * ----------------------------------------------------------------------- */
>>
>> /*
>> - * run_init(consoledev, realroot, init, initargs)
>> + * run_init(consoledev, realroot, drop_capabilities, init, initargs)
>> *
>> * This function should be called as the last thing in kinit,
>> * from initramfs, it does the following:
>> @@ -35,6 +35,7 @@
>> * - Remounts /real-root onto the root filesystem;
>> * - Chroots;
>> * - Opens /dev/console;
>> + * - Drops capabilities if needed;
>> * - Spawns the specified init program (with arguments.)
>> *
>> * On failure, returns a human-readable error message.
>> @@ -52,7 +53,9 @@
>> #include <sys/stat.h>
>> #include <sys/types.h>
>> #include <sys/vfs.h>
>> +
>> #include "run-init.h"
>> +#include "capabilities.h"
>>
>> /* Make it possible to compile on glibc by including constants that the
>> always-behind shipped glibc headers may not include. Classic example
>> @@ -154,7 +157,8 @@ static int nuke(const char *what)
>> }
>>
>> const char *run_init(const char *realroot, const char *console,
>> - const char *init, char **initargs)
>> + const char *drop_capabilities, const char *init,
>> + char **initargs)
>> {
>> struct stat rst, cst;
>> struct statfs sfs;
>> @@ -203,6 +207,9 @@ const char *run_init(const char *realroot, const char *console,
>> dup2(confd, 2);
>> close(confd);
>>
>> + /* Drop capabilities */
>> + do_capabilities(drop_capabilities);
>> +
>> /* Spawn init */
>> execv(init, initargs);
>> return init; /* Failed to spawn init */
> --
> maks
>
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