Re: [PATCH v35 28/29] LSM: Add /proc attr entry for full LSM context

John Johansen
2022-04-22T08:37:51Z

On 4/18/22 07:59, Casey Schaufler wrote:
> Add an entry /proc/.../attr/context which displays the full
> process security "context" in compound format:
> lsm1\0value\0lsm2\0value\0...
> This entry is not writable.
>
> A security module may decide that its policy does not allow
> this information to be displayed. In this case none of the
> information will be displayed.
>
> Reviewed-by: Kees Cook <keescook@chromium.org>
> Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>

Acked-by: John Johansen <john.johansen@canonical.com>

> Cc: linux-api@vger.kernel.org
> Cc: linux-doc@vger.kernel.org
> ---
> Documentation/security/lsm.rst | 14 +++++
> fs/proc/base.c | 1 +
> include/linux/lsm_hooks.h | 6 +++
> security/apparmor/include/procattr.h | 2 +-
> security/apparmor/lsm.c | 8 ++-
> security/apparmor/procattr.c | 22 ++++----
> security/security.c | 79 ++++++++++++++++++++++++++++
> security/selinux/hooks.c | 2 +-
> security/smack/smack_lsm.c | 2 +-
> 9 files changed, 121 insertions(+), 15 deletions(-)
>
> diff --git a/Documentation/security/lsm.rst b/Documentation/security/lsm.rst
> index b77b4a540391..070225ae6ceb 100644
> --- a/Documentation/security/lsm.rst
> +++ b/Documentation/security/lsm.rst
> @@ -143,3 +143,17 @@ separated list of the active security modules.
> The file ``/proc/pid/attr/interface_lsm`` contains the name of the security
> module for which the ``/proc/pid/attr/current`` interface will
> apply. This interface can be written to.
> +
> +The infrastructure does provide an interface for the special
> +case where multiple security modules provide a process context.
> +This is provided in compound context format.
> +
> +- `lsm\0value\0lsm\0value\0`
> +
> +The `lsm` and `value` fields are NUL-terminated bytestrings.
> +Each field may contain whitespace or non-printable characters.
> +The NUL bytes are included in the size of a compound context.
> +The context ``Bell\0Secret\0Biba\0Loose\0`` has a size of 23.
> +
> +The file ``/proc/pid/attr/context`` provides the security
> +context of the identified process.
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index f2d15348bdff..f8aed4404e7e 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -2828,6 +2828,7 @@ static const struct pid_entry attr_dir_stuff[] = {
> ATTR(NULL, "keycreate", 0666),
> ATTR(NULL, "sockcreate", 0666),
> ATTR(NULL, "interface_lsm", 0666),
> + ATTR(NULL, "context", 0444),
> #ifdef CONFIG_SECURITY_SMACK
> DIR("smack", 0555,
> proc_smack_attr_dir_inode_ops, proc_smack_attr_dir_ops),
> diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
> index fd63ae215104..425538ebc606 100644
> --- a/include/linux/lsm_hooks.h
> +++ b/include/linux/lsm_hooks.h
> @@ -1401,6 +1401,12 @@
> * @pages contains the number of pages.
> * Return 0 if permission is granted.
> *
> + * @getprocattr:
> + * Provide the named process attribute for display in special files in
> + * the /proc/.../attr directory. Attribute naming and the data displayed
> + * is at the discretion of the security modules. The exception is the
> + * "context" attribute, which will contain the security context of the
> + * task as a nul terminated text string without trailing whitespace.
> * @ismaclabel:
> * Check if the extended attribute specified by @name
> * represents a MAC label. Returns 1 if name is a MAC
> diff --git a/security/apparmor/include/procattr.h b/security/apparmor/include/procattr.h
> index 31689437e0e1..03dbfdb2f2c0 100644
> --- a/security/apparmor/include/procattr.h
> +++ b/security/apparmor/include/procattr.h
> @@ -11,7 +11,7 @@
> #ifndef __AA_PROCATTR_H
> #define __AA_PROCATTR_H
>
> -int aa_getprocattr(struct aa_label *label, char **string);
> +int aa_getprocattr(struct aa_label *label, char **string, bool newline);
> int aa_setprocattr_changehat(char *args, size_t size, int flags);
>
> #endif /* __AA_PROCATTR_H */
> diff --git a/security/apparmor/lsm.c b/security/apparmor/lsm.c
> index 29181bc8c693..1ee58c1491ab 100644
> --- a/security/apparmor/lsm.c
> +++ b/security/apparmor/lsm.c
> @@ -602,6 +602,7 @@ static int apparmor_getprocattr(struct task_struct *task, char *name,
> const struct cred *cred = get_task_cred(task);
> struct aa_task_ctx *ctx = task_ctx(current);
> struct aa_label *label = NULL;
> + bool newline = true;
>
> if (strcmp(name, "current") == 0)
> label = aa_get_newest_label(cred_label(cred));
> @@ -609,11 +610,14 @@ static int apparmor_getprocattr(struct task_struct *task, char *name,
> label = aa_get_newest_label(ctx->previous);
> else if (strcmp(name, "exec") == 0 && ctx->onexec)
> label = aa_get_newest_label(ctx->onexec);
> - else
> + else if (strcmp(name, "context") == 0) {
> + label = aa_get_newest_label(cred_label(cred));
> + newline = false;
> + } else
> error = -EINVAL;
>
> if (label)
> - error = aa_getprocattr(label, value);
> + error = aa_getprocattr(label, value, newline);
>
> aa_put_label(label);
> put_cred(cred);
> diff --git a/security/apparmor/procattr.c b/security/apparmor/procattr.c
> index fde332e0ea7d..172550f67fc0 100644
> --- a/security/apparmor/procattr.c
> +++ b/security/apparmor/procattr.c
> @@ -20,6 +20,7 @@
> * aa_getprocattr - Return the profile information for @profile
> * @profile: the profile to print profile info about (NOT NULL)
> * @string: Returns - string containing the profile info (NOT NULL)
> + * @newline: Should a newline be added to @string.
> *
> * Requires: profile != NULL
> *
> @@ -28,20 +29,21 @@
> *
> * Returns: size of string placed in @string else error code on failure
> */
> -int aa_getprocattr(struct aa_label *label, char **string)
> +int aa_getprocattr(struct aa_label *label, char **string, bool newline)
> {
> struct aa_ns *ns = labels_ns(label);
> struct aa_ns *current_ns = aa_get_current_ns();
> + int flags = FLAG_VIEW_SUBNS | FLAG_HIDDEN_UNCONFINED;
> int len;
>
> if (!aa_ns_visible(current_ns, ns, true)) {
> aa_put_ns(current_ns);
> return -EACCES;
> }
> + if (newline)
> + flags |= FLAG_SHOW_MODE;
>
> - len = aa_label_snxprint(NULL, 0, current_ns, label,
> - FLAG_SHOW_MODE | FLAG_VIEW_SUBNS |
> - FLAG_HIDDEN_UNCONFINED);
> + len = aa_label_snxprint(NULL, 0, current_ns, label, flags);
> AA_BUG(len < 0);
>
> *string = kmalloc(len + 2, GFP_KERNEL);
> @@ -50,19 +52,19 @@ int aa_getprocattr(struct aa_label *label, char **string)
> return -ENOMEM;
> }
>
> - len = aa_label_snxprint(*string, len + 2, current_ns, label,
> - FLAG_SHOW_MODE | FLAG_VIEW_SUBNS |
> - FLAG_HIDDEN_UNCONFINED);
> + len = aa_label_snxprint(*string, len + 2, current_ns, label, flags);
> if (len < 0) {
> aa_put_ns(current_ns);
> return len;
> }
>
> - (*string)[len] = '\n';
> - (*string)[len + 1] = 0;
> + if (newline) {
> + (*string)[len] = '\n';
> + (*string)[++len] = 0;
> + }
>
> aa_put_ns(current_ns);
> - return len + 1;
> + return len;
> }
>
> /**
> diff --git a/security/security.c b/security/security.c
> index be6682768760..4c6d4171ded6 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -802,6 +802,57 @@ static int lsm_superblock_alloc(struct super_block *sb)
> return 0;
> }
>
> +/**
> + * append_ctx - append a lsm/context pair to a compound context
> + * @ctx: the existing compound context
> + * @ctxlen: size of the old context, including terminating nul byte
> + * @lsm: new lsm name, nul terminated
> + * @new: new context, possibly nul terminated
> + * @newlen: maximum size of @new
> + *
> + * replace @ctx with a new compound context, appending @newlsm and @new
> + * to @ctx. On exit the new data replaces the old, which is freed.
> + * @ctxlen is set to the new size, which includes a trailing nul byte.
> + *
> + * Returns 0 on success, -ENOMEM if no memory is available.
> + */
> +static int append_ctx(char **ctx, int *ctxlen, const char *lsm, char *new,
> + int newlen)
> +{
> + char *final;
> + size_t llen;
> + size_t nlen;
> + size_t flen;
> +
> + llen = strlen(lsm) + 1;
> + /*
> + * A security module may or may not provide a trailing nul on
> + * when returning a security context. There is no definition
> + * of which it should be, and there are modules that do it
> + * each way.
> + */
> + nlen = strnlen(new, newlen);
> +
> + flen = *ctxlen + llen + nlen + 1;
> + final = kzalloc(flen, GFP_KERNEL);
> +
> + if (final == NULL)
> + return -ENOMEM;
> +
> + if (*ctxlen)
> + memcpy(final, *ctx, *ctxlen);
> +
> + memcpy(final + *ctxlen, lsm, llen);
> + memcpy(final + *ctxlen + llen, new, nlen);
> +
> + kfree(*ctx);
> +
> + *ctx = final;
> + *ctxlen = flen;
> +
> + return 0;
> +}
> +
> /*
> * The default value of the LSM hook is defined in linux/lsm_hook_defs.h and
> * can be accessed with:
> @@ -2223,6 +2274,10 @@ int security_getprocattr(struct task_struct *p, const char *lsm, char *name,
> char **value)
> {
> struct security_hook_list *hp;
> + char *final = NULL;
> + char *cp;
> + int rc = 0;
> + int finallen = 0;
> int ilsm = lsm_task_ilsm(current);
> int slot = 0;
>
> @@ -2250,6 +2305,30 @@ int security_getprocattr(struct task_struct *p, const char *lsm, char *name,
> return -ENOMEM;
> }
>
> + if (!strcmp(name, "context")) {
> + hlist_for_each_entry(hp, &security_hook_heads.getprocattr,
> + list) {
> + rc = hp->hook.getprocattr(p, "context", &cp);
> + if (rc == -EINVAL)
> + continue;
> + if (rc < 0) {
> + kfree(final);
> + return rc;
> + }
> + rc = append_ctx(&final, &finallen, hp->lsmid->lsm,
> + cp, rc);
> + kfree(cp);
> + if (rc < 0) {
> + kfree(final);
> + return rc;
> + }
> + }
> + if (final == NULL)
> + return -EINVAL;
> + *value = final;
> + return finallen;
> + }
> +
> hlist_for_each_entry(hp, &security_hook_heads.getprocattr, list) {
> if (lsm != NULL && strcmp(lsm, hp->lsmid->lsm))
> continue;
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 653dd2e236f1..cd977493f734 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -6346,7 +6346,7 @@ static int selinux_getprocattr(struct task_struct *p,
> goto bad;
> }
>
> - if (!strcmp(name, "current"))
> + if (!strcmp(name, "current") || !strcmp(name, "context"))
> sid = __tsec->sid;
> else if (!strcmp(name, "prev"))
> sid = __tsec->osid;
> diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
> index 2190c03ae3d0..9e442c4495bf 100644
> --- a/security/smack/smack_lsm.c
> +++ b/security/smack/smack_lsm.c
> @@ -3485,7 +3485,7 @@ static int smack_getprocattr(struct task_struct *p, char *name, char **value)
> char *cp;
> int slen;
>
> - if (strcmp(name, "current") != 0)
> + if (strcmp(name, "current") != 0 && strcmp(name, "context") != 0)
> return -EINVAL;
>
> cp = kstrdup(skp->smk_known, GFP_KERNEL);