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Message-ID: <CAHse=S-ECjFoZprku+Qb2iDp9auQgWLvYoxxC7-dquOqPWbeTg@mail.gmail.com> Date: Fri, 7 Nov 2014 13:20:58 +0000 From: David Drysdale <drysdale@...gle.com> To: Andy Lutomirski <luto@...capital.net> Cc: "Eric W. Biederman" <ebiederm@...ssion.com>, Alexander Viro <viro@...iv.linux.org.uk>, Meredydd Luff <meredydd@...atehouse.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>, Andrew Morton <akpm@...ux-foundation.org>, Kees Cook <keescook@...omium.org>, Arnd Bergmann <arnd@...db.de>, Rich Felker <dalias@...ifal.cx>, Christoph Hellwig <hch@...radead.org>, X86 ML <x86@...nel.org>, linux-arch <linux-arch@...r.kernel.org>, Linux API <linux-api@...r.kernel.org> Subject: Re: [PATCHv6 RFC 0/3] syscalls,x86: Add execveat() system call On Thu, Nov 6, 2014 at 4:55 PM, Andy Lutomirski <luto@...capital.net> wrote: > On Thu, Nov 6, 2014 at 8:07 AM, David Drysdale <drysdale@...gle.com> 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. Yes, the same checks will happen for an O_PATH fd as for a normal fd. I'll add an explicit test case for that too. > --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. My bad, it doesn't -- I forgot to update that part of the commit description, it's left over from the version that used d_path(). > --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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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