Re: [PATCH v7 4/7] fs: Introduce O_MAYEXEC flag for openat2(2)

[Eric W. Biederman]

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 */