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Message-ID: <CALCETrXFp-apnfRPg9qx6E7g3KDU8DanBW1pMUnA1zShrB5xKg@mail.gmail.com> Date: Wed, 19 Apr 2017 16:15:28 -0700 From: Andy Lutomirski <luto@...nel.org> To: Djalal Harouni <tixxdz@...il.com> Cc: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Andy Lutomirski <luto@...nel.org>, Kees Cook <keescook@...omium.org>, Andrew Morton <akpm@...ux-foundation.org>, "Serge E. Hallyn" <serge@...lyn.com>, "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, LSM List <linux-security-module@...r.kernel.org>, Linux API <linux-api@...r.kernel.org>, Dongsu Park <dpark@...teo.net>, Casey Schaufler <casey@...aufler-ca.com>, James Morris <james.l.morris@...cle.com>, Paul Moore <paul@...l-moore.com>, Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Jonathan Corbet <corbet@....net>, Jessica Yu <jeyu@...hat.com>, Rusty Russell <rusty@...tcorp.com.au>, Arnaldo Carvalho de Melo <acme@...hat.com>, Mauro Carvalho Chehab <mchehab@...nel.org>, Ingo Molnar <mingo@...nel.org>, zendyani@...il.com, Peter Zijlstra <peterz@...radead.org> Subject: Re: [PATCH v3 2/2] modules:capabilities: add a per-task modules autoload restriction On Wed, Apr 19, 2017 at 3:20 PM, Djalal Harouni <tixxdz@...il.com> wrote: > Previous patches added the global "modules_autoload" restriction. This patch > make it possible to support process trees, containers, and sandboxes by > providing an inherited per-task "modules_autoload" flag that cannot be > re-enabled once disabled. This allows to restrict automatic module > loading without affecting the rest of the system. > > Any task can set its "modules_autoload". Once set, this setting is inherited > across fork, clone and execve. With "modules_autoload" set, automatic > module loading will have first to satisfy the per-task access permissions > before attempting to implicitly load the module. For example, automatic > loading of modules that contain bugs or vulnerabilities can be > restricted and untrusted users can no longer abuse such interfaces > > To set modules_autoload, use prctl(PR_SET_MODULES_AUTOLOAD, value, 0, 0, 0). > > When value is (0), the default, automatic modules loading is allowed. > > When value is (1), task must have CAP_SYS_MODULE to be able to trigger a > module auto-load operation, or CAP_NET_ADMIN for modules with a > 'netdev-%s' alias. > > When value is (2), automatic modules loading is disabled for the current > task. > > The 'modules_autoload' value may only be increased, never decreased, thus > ensuring that once applied, processes can never relax their setting. > > When a request to a kernel module is denied, the module name with the > corresponding process name and its pid are logged. Administrators can use > such information to explicitly load the appropriate modules. > > The per-task "modules_autoload" restriction: > > Before: > $ lsmod | grep ipip - > $ sudo ip tunnel add mytun mode ipip remote 10.0.2.100 local 10.0.2.15 ttl 255 > $ lsmod | grep ipip - > ipip 16384 0 > tunnel4 16384 1 ipip > ip_tunnel 28672 1 ipip > > After: > $ lsmod | grep ipip - > $ ./pr_modules_autoload > $ grep "Modules" /proc/self/status > ModulesAutoload: 2 > $ cat /proc/sys/kernel/modules_autoload > 0 > $ sudo ip tunnel add mytun mode ipip remote 10.0.2.100 local 10.0.2.15 ttl 255 > add tunnel "tunl0" failed: No such device > $ lsmod | grep ipip > $ dmesg | tail -3 > [ 16.363903] virbr0: port 1(virbr0-nic) entered disabled state > [ 823.565958] Automatic module loading of netdev-tunl0 by "ip"[1362] was denied > [ 823.565967] Automatic module loading of tunl0 by "ip"[1362] was denied > > Cc: Serge Hallyn <serge@...lyn.com> > Cc: Andy Lutomirski <luto@...nel.org> > Suggested-by: Kees Cook <keescook@...omium.org> > Signed-off-by: Djalal Harouni <tixxdz@...il.com> > --- > Documentation/filesystems/proc.txt | 3 ++ > Documentation/prctl/modules_autoload.txt | 49 +++++++++++++++++++++++++++++++ > fs/proc/array.c | 6 ++++ > include/linux/module.h | 48 ++++++++++++++++++++++++++++-- > include/linux/sched.h | 5 ++++ > include/linux/security.h | 2 +- > include/uapi/linux/prctl.h | 8 +++++ > kernel/fork.c | 4 +++ > kernel/module.c | 17 +++++++---- > security/commoncap.c | 50 ++++++++++++++++++++++++++++---- > 10 files changed, 178 insertions(+), 14 deletions(-) > create mode 100644 Documentation/prctl/modules_autoload.txt > > diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt > index 4cddbce..df4d145 100644 > --- a/Documentation/filesystems/proc.txt > +++ b/Documentation/filesystems/proc.txt > @@ -194,6 +194,7 @@ read the file /proc/PID/status: > CapBnd: ffffffffffffffff > NoNewPrivs: 0 > Seccomp: 0 > + ModulesAutoload: 0 > voluntary_ctxt_switches: 0 > nonvoluntary_ctxt_switches: 1 > > @@ -267,6 +268,8 @@ Table 1-2: Contents of the status files (as of 4.8) > CapBnd bitmap of capabilities bounding set > NoNewPrivs no_new_privs, like prctl(PR_GET_NO_NEW_PRIV, ...) > Seccomp seccomp mode, like prctl(PR_GET_SECCOMP, ...) > + ModulesAutoload modules autoload, like > + prctl(PR_GET_MODULES_AUTOLOAD, ...) > Cpus_allowed mask of CPUs on which this process may run > Cpus_allowed_list Same as previous, but in "list format" > Mems_allowed mask of memory nodes allowed to this process > diff --git a/Documentation/prctl/modules_autoload.txt b/Documentation/prctl/modules_autoload.txt > new file mode 100644 > index 0000000..242852e > --- /dev/null > +++ b/Documentation/prctl/modules_autoload.txt > @@ -0,0 +1,49 @@ > +A request to a kernel feature that is implemented by a module that is > +not loaded may trigger the module auto-load feature, allowing to > +transparently satisfy userspace. In this case an implicit kernel module > +load operation happens. > + > +Usually to load or unload a kernel module, an explicit operation happens > +where programs are required to have some capabilities in order to perform > +such operations. However, with the implicit module loading, no > +capabilities are required, anyone who is able to request a certain kernel > +feature, may also implicitly load its corresponding kernel module. This > +operation can be abused by unprivileged users to expose kernel interfaces > +that maybe privileged users did not want to be made available for various > +reasons: resources, bugs, vulnerabilties, etc. The DCCP vulnerability is > +(CVE-2017-6074) is one real example. > + > +The new per-task "modules_autoload" flag, is a new way to restrict > +automatic module loading, preventing the kernel from exposing more of > +its interface. This particularly useful for containers and sandboxes > +where sandboxed processes should affect the rest of the system. > + > +Any task can set "modules_autoload". Once set, this setting is inherited > +across fork, clone and execve. With "modules_autoload" set, automatic > +module loading will have first to satisfy the per-task access permissions > +before attempting to implicitly load the module. For example, automatic > +loading of modules that contain bugs or vulnerabilities can be > +restricted and imprivileged users can no longer abuse such interfaces. > + > +To set modules_autoload, use prctl(PR_SET_MODULES_AUTOLOAD, value, 0, 0, 0). > + > +When value is (0), the default, automatic modules loading is allowed. > + > +When value is (1), task must have CAP_SYS_MODULE to be able to trigger a > +module auto-load operation, or CAP_NET_ADMIN for modules with a > +'netdev-%s' alias. > + > +When value is (2), automatic modules loading is disabled for the current > +task. > + > +The 'modules_autoload' value may only be increased, never decreased, thus > +ensuring that once applied, processes can never relax their setting. > + > +When a request to a kernel module is denied, the module name with the > +corresponding process name and its pid are logged. Administrators can use > +such information to explicitly load the appropriate modules. > + > +Please note that even if the per-task "modules_autoload" value allows to > +auto-load the corresponding module, automatic module loading may still > +fail due to the global "modules_autoload" sysctl. For more details please > +see "modules_autoload" in Documentation/sysctl/kernel.txt > diff --git a/fs/proc/array.c b/fs/proc/array.c > index 88c3555..cbcf087 100644 > --- a/fs/proc/array.c > +++ b/fs/proc/array.c > @@ -88,6 +88,7 @@ > #include <linux/string_helpers.h> > #include <linux/user_namespace.h> > #include <linux/fs_struct.h> > +#include <linux/module.h> > > #include <asm/pgtable.h> > #include <asm/processor.h> > @@ -346,10 +347,15 @@ static inline void task_cap(struct seq_file *m, struct task_struct *p) > > static inline void task_seccomp(struct seq_file *m, struct task_struct *p) > { > + int autoload = task_modules_autoload(p); > + > seq_put_decimal_ull(m, "NoNewPrivs:\t", task_no_new_privs(p)); > #ifdef CONFIG_SECCOMP > seq_put_decimal_ull(m, "\nSeccomp:\t", p->seccomp.mode); > #endif > + if (autoload != -ENOSYS) > + seq_put_decimal_ull(m, "\nModulesAutoload:\t", autoload); > + > seq_putc(m, '\n'); > } > > diff --git a/include/linux/module.h b/include/linux/module.h > index 4b96c10..595800f 100644 > --- a/include/linux/module.h > +++ b/include/linux/module.h > @@ -13,6 +13,7 @@ > #include <linux/kmod.h> > #include <linux/init.h> > #include <linux/elf.h> > +#include <linux/sched.h> > #include <linux/stringify.h> > #include <linux/kobject.h> > #include <linux/moduleparam.h> > @@ -506,7 +507,33 @@ bool __is_module_percpu_address(unsigned long addr, unsigned long *can_addr); > bool is_module_percpu_address(unsigned long addr); > bool is_module_text_address(unsigned long addr); > > -int modules_autoload_access(char *kmod_name); > +int modules_autoload_access(struct task_struct *task, char *kmod_name); > + > +/* Sets task's modules_autoload */ > +static inline int task_set_modules_autoload(struct task_struct *task, > + unsigned long value) > +{ > + if (value > MODULES_AUTOLOAD_DISABLED) > + return -EINVAL; > + else if (task->modules_autoload > value) > + return -EPERM; > + else if (task->modules_autoload < value) > + task->modules_autoload = value; > + > + return 0; > +} This needs to be more locked down. Otherwise someone could set this and then run a setuid program. Admittedly, it would be quite odd if this particular thing causes a problem, but the issue exists nonetheless. More generally, I think this feature would fit in fairly nicely with my "implicit rights" idea. Unfortunately, Linus hated it, but maybe if I actually implemented it, he wouldn't hate it so much.
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