Re: [PATCH v20 1/6] exec: Add a new AT_CHECK flag to execveat(2)

[Amir Goldstein]

On Mon, Oct 14, 2024 at 9:39 AM Mickaël Salaün <mic@xxxxxxxxxxx> wrote:
>
> On Sun, Oct 13, 2024 at 11:25:11AM +0200, Amir Goldstein wrote:
> > On Fri, Oct 11, 2024 at 8:45 PM Mickaël Salaün <mic@xxxxxxxxxxx> wrote:
> > >
> > > Add a new AT_CHECK flag to execveat(2) to check if a file would be
> > > allowed for execution.  The main use case is for script interpreters and
> > > dynamic linkers to check execution permission according to the kernel's
> > > security policy. Another use case is to add context to access logs e.g.,
> > > which script (instead of interpreter) accessed a file.  As any
> > > executable code, scripts could also use this check [1].
> > >
> > > This is different from faccessat(2) + X_OK which only checks a subset of
> > > access rights (i.e. inode permission and mount options for regular
> > > files), but not the full context (e.g. all LSM access checks).  The main
> > > use case for access(2) is for SUID processes to (partially) check access
> > > on behalf of their caller.  The main use case for execveat(2) + AT_CHECK
> > > is to check if a script execution would be allowed, according to all the
> > > different restrictions in place.  Because the use of AT_CHECK follows
> > > the exact kernel semantic as for a real execution, user space gets the
> > > same error codes.
> > >
> > > An interesting point of using execveat(2) instead of openat2(2) is that
> > > it decouples the check from the enforcement.  Indeed, the security check
> > > can be logged (e.g. with audit) without blocking an execution
> > > environment not yet ready to enforce a strict security policy.
> > >
> > > LSMs can control or log execution requests with
> > > security_bprm_creds_for_exec().  However, to enforce a consistent and
> > > complete access control (e.g. on binary's dependencies) LSMs should
> > > restrict file executability, or mesure executed files, with
> > > security_file_open() by checking file->f_flags & __FMODE_EXEC.
> > >
> > > Because AT_CHECK is dedicated to user space interpreters, it doesn't
> > > make sense for the kernel to parse the checked files, look for
> > > interpreters known to the kernel (e.g. ELF, shebang), and return ENOEXEC
> > > if the format is unknown.  Because of that, security_bprm_check() is
> > > never called when AT_CHECK is used.
> > >
> > > It should be noted that script interpreters cannot directly use
> > > execveat(2) (without this new AT_CHECK flag) because this could lead to
> > > unexpected behaviors e.g., `python script.sh` could lead to Bash being
> > > executed to interpret the script.  Unlike the kernel, script
> > > interpreters may just interpret the shebang as a simple comment, which
> > > should not change for backward compatibility reasons.
> > >
> > > Because scripts or libraries files might not currently have the
> > > executable permission set, or because we might want specific users to be
> > > allowed to run arbitrary scripts, the following patch provides a dynamic
> > > configuration mechanism with the SECBIT_EXEC_RESTRICT_FILE and
> > > SECBIT_EXEC_DENY_INTERACTIVE securebits.
> > >
> > > This is a redesign of the CLIP OS 4's O_MAYEXEC:
> > > https://github.com/clipos-archive/src_platform_clip-patches/blob/f5cb330d6b684752e403b4e41b39f7004d88e561/1901_open_mayexec.patch
> > > This patch has been used for more than a decade with customized script
> > > interpreters.  Some examples can be found here:
> > > https://github.com/clipos-archive/clipos4_portage-overlay/search?q=O_MAYEXEC
> > >
> > > Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> > > Cc: Christian Brauner <brauner@xxxxxxxxxx>
> > > Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> > > Cc: Paul Moore <paul@xxxxxxxxxxxxxx>
> > > Cc: Serge Hallyn <serge@xxxxxxxxxx>
> > > Link: https://docs.python.org/3/library/io.html#io.open_code [1]
> > > Signed-off-by: Mickaël Salaün <mic@xxxxxxxxxxx>
> > > Link: https://lore.kernel.org/r/20241011184422.977903-2-mic@xxxxxxxxxxx
> > > ---
> > >
> > > Changes since v19:
> > > * Remove mention of "role transition" as suggested by Andy.
> > > * Highlight the difference between security_bprm_creds_for_exec() and
> > >   the __FMODE_EXEC check for LSMs (in commit message and LSM's hooks) as
> > >   discussed with Jeff.
> > > * Improve documentation both in UAPI comments and kernel comments
> > >   (requested by Kees).
> > >
> > > New design since v18:
> > > https://lore.kernel.org/r/20220104155024.48023-3-mic@xxxxxxxxxxx
> > > ---
> > >  fs/exec.c                  | 18 ++++++++++++++++--
> > >  include/linux/binfmts.h    |  7 ++++++-
> > >  include/uapi/linux/fcntl.h | 31 +++++++++++++++++++++++++++++++
> > >  kernel/audit.h             |  1 +
> > >  kernel/auditsc.c           |  1 +
> > >  security/security.c        | 10 ++++++++++
> > >  6 files changed, 65 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/fs/exec.c b/fs/exec.c
> > > index 6c53920795c2..163c659d9ae6 100644
> > > --- a/fs/exec.c
> > > +++ b/fs/exec.c
> > > @@ -891,7 +891,7 @@ static struct file *do_open_execat(int fd, struct filename *name, int flags)
> > >                 .lookup_flags = LOOKUP_FOLLOW,
> > >         };
> > >
> > > -       if ((flags & ~(AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH)) != 0)
> > > +       if ((flags & ~(AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH | AT_CHECK)) != 0)
> > >                 return ERR_PTR(-EINVAL);
> > >         if (flags & AT_SYMLINK_NOFOLLOW)
> > >                 open_exec_flags.lookup_flags &= ~LOOKUP_FOLLOW;
> > > @@ -1545,6 +1545,20 @@ static struct linux_binprm *alloc_bprm(int fd, struct filename *filename, int fl
> > >         }
> > >         bprm->interp = bprm->filename;
> > >
> > > +       /*
> > > +        * At this point, security_file_open() has already been called (with
> > > +        * __FMODE_EXEC) and access control checks for AT_CHECK will stop just
> > > +        * after the security_bprm_creds_for_exec() call in bprm_execve().
> > > +        * Indeed, the kernel should not try to parse the content of the file
> > > +        * with exec_binprm() nor change the calling thread, which means that
> > > +        * the following security functions will be not called:
> > > +        * - security_bprm_check()
> > > +        * - security_bprm_creds_from_file()
> > > +        * - security_bprm_committing_creds()
> > > +        * - security_bprm_committed_creds()
> > > +        */
> > > +       bprm->is_check = !!(flags & AT_CHECK);
> > > +
> > >         retval = bprm_mm_init(bprm);
> > >         if (!retval)
> > >                 return bprm;
> > > @@ -1839,7 +1853,7 @@ static int bprm_execve(struct linux_binprm *bprm)
> > >
> > >         /* Set the unchanging part of bprm->cred */
> > >         retval = security_bprm_creds_for_exec(bprm);
> > > -       if (retval)
> > > +       if (retval || bprm->is_check)
> > >                 goto out;
> > >
> > >         retval = exec_binprm(bprm);
> > > diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
> > > index e6c00e860951..8ff0eb3644a1 100644
> > > --- a/include/linux/binfmts.h
> > > +++ b/include/linux/binfmts.h
> > > @@ -42,7 +42,12 @@ struct linux_binprm {
> > >                  * Set when errors can no longer be returned to the
> > >                  * original userspace.
> > >                  */
> > > -               point_of_no_return:1;
> > > +               point_of_no_return:1,
> > > +               /*
> > > +                * Set by user space to check executability according to the
> > > +                * caller's environment.
> > > +                */
> > > +               is_check:1;
> > >         struct file *executable; /* Executable to pass to the interpreter */
> > >         struct file *interpreter;
> > >         struct file *file;
> > > diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
> > > index 87e2dec79fea..e606815b1c5a 100644
> > > --- a/include/uapi/linux/fcntl.h
> > > +++ b/include/uapi/linux/fcntl.h
> > > @@ -154,6 +154,37 @@
> > >                                            usable with open_by_handle_at(2). */
> > >  #define AT_HANDLE_MNT_ID_UNIQUE        0x001   /* Return the u64 unique mount ID. */
> > >
> > > +/*
> > > + * AT_CHECK only performs a check on a regular file and returns 0 if execution
> > > + * of this file would be allowed, ignoring the file format and then the related
> > > + * interpreter dependencies (e.g. ELF libraries, script's shebang).
> > > + *
> > > + * Programs should always perform this check to apply kernel-level checks
> > > + * against files that are not directly executed by the kernel but passed to a
> > > + * user space interpreter instead.  All files that contain executable code,
> > > + * from the point of view of the interpreter, should be checked.  However the
> > > + * result of this check should only be enforced according to
> > > + * SECBIT_EXEC_RESTRICT_FILE or SECBIT_EXEC_DENY_INTERACTIVE.  See securebits.h
> > > + * documentation and the samples/check-exec/inc.c example.
> > > + *
> > > + * The main purpose of this flag is to improve the security and consistency of
> > > + * an execution environment to ensure that direct file execution (e.g.
> > > + * `./script.sh`) and indirect file execution (e.g. `sh script.sh`) lead to the
> > > + * same result.  For instance, this can be used to check if a file is
> > > + * trustworthy according to the caller's environment.
> > > + *
> > > + * In a secure environment, libraries and any executable dependencies should
> > > + * also be checked.  For instance, dynamic linking should make sure that all
> > > + * libraries are allowed for execution to avoid trivial bypass (e.g. using
> > > + * LD_PRELOAD).  For such secure execution environment to make sense, only
> > > + * trusted code should be executable, which also requires integrity guarantees.
> > > + *
> > > + * To avoid race conditions leading to time-of-check to time-of-use issues,
> > > + * AT_CHECK should be used with AT_EMPTY_PATH to check against a file
> > > + * descriptor instead of a path.
> > > + */
> >
> > If you ask me, the very elaborate comment above belongs to execveat(2)
> > man page and is way too verbose for a uapi header.
>
> OK, but since this new flags raised a lot of questions, I guess a
> dedicated Documentation/userspace-api/check-exec.rst file with thit
> AT*_CHECK and the related securebits would be useful instead of the
> related inlined documentation.
>
> >
> > > +#define AT_CHECK               0x10000
> >
> > Please see the comment "Per-syscall flags for the *at(2) family of syscalls."
> > above. If this is a per-syscall flag please use one of the per-syscall
> > flags, e.g.:
> >
> > /* Flags for execveat2(2) */
> > #define AT_EXECVE_CHECK     0x0001   /* Only perform a check if
> > execution would be allowed */
>
> I missed this part, this prefix makes sense, thanks.
>

Not only the prefix, also the overloaded value.

Thanks,
Amir.