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Date: Fri, 11 Sep 2015 08:40:17 -0600
From: Tycho Andersen <tycho.andersen@...onical.com>
To: Daniel Borkmann <daniel@...earbox.net>
Cc: Kees Cook <keescook@...omium.org>,
Alexei Starovoitov <ast@...nel.org>,
"David S. Miller" <davem@...emloft.net>,
Will Drewry <wad@...omium.org>,
Oleg Nesterov <oleg@...hat.com>,
Andy Lutomirski <luto@...capital.net>,
Pavel Emelyanov <xemul@...allels.com>,
"Serge E. Hallyn" <serge.hallyn@...ntu.com>,
linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
linux-api@...r.kernel.org
Subject: Re: [PATCH v2 5/5] seccomp: add a way to attach a filter via eBPF fd
On Fri, Sep 11, 2015 at 02:37:59PM +0200, Daniel Borkmann wrote:
> On 09/11/2015 02:21 AM, Tycho Andersen wrote:
> >This is the final bit needed to support seccomp filters created via the bpf
> >syscall. The patch adds a new seccomp operation SECCOMP_MODE_FILTER_EBPF,
> >which takes exactly one command (presumably to be expanded upon later when
> >seccomp EBPFs support more interesting things) and an argument struct
> >similar to that of bpf(), although the size is explicit in the struct to
> >avoid changing the signature of seccomp().
> >
> >v2: Don't abuse seccomp's third argument; use a separate command and a
> > pointer to a structure instead.
>
> Comments below ...
>
> >Signed-off-by: Tycho Andersen <tycho.andersen@...onical.com>
> >CC: Kees Cook <keescook@...omium.org>
> >CC: Will Drewry <wad@...omium.org>
> >CC: Oleg Nesterov <oleg@...hat.com>
> >CC: Andy Lutomirski <luto@...capital.net>
> >CC: Pavel Emelyanov <xemul@...allels.com>
> >CC: Serge E. Hallyn <serge.hallyn@...ntu.com>
> >CC: Alexei Starovoitov <ast@...nel.org>
> >CC: Daniel Borkmann <daniel@...earbox.net>
> >---
> > include/uapi/linux/seccomp.h | 16 +++++
> > kernel/seccomp.c | 135 ++++++++++++++++++++++++++++++++++++++-----
> > 2 files changed, 138 insertions(+), 13 deletions(-)
> >
> >diff --git a/include/uapi/linux/seccomp.h b/include/uapi/linux/seccomp.h
> >index 0f238a4..a8694e2 100644
> >--- a/include/uapi/linux/seccomp.h
> >+++ b/include/uapi/linux/seccomp.h
> >@@ -13,10 +13,14 @@
> > /* Valid operations for seccomp syscall. */
> > #define SECCOMP_SET_MODE_STRICT 0
> > #define SECCOMP_SET_MODE_FILTER 1
> >+#define SECCOMP_MODE_FILTER_EBPF 2
>
> Should this be SECCOMP_SET_MODE_FILTER_EBPF or just SECCOMP_SET_MODE_EBPF?
I just stole the name Kees gave it in the previous thread, but I think
that perhaps there are other plans for manipulating seccomp ebpfs (?).
The command is SECCOMP_EBPF_ADD_FD, so it seems like we could add a
command like SECCOMP_EBPF_SOMETHING in the future.
> > /* Valid flags for SECCOMP_SET_MODE_FILTER */
> > #define SECCOMP_FILTER_FLAG_TSYNC 1
> >
> >+/* Valid cmds for SECCOMP_MODE_FILTER_EBPF */
> >+#define SECCOMP_EBPF_ADD_FD 0
> >+
> > /*
> > * All BPF programs must return a 32-bit value.
> > * The bottom 16-bits are for optional return data.
> >@@ -51,4 +55,16 @@ struct seccomp_data {
> > __u64 args[6];
> > };
> >
> >+struct seccomp_ebpf {
> >+ unsigned int size;
> >+
> >+ union {
> >+ /* SECCOMP_EBPF_ADD_FD */
> >+ struct {
> >+ unsigned int add_flags;
> >+ __u32 add_fd;
> >+ };
> >+ };
> >+};
> >+
> > #endif /* _UAPI_LINUX_SECCOMP_H */
> >diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> >index 1856f69..e78175a 100644
> >--- a/kernel/seccomp.c
> >+++ b/kernel/seccomp.c
> >@@ -65,6 +65,9 @@ struct seccomp_filter {
> > /* Limit any path through the tree to 256KB worth of instructions. */
> > #define MAX_INSNS_PER_PATH ((1 << 18) / sizeof(struct sock_filter))
> >
> >+static long seccomp_install_filter(unsigned int flags,
> >+ struct seccomp_filter *prepared);
> >+
> > /*
> > * Endianness is explicitly ignored and left for BPF program authors to manage
> > * as per the specific architecture.
> >@@ -356,17 +359,6 @@ static struct seccomp_filter *seccomp_prepare_filter(struct sock_fprog *fprog)
> >
> > BUG_ON(INT_MAX / fprog->len < sizeof(struct sock_filter));
> >
> >- /*
> >- * Installing a seccomp filter requires that the task has
> >- * CAP_SYS_ADMIN in its namespace or be running with no_new_privs.
> >- * This avoids scenarios where unprivileged tasks can affect the
> >- * behavior of privileged children.
> >- */
> >- if (!task_no_new_privs(current) &&
> >- security_capable_noaudit(current_cred(), current_user_ns(),
> >- CAP_SYS_ADMIN) != 0)
> >- return ERR_PTR(-EACCES);
> >-
> > /* Allocate a new seccomp_filter */
> > sfilter = kzalloc(sizeof(*sfilter), GFP_KERNEL | __GFP_NOWARN);
> > if (!sfilter)
> >@@ -510,8 +502,105 @@ static void seccomp_send_sigsys(int syscall, int reason)
> > info.si_syscall = syscall;
> > force_sig_info(SIGSYS, &info, current);
> > }
> >+
> > #endif /* CONFIG_SECCOMP_FILTER */
> >
> >+#if defined(CONFIG_BPF_SYSCALL) && defined(CONFIG_SECCOMP_FILTER)
> >+static struct seccomp_filter *seccomp_prepare_ebpf(int fd)
> >+{
> >+ struct seccomp_filter *ret;
> >+ struct bpf_prog *prog;
> >+
> >+ prog = bpf_prog_get(fd);
> >+ if (IS_ERR(prog))
> >+ return (struct seccomp_filter *) prog;
>
> ERR_CAST()
>
> >+
> >+ if (prog->type != BPF_PROG_TYPE_SECCOMP) {
> >+ bpf_prog_put(prog);
> >+ return ERR_PTR(-EINVAL);
> >+ }
> >+
> >+ ret = kzalloc(sizeof(*ret), GFP_KERNEL | __GFP_NOWARN);
> >+ if (!ret) {
> >+ bpf_prog_put(prog);
> >+ return ERR_PTR(-ENOMEM);
> >+ }
> >+
> >+ ret->prog = prog;
> >+ atomic_set(&ret->usage, 1);
> >+
> >+ /* Intentionally don't bpf_prog_put() here, because the underlying prog
> >+ * is refcounted too and we're holding a reference from the struct
> >+ * seccomp_filter object.
> >+ */
> >+ return ret;
> >+}
> >+
> >+static long seccomp_ebpf_add_fd(struct seccomp_ebpf *ebpf)
> >+{
> >+ struct seccomp_filter *prepared;
> >+
> >+ prepared = seccomp_prepare_ebpf(ebpf->add_fd);
> >+ if (IS_ERR(prepared))
> >+ return PTR_ERR(prepared);
> >+
> >+ return seccomp_install_filter(ebpf->add_flags, prepared);
> >+}
> >+
> >+static long seccomp_mode_filter_ebpf(unsigned int cmd, const char __user *uargs)
> >+{
> >+ const struct seccomp_ebpf __user *uebpf;
> >+ struct seccomp_ebpf ebpf;
> >+ unsigned int size;
> >+ long ret = -EFAULT;
> >+
> >+ uebpf = (const struct seccomp_ebpf __user *) uargs;
> >+
> >+ if (get_user(size, &uebpf->size) != 0)
> >+ return -EFAULT;
> >+
> >+ /* If we're handed a bigger struct than we know of,
> >+ * ensure all the unknown bits are 0 - i.e. new
> >+ * user-space does not rely on any kernel feature
> >+ * extensions we dont know about yet.
> >+ */
> >+ if (size > sizeof(ebpf)) {
> >+ unsigned char __user *addr;
> >+ unsigned char __user *end;
> >+ unsigned char val;
> >+
> >+ addr = (void __user *)uebpf + sizeof(ebpf);
> >+ end = (void __user *)uebpf + size;
> >+
> >+ for (; addr < end; addr++) {
> >+ int err = get_user(val, addr);
> >+
> >+ if (err)
> >+ return err;
> >+ if (val)
> >+ return -E2BIG;
> >+ }
> >+ size = sizeof(ebpf);
> >+ }
> >+
> >+ if (copy_from_user(&ebpf, uebpf, size) != 0)
> >+ return -EFAULT;
>
> Not sure it's worth adding all this bpf(2)-alike interface complexity into
> this, but fair enough, I guess there are some very good reasons and bigger
> additions coming then ...
I'm not sure what bigger additions are coming, although it seems Andy
might have something. I think this is just an attempt to future proof
things.
> >+ switch (cmd) {
> >+ case SECCOMP_EBPF_ADD_FD:
> >+ ret = seccomp_ebpf_add_fd(&ebpf);
> >+ break;
> >+ }
> >+
> >+ return ret;
> >+}
> >+#else
> >+static long seccomp_mode_filter_ebpf(unsigned int cmd, const char __user *uargs)
> >+{
> >+ return -EINVAL;
> >+}
> >+#endif
> >+
> > /*
> > * Secure computing mode 1 allows only read/write/exit/sigreturn.
> > * To be fully secure this must be combined with rlimit
> >@@ -760,9 +849,7 @@ out:
> > static long seccomp_set_mode_filter(unsigned int flags,
> > const char __user *filter)
> > {
> >- const unsigned long seccomp_mode = SECCOMP_MODE_FILTER;
> > struct seccomp_filter *prepared = NULL;
> >- long ret = -EINVAL;
> >
> > /* Validate flags. */
> > if (flags & ~SECCOMP_FILTER_FLAG_MASK)
> >@@ -773,6 +860,26 @@ static long seccomp_set_mode_filter(unsigned int flags,
> > if (IS_ERR(prepared))
> > return PTR_ERR(prepared);
> >
> >+ return seccomp_install_filter(flags, prepared);
>
> I (truly) hope, I'm overseeing something ;) ...
>
> ... but why doing all the (classic) seccomp-BPF preparation work (which is rather
> a lot) up to this point, where you have it ready, only to *then* find out we don't
> have the actual permissions ?!
Yes, this seems dumb. I was trying to avoid having the check in two
places, but that's probably what's necessary.
> Plus, when seccomp_install_filter() fails with -EACCES, who is releasing all the
> allocated foo resp. dropping taken program refs !?
Yes, seccomp_install_filter is /supposed/ to free things if the
install fails, although it doesn't in the permissions case because
of the copy paste error, doh.
> I see the same in seccomp_ebpf_add_fd().
Same as above, seccomp_install_filter is supposed to call
seccomp_filter_free in case of an error, but it doesn't.
Thanks for the look. I'll make the changes for the next set.
Tycho
> So, an unprivileged child could increase the parent's bpf_prog's reference count
> w/o having the actual permissions to do so, and thus controlling it to the point
> where the next bpf_prog_put() would unintentionally release it?
>
> (So yeah, I'm hoping I misread something ... ;))
>
> >+}
> >+
> >+static long seccomp_install_filter(unsigned int flags,
> >+ struct seccomp_filter *prepared)
> >+{
> >+ const unsigned long seccomp_mode = SECCOMP_MODE_FILTER;
> >+ long ret = -EINVAL;
> >+
> >+ /*
> >+ * Installing a seccomp filter requires that the task has
> >+ * CAP_SYS_ADMIN in its namespace or be running with no_new_privs.
> >+ * This avoids scenarios where unprivileged tasks can affect the
> >+ * behavior of privileged children.
> >+ */
> >+ if (!task_no_new_privs(current) &&
> >+ security_capable_noaudit(current_cred(), current_user_ns(),
> >+ CAP_SYS_ADMIN) != 0)
> >+ return -EACCES;
> >+
> > /*
> > * Make sure we cannot change seccomp or nnp state via TSYNC
> > * while another thread is in the middle of calling exec.
> >@@ -875,6 +982,8 @@ static long do_seccomp(unsigned int op, unsigned int flags,
> > return seccomp_set_mode_strict();
> > case SECCOMP_SET_MODE_FILTER:
> > return seccomp_set_mode_filter(flags, uargs);
> >+ case SECCOMP_MODE_FILTER_EBPF:
> >+ return seccomp_mode_filter_ebpf(flags, uargs);
> > default:
> > return -EINVAL;
> > }
> >
>
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