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). 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. A simple security policy implementation, configured through a dedicated sysctl, is available in a following patch. 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 11 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 Signed-off-by: Mickaël Salaün <mic@xxxxxxxxxxx> Signed-off-by: Thibaut Sautereau <thibaut.sautereau@xxxxxxxxxxx> Signed-off-by: Vincent Strubel <vincent.strubel@xxxxxxxxxxx> Cc: Aleksa Sarai <cyphar@xxxxxxxxxx> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx> Cc: Kees Cook <keescook@xxxxxxxxxxxx> --- 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). --- 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 719b320ede52..f3f08a36d1d2 100644 --- a/fs/open.c +++ b/fs/open.c @@ -962,6 +962,8 @@ 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; @@ -1029,6 +1031,12 @@ 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; + } + 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 4f6f59b4f22a..313c934de9ee 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 */ -- 2.26.2
