Re: [RFC PATCH v19 1/5] exec: Add a new AT_CHECK flag to execveat(2)

Jeff Xu
2024-07-18T02:08:56Z

On Wed, Jul 17, 2024 at 3:01 AM Mickaël Salaün <mic@digikod.net> wrote:
>
> On Tue, Jul 16, 2024 at 11:33:55PM -0700, Jeff Xu wrote:
> > On Thu, Jul 4, 2024 at 12:02 PM Mickaël Salaün <mic@digikod.net> 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 than faccessat(2) which only checks file access
> > > rights, but not the full context e.g. mount point's noexec, stack limit,
> > > and all potential LSM extra checks (e.g. argv, envp, credentials).
> > > Since the use of AT_CHECK follows the exact kernel semantic as for a
> > > real execution, user space gets the same error codes.
> > >
> > So we concluded that execveat(AT_CHECK) will be used to check the
> > exec, shared object, script and config file (such as seccomp config),
>
> "config file" that contains executable code.
>
Is seccomp config considered as "contains executable code", seccomp
config is translated into bpf, so maybe yes ? but bpf is running in
the kernel.

> > I'm still thinking execveat(AT_CHECK) vs faccessat(AT_CHECK) in
> > different use cases:
> >
> > execveat clearly has less code change, but that also means: we can't
> > add logic specific to exec (i.e. logic that can't be applied to
> > config) for this part (from do_execveat_common to
> > security_bprm_creds_for_exec) in future. This would require some
> > agreement/sign-off, I'm not sure from whom.
>
> I'm not sure to follow. We could still add new flags, but for now I
> don't see use cases. This patch series is not meant to handle all
> possible "trust checks", only executable code, which makes sense for the
> kernel.
>
I guess the "configfile" discussion is where I get confused, at one
point, I think this would become a generic "trust checks" api for
everything related to "generating executable code", e.g. javascript,
java code, and more.
We will want to clearly define the scope of execveat(AT_CHECK)

> If we want other checks, we'll need to clearly define their semantic and
> align with the kernel. faccessat2(2) might be used to check other file
> properties, but the executable property is not only defined by the file
> attributes.
>
Agreed.

> >
> > --------------------------
> > now looked at user cases (focus on elf for now)
> >
> > 1> ld.so /tmp/a.out, /tmp/a.out is on non-exec mount
> > dynamic linker will first call execveat(fd, AT_CHECK) then execveat(fd)
> >
> > 2> execve(/usr/bin/some.out) and some.out has dependency on /tmp/a.so
> > /usr/bin/some.out will pass AT_CHECK
> >
> > 3> execve(usr/bin/some.out) and some.out uses custom /tmp/ld.so
> > /usr/bin/some.out will pass AT_CHECK, however, it uses a custom
> > /tmp/ld.so (I assume this is possible for elf header will set the
> > path for ld.so because kernel has no knowledge of that, and
> > binfmt_elf.c allocate memory for ld.so during execveat call)
> >
> > 4> dlopen(/tmp/a.so)
> > I assume dynamic linker will call execveat(AT_CHECK), before map a.so
> > into memory.
> >
> > For case 1>
> > Alternative solution: Because AT_CHECK is always called, I think we
> > can avoid the first AT_CHECK call, and check during execveat(fd),
>
> There is no need to use AT_CHECK if we're going to call execveat(2) on
> the same file descriptor. By design, AT_CHECK is implicit for any
> execve(2).
>
Yes. I realized I was wrong to say that ld.so will call execve() for
/tmp/a.out, there is no execve() call, otherwise it would have been
blocked already today.
The ld.so will mmap the /tmp/a.out directly. So case 1 is no
different than case 2 and 4. ( the elf objects are mapped to memory
by dynamic linker.)
I'm not familiar with dynamic linker, Florian is on this thread, and
can help to correct me if my guess is wrong.

> > this means the kernel will enforce SECBIT_EXEC_RESTRICT_FILE = 1, the
> > benefit is that there is no TOCTOU and save one round trip of syscall
> > for a succesful execveat() case.
>
> As long as user space uses the same file descriptor, there is no TOCTOU.
>
> SECBIT_EXEC_RESTRICT_FILE only makes sense for user space: it defines
> the user space security policy. The kernel already enforces the same
> security policy for any execve(2), whatever are the calling process's
> securebits.
>
> >
> > For case 2>
> > dynamic linker will call execve(AT_CHECK), then mmap(fd) into memory.
> > However, the process can all open then mmap() directly, it seems
> > minimal effort for an attacker to walk around such a defence from
> > dynamic linker.
>
> Which process? What do you mean by "can all open then mmap() directly"?
>
> In this context the dynamic linker (like its parent processes) is
> trusted (guaranteed by the system).
>
> For case 2, the dynamic linker must check with AT_CHECK all files that
> will be mapped, which include /usr/bin/some.out and /tmp/a.so
>
My point is that the process can work around this by mmap() the file directly.

> >
> > Alternative solution:
> > dynamic linker call AT_CHECK for each .so, kernel will save the state
> > (associated with fd)
> > kernel will check fd state at the time of mmap(fd, executable memory)
> > and enforce SECBIT_EXEC_RESTRICT_FILE = 1
>
> The idea with AT_CHECK is that there is no kernel side effect, no extra
> kernel state, and the semantic is the same as with execve(2).
>
> This also enables us to check file's executable permission and ignore
> it, which is useful in a "permissive mode" when preparing for a
> migration without breaking a system, or to do extra integrity checks.
For preparing a migration (detect all violations), this is useful.
But as a defense mechanism (SECBIT_EXEC_RESTRICT_FILE = 1) , this
seems to be weak, at least for elf loading case.

> BTW, this use case would also be more complex with a new openat2(2) flag
> like the original O_MAYEXEC.
>
> >
> > Alternative solution 2:
> > a new syscall to load the .so and enforce the AT_CHECK in kernel
>
> A new syscall would be overkill for this feature. Please see Linus's
> comment.
>
maybe, I was thinking on how to prevent "/tmp/a.o" from getting mmap()
to executable memory.

> >
> > This also means, for the solution to be complete, we might want to
> > block creation of executable anonymous memory (e.g. by seccomp, ),
>
> How seccomp could create anonymous memory in user space?
> seccomp filters should be treated (and checked with AT_CHECK) as
> executable code anyway.
>
> > unless the user space can harden the creation of executable anonymous
> > memory in some way.
>
> User space is already in charge of mmapping its own memory. I don't see
> what is missing.
>
> >
> > For case 3>
> > I think binfmt_elf.c in the kernel needs to check the ld.so to make
> > sure it passes AT_CHECK, before loading it into memory.
>
> All ELF dependencies are opened and checked with open_exec(), which
> perform the main executability checks (with the __FMODE_EXEC flag).
> Did I miss something?
>
I mean the ld-linux-x86-64.so.2 which is loaded by binfmt in the kernel.
The app can choose its own dynamic linker path during build, (maybe
even statically link one ?) This is another reason that relying on a
userspace only is not enough.

> However, we must be careful with programs using the (deprecated)
> uselib(2). They should also check with AT_CHECK because this syscall
> opens the shared library without __FMODE_EXEC (similar to a simple file
> open). See
> https://lore.kernel.org/all/CAHk-=wiUwRG7LuR=z5sbkFVGQh+7qVB6_1NM0Ny9SVNL1Un4Sw@mail.gmail.com/
>
> >
> > For case 4>
> > same as case 2.
> >
> > Consider those cases: I think:
> > a> relying purely on userspace for enforcement does't seem to be
> > effective, e.g. it is trivial to call open(), then mmap() it into
> > executable memory.
>
> As Steve explained (and is also explained in the patches), it is trivial
> if the attacker can already execute its own code, which is too late to
> enforce any script execution control.
>
> > b> if both user space and kernel need to call AT_CHECK, the faccessat
> > seems to be a better place for AT_CHECK, e.g. kernel can call
> > do_faccessat(AT_CHECK) and userspace can call faccessat(). This will
> > avoid complicating the execveat() code path.
>
> A previous version of this patches series already patched faccessat(2),
> but this is not the right place. faccessat2(2) is dedicated to check
> file permissions, not executability (e.g. with mount's noexec).
>
> >
> > What do you think ?
>
> I think there are some misunderstandings. Please let me know if it's
> clearer now.
>
I'm still not sure about the user case for dynamic linker (elf
loading) case. Maybe this patch is more suitable for scripts?
A detailed user case will help demonstrate the use case for dynamic
linker, e.g. what kind of app will benefit from
SECBIT_EXEC_RESTRICT_FILE = 1, what kind of threat model are we
dealing with , what kind of attack chain we blocked as a result.

> >
> > Thanks
> > -Jeff
> >
> > > With the information that a script interpreter is about to interpret a
> > > script, an LSM security policy can adjust caller's access rights or log
> > > execution request as for native script execution (e.g. role transition).
> > > This is possible thanks to the call to security_bprm_creds_for_exec().
> > >
> > > Because LSMs may only change bprm's credentials, use of AT_CHECK with
> > > current kernel code should not be a security issue (e.g. unexpected role
> > > transition). LSMs willing to update the caller's credential could now
> > > do so when bprm->is_check is set. Of course, such policy change should
> > > be in line with the new user space code.
> > >
> > > 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_SHOULD_EXEC_CHECK and
> > > SECBIT_SHOULD_EXEC_RESTRICT 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@zeniv.linux.org.uk>
> > > Cc: Christian Brauner <brauner@kernel.org>
> > > Cc: Kees Cook <keescook@chromium.org>
> > > Cc: Paul Moore <paul@paul-moore.com>
> > > Link: https://docs.python.org/3/library/io.html#io.open_code [1]
> > > Signed-off-by: Mickaël Salaün <mic@digikod.net>
> > > Link: https://lore.kernel.org/r/20240704190137.696169-2-mic@digikod.net
> > > ---
> > >
> > > New design since v18:
> > > https://lore.kernel.org/r/20220104155024.48023-3-mic@digikod.net
> > > ---
> > > fs/exec.c | 5 +++--
> > > include/linux/binfmts.h | 7 ++++++-
> > > include/uapi/linux/fcntl.h | 30 ++++++++++++++++++++++++++++++
> > > kernel/audit.h | 1 +
> > > kernel/auditsc.c | 1 +
> > > 5 files changed, 41 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/fs/exec.c b/fs/exec.c
> > > index 40073142288f..ea2a1867afdc 100644
> > > --- a/fs/exec.c
> > > +++ b/fs/exec.c
> > > @@ -931,7 +931,7 @@ 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;
> > > @@ -1595,6 +1595,7 @@ static struct linux_binprm *alloc_bprm(int fd, struct filename *filename, int fl
> > > bprm->filename = bprm->fdpath;
> > > }
> > > bprm->interp = bprm->filename;
> > > + bprm->is_check = !!(flags & AT_CHECK);
> > >
> > > retval = bprm_mm_init(bprm);
> > > if (!retval)
> > > @@ -1885,7 +1886,7 @@ 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 70f97f685bff..8ff9c9e33aed 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 c0bcc185fa48..bcd05c59b7df 100644
> > > --- a/include/uapi/linux/fcntl.h
> > > +++ b/include/uapi/linux/fcntl.h
> > > @@ -118,6 +118,36 @@
> > > #define AT_HANDLE_FID AT_REMOVEDIR /* file handle is needed to
> > > compare object identity and may not
> > > be usable to open_by_handle_at(2) */
> > > +
> > > +/*
> > > + * 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). AT_CHECK
> > > + * should only be used if SECBIT_SHOULD_EXEC_CHECK is set for the calling
> > > + * thread. See securebits.h documentation.
> > > + *
> > > + * Programs should use this check to apply kernel-level checks against files
> > > + * that are not directly executed by the kernel but directly passed to a user
> > > + * space interpreter instead. All files that contain executable code, from the
> > > + * point of view of the interpreter, should be checked. 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.
> > > + */
> > > +#define AT_CHECK 0x10000
> > > +
> > > #if defined(__KERNEL__)
> > > #define AT_GETATTR_NOSEC 0x80000000
> > > #endif
> > > diff --git a/kernel/audit.h b/kernel/audit.h
> > > index a60d2840559e..8ebdabd2ab81 100644
> > > --- a/kernel/audit.h
> > > +++ b/kernel/audit.h
> > > @@ -197,6 +197,7 @@ struct audit_context {
> > > struct open_how openat2;
> > > struct {
> > > int argc;
> > > + bool is_check;
> > > } execve;
> > > struct {
> > > char *name;
> > > diff --git a/kernel/auditsc.c b/kernel/auditsc.c
> > > index 6f0d6fb6523f..b6316e284342 100644
> > > --- a/kernel/auditsc.c
> > > +++ b/kernel/auditsc.c
> > > @@ -2662,6 +2662,7 @@ void __audit_bprm(struct linux_binprm *bprm)
> > >
> > > context->type = AUDIT_EXECVE;
> > > context->execve.argc = bprm->argc;
> > > + context->execve.is_check = bprm->is_check;
> > > }
> > >
> > >
> > > --
> > > 2.45.2
> > >
> >