Re: [PATCHv6 RFC 0/3] syscalls,x86: Add execveat() system call

[Andy Lutomirski]

On Thu, Nov 6, 2014 at 8:07 AM, David Drysdale <drysdale@xxxxxxxxxx> wrote:
> Here's another pass at this. Some things to discuss in particular:
>
> 1) The current approach for interpreted execs (i.e. mostly "#!" scripts)
>    gives them an argv[1] filename like "/dev/fd/<fd>/<path>".  This
>    means that script execution in a /proc-less system isn't going to
>    work, at least until interpreters get smart enough to spot and
>    special-case the leading "/dev/fd/<fd>", or until there's something
>    to use in place of /dev/fd -> /proc/self/fd (e.g. Al's dupfs
>    suggestion, https://lkml.org/lkml/2014/10/19/141).
>
>    So is an execveat(2) that (currently) only works for non-interpreted
>    programs still useful?

I think it is.  I would make sure to return a distinguishable error
code in the event that the failure happens because of one of the
unsupported cases.

>
> 2) I don't like having to add a new LOOKUP_EMPTY_NOPATH flag
>    just to prevent O_PATH fds from being fexecve()ed -- alternative
>    suggestions welcomed.  (More generally, I don't have a great
>    feel for what O_PATH is for; how bad would it be to just allow
>    them to be fexecve()ed?)

If you fexecve an O_PATH fd, does it at least check that you have
execute permission on the inode?  If so, it seems okay to allow it.

--Andy

>
> .........
>
> This patch set adds execveat(2) for x86, and is derived from Meredydd
> Luff's patch from Sept 2012 (https://lkml.org/lkml/2012/9/11/528).
>
> The primary aim of adding an execveat syscall is to allow an
> implementation of fexecve(3) that does not rely on the /proc
> filesystem, at least for executables (rather than scripts).  The
> current glibc version of fexecve(3) is implemented via /proc, which
> causes problems in sandboxed or otherwise restricted environments.
>
> Given the desire for a /proc-free fexecve() implementation, HPA
> suggested (https://lkml.org/lkml/2006/7/11/556) that an execveat(2)
> syscall would be an appropriate generalization.
>
> Also, having a new syscall means that it can take a flags argument
> without back-compatibility concerns.  The current implementation just
> defines the AT_EMPTY_PATH and AT_SYMLINK_NOFOLLOW flags, but other
> flags could be added in future -- for example, flags for new namespaces
> (as suggested at https://lkml.org/lkml/2006/7/11/474).
>
> Related history:
>  - https://lkml.org/lkml/2006/12/27/123 is an example of someone
>    realizing that fexecve() is likely to fail in a chroot environment.
>  - http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=514043 covered
>    documenting the /proc requirement of fexecve(3) in its manpage, to
>    "prevent other people from wasting their time".
>  - https://bugzilla.kernel.org/show_bug.cgi?id=74481 documented that
>    it's not possible to fexecve() a file descriptor for a script with
>    close-on-exec set (which is possible with the implementation here).

Confused.  How does it work for a close-on-exec script?  I understand
how it works for a close-on-exec ELF binary.

--Andy

>  - https://bugzilla.redhat.com/show_bug.cgi?id=241609 described a
>    problem where a process that did setuid() could not fexecve()
>    because it no longer had access to /proc/self/fd; this has since
>    been fixed.
>
>
> Changes since v5:
>  - Set new flag in bprm->interp_flags for O_CLOEXEC fds, so that binfmts
>    that invoke an interpreter fail the exec (as they will not be able
>    to access the invoked file). [Andy Lutomirski]
>  - Don't truncate long paths. [Andy Lutomirski]
>  - Commonize code to open the executed file. [Eric W. Biederman]
>  - Mark O_PATH file descriptors so they cannot be fexecve()ed.
>  - Make self-test more helpful, and add additional cases:
>      - file offset non-zero
>      - binary file without execute bit
>      - O_CLOEXEC fds
>
> Changes since v4, suggested by Eric W. Biederman:
>  - Use empty filename with AT_EMPTY_PATH flag rather than NULL
>    pathname to request fexecve-like behaviour.
>  - Build pathname as "/dev/fd/<fd>/<filename>" (or "/dev/fd/<fd>")
>    rather than using d_path().
>  - Patch against v3.17 (bfe01a5ba249)
>
> Changes since Meredydd's v3 patch:
>  - Added a selftest.
>  - Added a man page.
>  - Left open_exec() signature untouched to reduce patch impact
>    elsewhere (as suggested by Al Viro).
>  - Filled in bprm->filename with d_path() into a buffer, to avoid use
>    of potentially-ephemeral dentry->d_name.
>  - Patch against v3.14 (455c6fdbd21916).
>
>
> David Drysdale (2):
>   syscalls,x86: implement execveat() system call
>   syscalls,x86: add selftest for execveat(2)
>
>  arch/x86/ia32/audit.c                   |   1 +
>  arch/x86/ia32/ia32entry.S               |   1 +
>  arch/x86/kernel/audit_64.c              |   1 +
>  arch/x86/kernel/entry_64.S              |  28 +++
>  arch/x86/syscalls/syscall_32.tbl        |   1 +
>  arch/x86/syscalls/syscall_64.tbl        |   2 +
>  arch/x86/um/sys_call_table_64.c         |   1 +
>  fs/binfmt_em86.c                        |   4 +
>  fs/binfmt_misc.c                        |   4 +
>  fs/binfmt_script.c                      |  10 +
>  fs/exec.c                               | 115 ++++++++++--
>  fs/namei.c                              |   8 +-
>  include/linux/binfmts.h                 |   4 +
>  include/linux/compat.h                  |   3 +
>  include/linux/fs.h                      |   1 +
>  include/linux/namei.h                   |   1 +
>  include/linux/sched.h                   |   4 +
>  include/linux/syscalls.h                |   4 +
>  include/uapi/asm-generic/unistd.h       |   4 +-
>  kernel/sys_ni.c                         |   3 +
>  lib/audit.c                             |   3 +
>  tools/testing/selftests/Makefile        |   1 +
>  tools/testing/selftests/exec/.gitignore |   7 +
>  tools/testing/selftests/exec/Makefile   |  25 +++
>  tools/testing/selftests/exec/execveat.c | 321 ++++++++++++++++++++++++++++++++
>  25 files changed, 542 insertions(+), 15 deletions(-)
>  create mode 100644 tools/testing/selftests/exec/.gitignore
>  create mode 100644 tools/testing/selftests/exec/Makefile
>  create mode 100644 tools/testing/selftests/exec/execveat.c
>
> --
> 2.1.0.rc2.206.gedb03e5



-- 
Andy Lutomirski
AMA Capital Management, LLC
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