On 11/08/2020 21:51, Eric W. Biederman wrote: > 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? There is a switch case for each file type (in this patch and the next one). > > You are not requiring the opened script be executable? The (conditional) enforcement is in the next patch, with the rational. > > You are not requring path_noexec? Despite the original patch that > inspired this was checking path_noexec? This patch just introduces the new flag and its default behavior. See the next patch for a security policy configuration. > > 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. Yeah, but unfortunately this is not possible in practice because of general Linux distro, as explained in the next patch. > > As it is this patch appears wrong and dangerously confusing as it implies > execness but does not implement execness. Please see next patch. > > 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. As explained in this commit message, O_EXEC doesn't have the same semantic. Also, see next patch. > > 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 */
