Mickaël Salaün <mic@xxxxxxxxxxx> writes: > 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. When used with > openat2(2), the default behavior is only to forbid to open a directory. > > 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-567f6a4bdda1@xxxxxxxxxxx/ > > 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). You are allowing S_IFBLK, S_IFCHR, S_IFIFO, S_IFSOCK as targets for O_MAYEXEC? You are not requiring the opened script be executable? You are not requring path_noexec? Despite the original patch that inspired this was checking path_noexec? I honestly think this patch is buggy. If you could reuse MAY_EXEC in the kernel and mean what exec means when it says MAY_EXEC that would be useful. As it is this patch appears wrong and dangerously confusing as it implies execness but does not implement execness. If you were simply defining O_EXEC and reusing MAY_EXEC as it exists or exists with cleanups in the kernel this would be a small change that would seem to make reasonable sense. But as you are not reusing anything from MAY_EXEC this code does not make any sense as I am reading it. Eric > 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 O_MAYEXEC) can be found > here: > https://github.com/clipos-archive/clipos4_portage-overlay/search?q=O_MAYEXEC > > Co-developed-by: Vincent Strubel <vincent.strubel@xxxxxxxxxxx> > Signed-off-by: Vincent Strubel <vincent.strubel@xxxxxxxxxxx> > Co-developed-by: Thibaut Sautereau <thibaut.sautereau@xxxxxxxxxxx> > Signed-off-by: Thibaut Sautereau <thibaut.sautereau@xxxxxxxxxxx> > Signed-off-by: Mickaël Salaün <mic@xxxxxxxxxxx> > Cc: Aleksa Sarai <cyphar@xxxxxxxxxx> > Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx> > Cc: Deven Bowers <deven.desai@xxxxxxxxxxxxxxxxxxx> > Cc: Kees Cook <keescook@xxxxxxxxxxxx> > --- > > Changes since v6: > * Do not set __FMODE_EXEC for now because of inconsistent behavior: > https://lore.kernel.org/lkml/202007160822.CCDB5478@keescook/ > * Returns EISDIR when opening a directory with O_MAYEXEC. > * Removed Deven Bowers and Kees Cook Reviewed-by tags because of the > current update. > > 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.wuob7m5ofdewubui@xxxxxxxxxxxxxxxxxxxx/ > > 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.GA996@xxxxxxxxxxxxxxxxxxxxxx/ > --- > fs/fcntl.c | 2 +- > fs/namei.c | 4 ++-- > fs/open.c | 6 ++++++ > include/linux/fcntl.h | 2 +- > include/linux/fs.h | 2 ++ > include/uapi/asm-generic/fcntl.h | 7 +++++++ > 6 files changed, 19 insertions(+), 4 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/namei.c b/fs/namei.c > index ddc9b25540fe..3f074ec77390 100644 > --- a/fs/namei.c > +++ b/fs/namei.c > @@ -428,7 +428,7 @@ static int sb_permission(struct super_block *sb, struct inode *inode, int mask) > /** > * inode_permission - Check for access rights to a given inode > * @inode: Inode to check permission on > - * @mask: Right to check for (%MAY_READ, %MAY_WRITE, %MAY_EXEC) > + * @mask: Right to check for (%MAY_READ, %MAY_WRITE, %MAY_EXEC, %MAY_OPENEXEC) > * > * Check for read/write/execute permissions on an inode. We use fs[ug]id for > * this, letting us set arbitrary permissions for filesystem access without > @@ -2849,7 +2849,7 @@ static int may_open(const struct path *path, int acc_mode, int flag) > case S_IFLNK: > return -ELOOP; > case S_IFDIR: > - if (acc_mode & (MAY_WRITE | MAY_EXEC)) > + if (acc_mode & (MAY_WRITE | MAY_EXEC | MAY_OPENEXEC)) > return -EISDIR; > break; > case S_IFBLK: > diff --git a/fs/open.c b/fs/open.c > index 623b7506a6db..21c2c1020574 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 +1056,10 @@ 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; > + > op->open_flag = flags; > > /* O_TRUNC implies we need access checks for write permissions */ > diff --git a/include/linux/fcntl.h b/include/linux/fcntl.h > index 7bcdcf4f6ab2..e188a360fa5f 100644 > --- a/include/linux/fcntl.h > +++ b/include/linux/fcntl.h > @@ -10,7 +10,7 @@ > (O_RDONLY | O_WRONLY | O_RDWR | O_CREAT | O_EXCL | O_NOCTTY | O_TRUNC | \ > O_APPEND | O_NDELAY | O_NONBLOCK | O_NDELAY | __O_SYNC | O_DSYNC | \ > FASYNC | O_DIRECT | O_LARGEFILE | O_DIRECTORY | O_NOFOLLOW | \ > - O_NOATIME | O_CLOEXEC | O_PATH | __O_TMPFILE) > + O_NOATIME | O_CLOEXEC | O_PATH | __O_TMPFILE | O_MAYEXEC) > > /* List of all valid flags for the how->upgrade_mask argument: */ > #define VALID_UPGRADE_FLAGS \ > diff --git a/include/linux/fs.h b/include/linux/fs.h > index f5abba86107d..56f835c9a87a 100644 > --- a/include/linux/fs.h > +++ b/include/linux/fs.h > @@ -101,6 +101,8 @@ typedef int (dio_iodone_t)(struct kiocb *iocb, loff_t offset, > #define MAY_CHDIR 0x00000040 > /* called from RCU mode, don't block */ > #define MAY_NOT_BLOCK 0x00000080 > +/* the inode is opened with O_MAYEXEC */ > +#define MAY_OPENEXEC 0x00000100 > > /* > * flags in file.f_mode. Note that FMODE_READ and FMODE_WRITE must correspond > diff --git a/include/uapi/asm-generic/fcntl.h b/include/uapi/asm-generic/fcntl.h > index 9dc0bf0c5a6e..bca90620119f 100644 > --- a/include/uapi/asm-generic/fcntl.h > +++ b/include/uapi/asm-generic/fcntl.h > @@ -97,6 +97,13 @@ > #define O_NDELAY O_NONBLOCK > #endif > > +/* > + * Code execution from file is intended, checks such permission. A simple > + * policy can be enforced system-wide as explained in > + * Documentation/admin-guide/sysctl/fs.rst . > + */ > +#define O_MAYEXEC 040000000 > + > #define F_DUPFD 0 /* dup */ > #define F_GETFD 1 /* get close_on_exec */ > #define F_SETFD 2 /* set/clear close_on_exec */
