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

Mickaël Salaün
2020-08-13T14:37:06Z

On 11/08/2020 21:51, Eric W. Biederman wrote:
> Mickaël Salaün <mic@digikod.net> 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@digikod.net/
>>
>> 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@ssi.gouv.fr>
>> Signed-off-by: Vincent Strubel <vincent.strubel@ssi.gouv.fr>
>> Co-developed-by: Thibaut Sautereau <thibaut.sautereau@ssi.gouv.fr>
>> Signed-off-by: Thibaut Sautereau <thibaut.sautereau@ssi.gouv.fr>
>> Signed-off-by: Mickaël Salaün <mic@digikod.net>
>> Cc: Aleksa Sarai <cyphar@cyphar.com>
>> Cc: Al Viro <viro@zeniv.linux.org.uk>
>> Cc: Deven Bowers <deven.desai@linux.microsoft.com>
>> Cc: Kees Cook <keescook@chromium.org>
>> ---
>>
>> 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@yavin.dot.cyphar.com/
>>
>> 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@lithium.mbobrowski.org/
>> ---
>> 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 */