If the current task fails the check for the queried capability via
`capable(CAP_SYS_ADMIN)` LSMs like SELinux generate a denial message.
Issuing such denial messages unnecessarily can lead to a policy author
granting more privileges to a subject than needed to silence them.
Reorder CAP_SYS_ADMIN checks after the check whether the operation is
actually privileged.
Signed-off-by: Christian Göttsche <[email protected]>
---
security/keys/keyctl.c | 11 ++++++++---
1 file changed, 8 insertions(+), 3 deletions(-)
diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
index d54f73c558f7..19be69fa4d05 100644
--- a/security/keys/keyctl.c
+++ b/security/keys/keyctl.c
@@ -980,14 +980,19 @@ long keyctl_chown_key(key_serial_t id, uid_t user, gid_t group)
ret = -EACCES;
down_write(&key->sem);
- if (!capable(CAP_SYS_ADMIN)) {
+ {
+ bool is_privileged_op = false;
+
/* only the sysadmin can chown a key to some other UID */
if (user != (uid_t) -1 && !uid_eq(key->uid, uid))
- goto error_put;
+ is_privileged_op = true;
/* only the sysadmin can set the key's GID to a group other
* than one of those that the current process subscribes to */
if (group != (gid_t) -1 && !gid_eq(gid, key->gid) && !in_group_p(gid))
+ is_privileged_op = true;
+
+ if (is_privileged_op && !capable(CAP_SYS_ADMIN))
goto error_put;
}
@@ -1088,7 +1093,7 @@ long keyctl_setperm_key(key_serial_t id, key_perm_t perm)
down_write(&key->sem);
/* if we're not the sysadmin, we can only change a key that we own */
- if (capable(CAP_SYS_ADMIN) || uid_eq(key->uid, current_fsuid())) {
+ if (uid_eq(key->uid, current_fsuid()) || capable(CAP_SYS_ADMIN)) {
key->perm = perm;
notify_key(key, NOTIFY_KEY_SETATTR, 0);
ret = 0;
--
2.40.1
On Thu, May 11, 2023 at 8:33 AM Christian Göttsche
<[email protected]> wrote:
>
> If the current task fails the check for the queried capability via
> `capable(CAP_SYS_ADMIN)` LSMs like SELinux generate a denial message.
> Issuing such denial messages unnecessarily can lead to a policy author
> granting more privileges to a subject than needed to silence them.
>
> Reorder CAP_SYS_ADMIN checks after the check whether the operation is
> actually privileged.
>
> Signed-off-by: Christian Göttsche <[email protected]>
> ---
> security/keys/keyctl.c | 11 ++++++++---
> 1 file changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
> index d54f73c558f7..19be69fa4d05 100644
> --- a/security/keys/keyctl.c
> +++ b/security/keys/keyctl.c
> @@ -980,14 +980,19 @@ long keyctl_chown_key(key_serial_t id, uid_t user, gid_t group)
> ret = -EACCES;
> down_write(&key->sem);
>
> - if (!capable(CAP_SYS_ADMIN)) {
> + {
> + bool is_privileged_op = false;
> +
> /* only the sysadmin can chown a key to some other UID */
> if (user != (uid_t) -1 && !uid_eq(key->uid, uid))
> - goto error_put;
> + is_privileged_op = true;
>
> /* only the sysadmin can set the key's GID to a group other
> * than one of those that the current process subscribes to */
> if (group != (gid_t) -1 && !gid_eq(gid, key->gid) && !in_group_p(gid))
> + is_privileged_op = true;
> +
> + if (is_privileged_op && !capable(CAP_SYS_ADMIN))
> goto error_put;
> }
Hmm. Using braces just to create a new scope is a bit hacky; I'll
admit to using it to quickly create new local variables, but I only do
so in debug/test situations, not production code.
What if you move the CAP_SYS_ADMIN check down into the if-conditional
where the code checks to see if CAP_SYS_ADMIN is needed when changing
the UID? It should be possible to structure the CAP_SYS_ADMIN check
such that it is only executed if needed. It's a little more
complicated in the GID case, but I believe it should be doable.
--
paul-moore.com
On Fri, 19 May 2023 at 23:08, Paul Moore <[email protected]> wrote:
>
> On Thu, May 11, 2023 at 8:33 AM Christian Göttsche
> <[email protected]> wrote:
> >
> > If the current task fails the check for the queried capability via
> > `capable(CAP_SYS_ADMIN)` LSMs like SELinux generate a denial message.
> > Issuing such denial messages unnecessarily can lead to a policy author
> > granting more privileges to a subject than needed to silence them.
> >
> > Reorder CAP_SYS_ADMIN checks after the check whether the operation is
> > actually privileged.
> >
> > Signed-off-by: Christian Göttsche <[email protected]>
> > ---
> > security/keys/keyctl.c | 11 ++++++++---
> > 1 file changed, 8 insertions(+), 3 deletions(-)
> >
> > diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
> > index d54f73c558f7..19be69fa4d05 100644
> > --- a/security/keys/keyctl.c
> > +++ b/security/keys/keyctl.c
> > @@ -980,14 +980,19 @@ long keyctl_chown_key(key_serial_t id, uid_t user, gid_t group)
> > ret = -EACCES;
> > down_write(&key->sem);
> >
> > - if (!capable(CAP_SYS_ADMIN)) {
> > + {
> > + bool is_privileged_op = false;
> > +
> > /* only the sysadmin can chown a key to some other UID */
> > if (user != (uid_t) -1 && !uid_eq(key->uid, uid))
> > - goto error_put;
> > + is_privileged_op = true;
> >
> > /* only the sysadmin can set the key's GID to a group other
> > * than one of those that the current process subscribes to */
> > if (group != (gid_t) -1 && !gid_eq(gid, key->gid) && !in_group_p(gid))
> > + is_privileged_op = true;
> > +
> > + if (is_privileged_op && !capable(CAP_SYS_ADMIN))
> > goto error_put;
> > }
>
> Hmm. Using braces just to create a new scope is a bit hacky; I'll
> admit to using it to quickly create new local variables, but I only do
> so in debug/test situations, not production code.
>
> What if you move the CAP_SYS_ADMIN check down into the if-conditional
> where the code checks to see if CAP_SYS_ADMIN is needed when changing
> the UID? It should be possible to structure the CAP_SYS_ADMIN check
> such that it is only executed if needed. It's a little more
> complicated in the GID case, but I believe it should be doable.
This complication I exactly wanted to avoid. For me the inner scope
encapsulates the all the logic around the capability check just fine
and is quite readable. An alternative would be to create a new
function performing the checks and call it via
if (!chown_key_capable(key, user, uid, group, gid))
got error_put;
A minor inconvenience is the number of needed arguments (and the
actual code after inlining should be the same to the inner scope in
the end).
>
> --
> paul-moore.com
On Thu May 11, 2023 at 3:32 PM EEST, Christian Göttsche wrote:
> If the current task fails the check for the queried capability via
> `capable(CAP_SYS_ADMIN)` LSMs like SELinux generate a denial message.
> Issuing such denial messages unnecessarily can lead to a policy author
> granting more privileges to a subject than needed to silence them.
>
> Reorder CAP_SYS_ADMIN checks after the check whether the operation is
> actually privileged.
>
> Signed-off-by: Christian Göttsche <[email protected]>
> ---
> security/keys/keyctl.c | 11 ++++++++---
> 1 file changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
> index d54f73c558f7..19be69fa4d05 100644
> --- a/security/keys/keyctl.c
> +++ b/security/keys/keyctl.c
> @@ -980,14 +980,19 @@ long keyctl_chown_key(key_serial_t id, uid_t user, gid_t group)
> ret = -EACCES;
> down_write(&key->sem);
>
> - if (!capable(CAP_SYS_ADMIN)) {
> + {
> + bool is_privileged_op = false;
> +
> /* only the sysadmin can chown a key to some other UID */
> if (user != (uid_t) -1 && !uid_eq(key->uid, uid))
> - goto error_put;
> + is_privileged_op = true;
>
> /* only the sysadmin can set the key's GID to a group other
> * than one of those that the current process subscribes to */
> if (group != (gid_t) -1 && !gid_eq(gid, key->gid) && !in_group_p(gid))
> + is_privileged_op = true;
> +
> + if (is_privileged_op && !capable(CAP_SYS_ADMIN))
> goto error_put;
> }
>
> @@ -1088,7 +1093,7 @@ long keyctl_setperm_key(key_serial_t id, key_perm_t perm)
> down_write(&key->sem);
>
> /* if we're not the sysadmin, we can only change a key that we own */
> - if (capable(CAP_SYS_ADMIN) || uid_eq(key->uid, current_fsuid())) {
> + if (uid_eq(key->uid, current_fsuid()) || capable(CAP_SYS_ADMIN)) {
> key->perm = perm;
> notify_key(key, NOTIFY_KEY_SETATTR, 0);
> ret = 0;
> --
> 2.40.1
Acked-by: Jarkko Sakkinen <[email protected]>
BR, Jarkko
On Tue, May 23, 2023 at 2:33 PM Christian Göttsche
<[email protected]> wrote:
> On Fri, 19 May 2023 at 23:08, Paul Moore <[email protected]> wrote:
> > On Thu, May 11, 2023 at 8:33 AM Christian Göttsche
> > <[email protected]> wrote:
> > >
> > > If the current task fails the check for the queried capability via
> > > `capable(CAP_SYS_ADMIN)` LSMs like SELinux generate a denial message.
> > > Issuing such denial messages unnecessarily can lead to a policy author
> > > granting more privileges to a subject than needed to silence them.
> > >
> > > Reorder CAP_SYS_ADMIN checks after the check whether the operation is
> > > actually privileged.
> > >
> > > Signed-off-by: Christian Göttsche <[email protected]>
> > > ---
> > > security/keys/keyctl.c | 11 ++++++++---
> > > 1 file changed, 8 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
> > > index d54f73c558f7..19be69fa4d05 100644
> > > --- a/security/keys/keyctl.c
> > > +++ b/security/keys/keyctl.c
> > > @@ -980,14 +980,19 @@ long keyctl_chown_key(key_serial_t id, uid_t user, gid_t group)
> > > ret = -EACCES;
> > > down_write(&key->sem);
> > >
> > > - if (!capable(CAP_SYS_ADMIN)) {
> > > + {
> > > + bool is_privileged_op = false;
> > > +
> > > /* only the sysadmin can chown a key to some other UID */
> > > if (user != (uid_t) -1 && !uid_eq(key->uid, uid))
> > > - goto error_put;
> > > + is_privileged_op = true;
> > >
> > > /* only the sysadmin can set the key's GID to a group other
> > > * than one of those that the current process subscribes to */
> > > if (group != (gid_t) -1 && !gid_eq(gid, key->gid) && !in_group_p(gid))
> > > + is_privileged_op = true;
> > > +
> > > + if (is_privileged_op && !capable(CAP_SYS_ADMIN))
> > > goto error_put;
> > > }
> >
> > Hmm. Using braces just to create a new scope is a bit hacky; I'll
> > admit to using it to quickly create new local variables, but I only do
> > so in debug/test situations, not production code.
> >
> > What if you move the CAP_SYS_ADMIN check down into the if-conditional
> > where the code checks to see if CAP_SYS_ADMIN is needed when changing
> > the UID? It should be possible to structure the CAP_SYS_ADMIN check
> > such that it is only executed if needed. It's a little more
> > complicated in the GID case, but I believe it should be doable.
>
> This complication I exactly wanted to avoid. For me the inner scope
> encapsulates the all the logic around the capability check just fine
> and is quite readable. An alternative would be to create a new
> function performing the checks and call it via
>
> if (!chown_key_capable(key, user, uid, group, gid))
> got error_put;
>
> A minor inconvenience is the number of needed arguments (and the
> actual code after inlining should be the same to the inner scope in
> the end).
Well, lucky for you, Jarkko and David maintain the keys code, not me,
and Jarkko seems to like your patch just fine :)
Jarkko, I assume you'll be taking this via the keys tree?
--
paul-moore.com
On Thu, 2023-05-25 at 17:25 -0400, Paul Moore wrote:
> > A minor inconvenience is the number of needed arguments (and the
> > actual code after inlining should be the same to the inner scope in
> > the end).
>
> Well, lucky for you, Jarkko and David maintain the keys code, not me,
> and Jarkko seems to like your patch just fine :)
>
> Jarkko, I assume you'll be taking this via the keys tree?
I just picked it and mirrored to linux-next.
I think it is super important change because it tones down the human
error (a little bit at least). You could say improves user experience
kind of I guess :-)
BR, Jarkko
On Wed, 2023-05-21 at 02:04 +0300, Jarkko Sakkinen wrote:
> On Thu, 2023-05-25 at 17:25 -0400, Paul Moore wrote:
> > > A minor inconvenience is the number of needed arguments (and the
> > > actual code after inlining should be the same to the inner scope in
> > > the end).
> >
> > Well, lucky for you, Jarkko and David maintain the keys code, not me,
> > and Jarkko seems to like your patch just fine :)
> >
> > Jarkko, I assume you'll be taking this via the keys tree?
>
> I just picked it and mirrored to linux-next.
>
> I think it is super important change because it tones down the human
> error (a little bit at least). You could say improves user experience
> kind of I guess :-)
>
> BR, Jarkko
Kindly ping; I do not see this patch applied anywhere.
Regards,
Christian