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

[Mickaël Salaün]

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 */