Subject: Re: [PATCH v2 5/5] seccomp: add a way to attach a filter via eBPF fd

From: Tycho Andersen <[email-masked]>
Date: Fri, 11 Sep 2015 08:40:17 -0600

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 <[email-masked]>
> >CC: Kees Cook <[email-masked]>
> >CC: Will Drewry <[email-masked]>
> >CC: Oleg Nesterov <[email-masked]>
> >CC: Andy Lutomirski <[email-masked]>
> >CC: Pavel Emelyanov <[email-masked]>
> >CC: Serge E. Hallyn <serge.hallyn-GeWIH/[email-masked]>
> >CC: Alexei Starovoitov <[email-masked]>
> >CC: Daniel Borkmann <[email-masked]>
> >---
> >  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;
> >  	}
> >
>