Re: [PATCH v3 3/3] Documentation: prctl/seccomp_filter

[Randy Dunlap]

On 01/12/2012 03:38 PM, Will Drewry wrote:
> Documents how system call filtering using Berkeley Packet
> Filter programs works and how it may be used.
> Includes an example for x86 (32-bit).
> 
> v3: - call out BPF <-> Berkeley Packet Filter (rdunlap@xxxxxxxxxxxx)
>     - document use of tentative always-unprivileged
>     - guard sample compilation for i386 and x86_64
> v2: - move code to samples (corbet@xxxxxxx)
> 
> Signed-off-by: Will Drewry <wad@xxxxxxxxxxxx>
> ---
>  Documentation/prctl/seccomp_filter.txt |   94 ++++++++++++++++++++++++++++++++
>  samples/Makefile                       |    2 +-
>  samples/seccomp/Makefile               |   18 ++++++
>  samples/seccomp/bpf-example.c          |   74 +++++++++++++++++++++++++
>  4 files changed, 187 insertions(+), 1 deletions(-)
>  create mode 100644 Documentation/prctl/seccomp_filter.txt
>  create mode 100644 samples/seccomp/Makefile
>  create mode 100644 samples/seccomp/bpf-example.c
> 
> diff --git a/Documentation/prctl/seccomp_filter.txt b/Documentation/prctl/seccomp_filter.txt
> new file mode 100644
> index 0000000..2db8b89
> --- /dev/null
> +++ b/Documentation/prctl/seccomp_filter.txt
> @@ -0,0 +1,94 @@
> +		Seccomp filtering
> +		=================
> +
> +Introduction
> +------------
> +
> +A large number of system calls are exposed to every userland process
> +with many of them going unused for the entire lifetime of the process.
> +As system calls change and mature, bugs are found and eradicated.  A
> +certain subset of userland applications benefit by having a reduced set
> +of available system calls.  The resulting set reduces the total kernel
> +surface exposed to the application.  System call filtering is meant for
> +use with those applications.
> +
> +Seccomp filtering provides a means for a process to specify a filter
> +for incoming system calls.  The filter is expressed as a Berkeley Packet
> +Filter (BPF) program, as with socket filters, except that the data
> +operated on is the current user_regs_struct.  This allows for expressive
> +filtering of system calls using the pre-existing system call ABI and
> +using a filter program language with a long history of being exposed to
> +userland.  Additionally, BPF makes it impossible for users of seccomp to
> +fall prey to time-of-check-time-of-use (TOCTOU) attacks that are common
> +in system call interposition frameworks because the evaluated data is
> +solely register state just after system call entry.
> +
> +What it isn't
> +-------------
> +
> +System call filtering isn't a sandbox.  It provides a clearly defined
> +mechanism for minimizing the exposed kernel surface.  Beyond that,
> +policy for logical behavior and information flow should be managed with
> +a combinations of other system hardening techniques and, potentially, a

     combination                                                         an

> +LSM of your choosing.  Expressive, dynamic filters provide further options down
> +this path (avoiding pathological sizes or selecting which of the multiplexed
> +system calls in socketcall() is allowed, for instance) which could be
> +construed, incorrectly, as a more complete sandboxing solution.
> +
> +Usage
> +-----
> +
> +An additional seccomp mode is added, but they are not directly set by the
> +consuming process.  The new mode, '2', is only available if
> +CONFIG_SECCOMP_FILTER is set and enabled using prctl with the
> +PR_ATTACH_SECCOMP_FILTER argument.
> +
> +Interacting with seccomp filters is done using one prctl(2) call.
> +
> +PR_ATTACH_SECCOMP_FILTER:
> +	Allows the specification of a new filter using a BPF program.
> +	The BPF program will be executed over a user_regs_struct data
> +	reflecting system call time except with the system call number
> +	resident in orig_[register].  To allow a system call, the size
> +	of the data must be returned.  At present, all other return values
> +	result in the system call being blocked, but it is recommended to
> +	return 0 in those cases.  This will allow for future custom return
> +	values to be introduced, if ever desired.
> +
> +	Usage:
> +		prctl(PR_ATTACH_SECCOMP_FILTER, prog);
> +
> +	The 'prog' argument is a pointer to a struct sock_fprog which will
> +	contain the filter program.  If the program is invalid, the call
> +	will return -1 and set errno to -EINVAL.

	                                EINVAL.
(I think)

> +
> +	The struct user_regs_struct the @prog will see is based on the
> +	personality of the task at the time of this prctl call.  Additionally,
> +	is_compat_task is also tracked for the @prog.  This means that once set
> +	the calling task will have all of its system calls blocked if it
> +	switches its system call ABI (via personality or other means).
> +
> +	If fork/clone and execve are allowed by @prog, any child processes will
> +	be constrained to the same filters and syscal call ABI as the parent.

	                                       syscall

> +
> +	When called from an unprivileged process (lacking CAP_SYS_ADMIN), the
> +	"always_unprivileged" bit is enabled for the process.
> +
> +	Additionally, if prctl(2) is allowed by the attached filter,
> +	additional filters may be layered on which will increase evaluation
> +	time, but allow for further decreasing the attack surface during
> +	execution of a process.
> +
> +The above call returns 0 on success and non-zero on error.
> +
> +Example
> +-------
> +
> +samples/seccomp-bpf-example.c shows an example process that allows read from stdin,

   samples/seccomp/bpf-example.c

> +write to stdout/err, exit and signal returns for 32-bit x86.

                  /stderr,

> +
> +Adding architecture support
> +-----------------------
> +
> +Any platform with seccomp support will support seccomp filters
> +as long as CONFIG_SECCOMP_FILTER is enabled.



-- 
~Randy
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