On 15/07/2020 22:06, Kees Cook wrote: > On Tue, Jul 14, 2020 at 08:16:35PM +0200, Mickaël Salaün wrote: >> When the O_MAYEXEC flag is passed, openat2(2) may be subject to >> additional restrictions depending on a security policy managed by the >> kernel through a sysctl or implemented by an LSM thanks to the >> inode_permission hook. This new flag is ignored by open(2) and >> openat(2) because of their unspecified flags handling. >> >> The underlying idea is to be able to restrict scripts interpretation >> according to a policy defined by the system administrator. For this to >> be possible, script interpreters must use the O_MAYEXEC flag >> appropriately. To be fully effective, these interpreters also need to >> handle the other ways to execute code: command line parameters (e.g., >> option -e for Perl), module loading (e.g., option -m for Python), stdin, >> file sourcing, environment variables, configuration files, etc. >> According to the threat model, it may be acceptable to allow some script >> interpreters (e.g. Bash) to interpret commands from stdin, may it be a >> TTY or a pipe, because it may not be enough to (directly) perform >> syscalls. Further documentation can be found in a following patch. >> >> Even without enforced security policy, userland interpreters can set it >> to enforce the system policy at their level, knowing that it will not >> break anything on running systems which do not care about this feature. >> However, on systems which want this feature enforced, there will be >> knowledgeable people (i.e. sysadmins who enforced O_MAYEXEC >> deliberately) to manage it. A simple security policy implementation, >> configured through a dedicated sysctl, is available in a following >> patch. >> >> O_MAYEXEC should not be confused with the O_EXEC flag which is intended >> for execute-only, which obviously doesn't work for scripts. However, a >> similar behavior could be implemented in userland with O_PATH: >> https://lore.kernel.org/lkml/1e2f6913-42f2-3578-28ed-567f6a4bd...@digikod.net/ >> >> The implementation of O_MAYEXEC almost duplicates what execve(2) and >> uselib(2) are already doing: setting MAY_OPENEXEC in acc_mode (which can >> then be checked as MAY_EXEC, if enforced), and propagating FMODE_EXEC to >> _fmode via __FMODE_EXEC flag (which can then trigger a >> fanotify/FAN_OPEN_EXEC event). >> >> This is an updated subset of the patch initially written by Vincent >> Strubel for CLIP OS 4: >> https://github.com/clipos-archive/src_platform_clip-patches/blob/f5cb330d6b684752e403b4e41b39f7004d88e561/1901_open_mayexec.patch >> This patch has been used for more than 12 years with customized script >> interpreters. Some examples (with the original name O_MAYEXEC) can be >> found here: >> https://github.com/clipos-archive/clipos4_portage-overlay/search?q=O_MAYEXEC >> >> Co-developed-by: Vincent Strubel <vincent.stru...@ssi.gouv.fr> >> Signed-off-by: Vincent Strubel <vincent.stru...@ssi.gouv.fr> >> Co-developed-by: Thibaut Sautereau <thibaut.sauter...@ssi.gouv.fr> >> Signed-off-by: Thibaut Sautereau <thibaut.sauter...@ssi.gouv.fr> >> Signed-off-by: Mickaël Salaün <m...@digikod.net> >> Reviewed-by: Deven Bowers <deven.de...@linux.microsoft.com> >> Reviewed-by: Kees Cook <keesc...@chromium.org> >> Cc: Aleksa Sarai <cyp...@cyphar.com> >> Cc: Al Viro <v...@zeniv.linux.org.uk> >> --- >> >> Changes since v5: >> * Update commit message. >> >> Changes since v3: >> * Switch back to O_MAYEXEC, but only handle it with openat2(2) which >> checks unknown flags (suggested by Aleksa Sarai). Cf. >> >> https://lore.kernel.org/lkml/20200430015429.wuob7m5ofdewu...@yavin.dot.cyphar.com/ >> >> Changes since v2: >> * Replace O_MAYEXEC with RESOLVE_MAYEXEC from openat2(2). This change >> enables to not break existing application using bogus O_* flags that >> may be ignored by current kernels by using a new dedicated flag, only >> usable through openat2(2) (suggested by Jeff Layton). Using this flag >> will results in an error if the running kernel does not support it. >> User space needs to manage this case, as with other RESOLVE_* flags. >> The best effort approach to security (for most common distros) will >> simply consists of ignoring such an error and retry without >> RESOLVE_MAYEXEC. However, a fully controlled system may which to >> error out if such an inconsistency is detected. >> >> Changes since v1: >> * Set __FMODE_EXEC when using O_MAYEXEC to make this information >> available through the new fanotify/FAN_OPEN_EXEC event (suggested by >> Jan Kara and Matthew Bobrowski): >> https://lore.kernel.org/lkml/20181213094658.ga...@lithium.mbobrowski.org/ >> --- >> fs/fcntl.c | 2 +- >> fs/open.c | 8 ++++++++ >> include/linux/fcntl.h | 2 +- >> include/linux/fs.h | 2 ++ >> include/uapi/asm-generic/fcntl.h | 7 +++++++ >> 5 files changed, 19 insertions(+), 2 deletions(-) >> >> diff --git a/fs/fcntl.c b/fs/fcntl.c >> index 2e4c0fa2074b..0357ad667563 100644 >> --- a/fs/fcntl.c >> +++ b/fs/fcntl.c >> @@ -1033,7 +1033,7 @@ static int __init fcntl_init(void) >> * Exceptions: O_NONBLOCK is a two bit define on parisc; O_NDELAY >> * is defined as O_NONBLOCK on some platforms and not on others. >> */ >> - BUILD_BUG_ON(21 - 1 /* for O_RDONLY being 0 */ != >> + BUILD_BUG_ON(22 - 1 /* for O_RDONLY being 0 */ != >> HWEIGHT32( >> (VALID_OPEN_FLAGS & ~(O_NONBLOCK | O_NDELAY)) | >> __FMODE_EXEC | __FMODE_NONOTIFY)); >> diff --git a/fs/open.c b/fs/open.c >> index 623b7506a6db..38e434bdbbb6 100644 >> --- a/fs/open.c >> +++ b/fs/open.c >> @@ -987,6 +987,8 @@ inline struct open_how build_open_how(int flags, umode_t >> mode) >> .mode = mode & S_IALLUGO, >> }; >> >> + /* O_MAYEXEC is ignored by syscalls relying on build_open_how(). */ >> + how.flags &= ~O_MAYEXEC; >> /* O_PATH beats everything else. */ >> if (how.flags & O_PATH) >> how.flags &= O_PATH_FLAGS; >> @@ -1054,6 +1056,12 @@ inline int build_open_flags(const struct open_how >> *how, struct open_flags *op) >> if (flags & __O_SYNC) >> flags |= O_DSYNC; >> >> + /* Checks execution permissions on open. */ >> + if (flags & O_MAYEXEC) { >> + acc_mode |= MAY_OPENEXEC; >> + flags |= __FMODE_EXEC; >> + } > > Adding __FMODE_EXEC here will immediately change the behaviors of NFS > and fsnotify. If that's going to happen, I think it needs to be under > the control of the later patches doing the behavioral controls. > (specifically, NFS looks like it completely changes its access control > test when this is set and ignores the read/write checks entirely, which > is not what's wanted).
__FMODE_EXEC was suggested by Jan Kara and Matthew Bobrowski because of fsnotify. However, the NFS handling of SUID binaries [1] indeed leads to an unintended behavior. This also means that uselib(2) shouldn't work properly with NFS. I can remove the __FMODE_EXEC flag for now. [1] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f8d9a897d4384b77f13781ea813156568f68b83e
