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Message-ID: <20241120.Uy8ahtai5oku@digikod.net>
Date: Wed, 20 Nov 2024 10:42:37 +0100
From: Mickaël Salaün <mic@...ikod.net>
To: Jeff Xu <jeffxu@...omium.org>
Cc: Al Viro <viro@...iv.linux.org.uk>,
Christian Brauner <brauner@...nel.org>, Kees Cook <keescook@...omium.org>,
Paul Moore <paul@...l-moore.com>, Serge Hallyn <serge@...lyn.com>,
Adhemerval Zanella Netto <adhemerval.zanella@...aro.org>, Alejandro Colomar <alx@...nel.org>,
Aleksa Sarai <cyphar@...har.com>, Andrew Morton <akpm@...ux-foundation.org>,
Andy Lutomirski <luto@...nel.org>, Arnd Bergmann <arnd@...db.de>,
Casey Schaufler <casey@...aufler-ca.com>, Christian Heimes <christian@...hon.org>,
Dmitry Vyukov <dvyukov@...gle.com>, Elliott Hughes <enh@...gle.com>,
Eric Biggers <ebiggers@...nel.org>, Eric Chiang <ericchiang@...gle.com>,
Fan Wu <wufan@...ux.microsoft.com>, Florian Weimer <fweimer@...hat.com>,
Geert Uytterhoeven <geert@...ux-m68k.org>, James Morris <jamorris@...ux.microsoft.com>,
Jan Kara <jack@...e.cz>, Jann Horn <jannh@...gle.com>, Jeff Xu <jeffxu@...gle.com>,
Jonathan Corbet <corbet@....net>, Jordan R Abrahams <ajordanr@...gle.com>,
Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>, Linus Torvalds <torvalds@...ux-foundation.org>,
Luca Boccassi <bluca@...ian.org>, Luis Chamberlain <mcgrof@...nel.org>,
"Madhavan T . Venkataraman" <madvenka@...ux.microsoft.com>, Matt Bobrowski <mattbobrowski@...gle.com>,
Matthew Garrett <mjg59@...f.ucam.org>, Matthew Wilcox <willy@...radead.org>,
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Nicolas Bouchinet <nicolas.bouchinet@....gouv.fr>, Scott Shell <scottsh@...rosoft.com>,
Shuah Khan <shuah@...nel.org>, Stephen Rothwell <sfr@...b.auug.org.au>,
Steve Dower <steve.dower@...hon.org>, Steve Grubb <sgrubb@...hat.com>, Theodore Ts'o <tytso@....edu>,
Thibaut Sautereau <thibaut.sautereau@....gouv.fr>, Vincent Strubel <vincent.strubel@....gouv.fr>,
Xiaoming Ni <nixiaoming@...wei.com>, Yin Fengwei <fengwei.yin@...el.com>,
kernel-hardening@...ts.openwall.com, linux-api@...r.kernel.org, linux-fsdevel@...r.kernel.org,
linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-security-module@...r.kernel.org
Subject: Re: [PATCH v21 1/6] exec: Add a new AT_EXECVE_CHECK flag to
execveat(2)
On Tue, Nov 19, 2024 at 05:17:00PM -0800, Jeff Xu wrote:
> On Tue, Nov 12, 2024 at 11:22 AM Mickaël Salaün <mic@...ikod.net> wrote:
> >
> > Add a new AT_EXECVE_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_EXECVE_CHECK is to check if a script execution would be allowed,
> > according to all the different restrictions in place. Because the use
> > of AT_EXECVE_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_EXECVE_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_EXECVE_CHECK is used.
> >
> > It should be noted that script interpreters cannot directly use
> > execveat(2) (without this new AT_EXECVE_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@...iv.linux.org.uk>
> > Cc: Christian Brauner <brauner@...nel.org>
> > Cc: Kees Cook <keescook@...omium.org>
> > Cc: Paul Moore <paul@...l-moore.com>
> > Reviewed-by: Serge Hallyn <serge@...lyn.com>
> > Link: https://docs.python.org/3/library/io.html#io.open_code [1]
> > Signed-off-by: Mickaël Salaün <mic@...ikod.net>
> > Link: https://lore.kernel.org/r/20241112191858.162021-2-mic@digikod.net
> > ---
> >
> > Changes since v20:
> > * Rename AT_CHECK to AT_EXECVE_CHECK, requested by Amir Goldstein and
> > Serge Hallyn.
> > * Move the UAPI documentation to a dedicated RST file.
> > * Add Reviewed-by: Serge Hallyn
> >
> > 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@digikod.net
> > ---
> > Documentation/userspace-api/check_exec.rst | 34 ++++++++++++++++++++++
> > Documentation/userspace-api/index.rst | 1 +
> > fs/exec.c | 20 +++++++++++--
> > include/linux/binfmts.h | 7 ++++-
> > include/uapi/linux/fcntl.h | 4 +++
> > kernel/audit.h | 1 +
> > kernel/auditsc.c | 1 +
> > security/security.c | 10 +++++++
> > 8 files changed, 75 insertions(+), 3 deletions(-)
> > create mode 100644 Documentation/userspace-api/check_exec.rst
> >
> > diff --git a/Documentation/userspace-api/check_exec.rst b/Documentation/userspace-api/check_exec.rst
> > new file mode 100644
> > index 000000000000..ad1aeaa5f6c0
> > --- /dev/null
> > +++ b/Documentation/userspace-api/check_exec.rst
> > @@ -0,0 +1,34 @@
> > +===================
> > +Executability check
> > +===================
> > +
> > +AT_EXECVE_CHECK
> > +===============
> > +
> > +Passing the ``AT_EXECVE_CHECK`` flag to :manpage:`execveat(2)` 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.``.
> Regarding "should only"
> Userspace (e.g. libc) could decide to enforce even when
> SECBIT_EXEC_RESTRICT_FILE=0), i.e. if it determines not-enforcing
> doesn't make sense.
User space is always in control, but I don't think it would be wise to
not follow the configuration securebits (in a generic system) because
this could result to unattended behaviors (I don't have a specific one
in mind but...). That being said, configuration and checks are
standalones and specific/tailored systems are free to do the checks they
want.
> When SECBIT_EXEC_RESTRICT_FILE=1, userspace is bound to enforce.
>
> > +
> > +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_EXECVE_CHECK`` should be used with ``AT_EMPTY_PATH`` to check against a
> > +file descriptor instead of a path.
> > diff --git a/Documentation/userspace-api/index.rst b/Documentation/userspace-api/index.rst
> > index 274cc7546efc..6272bcf11296 100644
> > --- a/Documentation/userspace-api/index.rst
> > +++ b/Documentation/userspace-api/index.rst
> > @@ -35,6 +35,7 @@ Security-related interfaces
> > mfd_noexec
> > spec_ctrl
> > tee
> > + check_exec
> >
> > Devices and I/O
> > ===============
> > diff --git a/fs/exec.c b/fs/exec.c
> > index 6c53920795c2..bb83b6a39530 100644
> > --- a/fs/exec.c
> > +++ b/fs/exec.c
> > @@ -891,7 +891,8 @@ 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_EXECVE_CHECK)) != 0)
> > return ERR_PTR(-EINVAL);
> > if (flags & AT_SYMLINK_NOFOLLOW)
> > open_exec_flags.lookup_flags &= ~LOOKUP_FOLLOW;
> > @@ -1545,6 +1546,21 @@ 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_EXECVE_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_EXECVE_CHECK);
> > +
> > retval = bprm_mm_init(bprm);
> > if (!retval)
> > return bprm;
> > @@ -1839,7 +1855,7 @@ 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..2e87f2e3a79f 100644
> > --- a/include/uapi/linux/fcntl.h
> > +++ b/include/uapi/linux/fcntl.h
> > @@ -154,6 +154,10 @@
> > usable with open_by_handle_at(2). */
> > #define AT_HANDLE_MNT_ID_UNIQUE 0x001 /* Return the u64 unique mount ID. */
> >
> > +/* Flags for execveat2(2). */
> > +#define AT_EXECVE_CHECK 0x10000 /* Only perform a check if execution
> > + would be allowed. */
> > +
> > #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 cd57053b4a69..8d9ba5600cf2 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;
> Where is execve.is_check used ?
It is used in bprm_execve(), exposed to the audit framework, and
potentially used by LSMs.
>
>
> > }
> >
> >
> > diff --git a/security/security.c b/security/security.c
> > index c5981e558bc2..456361ec249d 100644
> > --- a/security/security.c
> > +++ b/security/security.c
> > @@ -1249,6 +1249,12 @@ int security_vm_enough_memory_mm(struct mm_struct *mm, long pages)
> > * to 1 if AT_SECURE should be set to request libc enable secure mode. @bprm
> > * contains the linux_binprm structure.
> > *
> > + * If execveat(2) is called with the AT_EXECVE_CHECK flag, bprm->is_check is
> > + * set. The result must be the same as without this flag even if the execution
> > + * will never really happen and @bprm will always be dropped.
> > + *
> > + * This hook must not change current->cred, only @bprm->cred.
> > + *
> > * Return: Returns 0 if the hook is successful and permission is granted.
> > */
> > int security_bprm_creds_for_exec(struct linux_binprm *bprm)
> > @@ -3100,6 +3106,10 @@ int security_file_receive(struct file *file)
> > * Save open-time permission checking state for later use upon file_permission,
> > * and recheck access if anything has changed since inode_permission.
> > *
> > + * We can check if a file is opened for execution (e.g. execve(2) call), either
> > + * directly or indirectly (e.g. ELF's ld.so) by checking file->f_flags &
> > + * __FMODE_EXEC .
> > + *
> > * Return: Returns 0 if permission is granted.
> > */
> > int security_file_open(struct file *file)
> > --
> > 2.47.0
> >
> >
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