Re: [PATCH v2 07/39] mount: attach mappings to mounts
On Tue, Nov 24, 2020 at 01:30:35PM +0100, Christian Brauner wrote:
> On Mon, Nov 23, 2020 at 11:24:28AM -0500, Tycho Andersen wrote:
> > On Mon, Nov 23, 2020 at 10:47:19AM -0500, Tycho Andersen wrote:
> > > On Sun, Nov 15, 2020 at 11:36:46AM +0100, Christian Brauner wrote:
> > > > +static inline struct user_namespace *mnt_user_ns(const struct vfsmount
> > > > *mnt)
> > > > +{
> > > > + return mnt->mnt_user_ns;
> > > > +}
> > >
> > > I think you might want a READ_ONCE() here. Right now it seems ok, since
> > > the
> > > mnt_user_ns can't change, but if we ever allow it to change (and I see
> > > you have
> > > a idmapped_mounts_wip_v2_allow_to_change_idmapping branch on your public
> > > tree
> > > :D), the pattern of,
> > >
> > > user_ns = mnt_user_ns(path->mnt);
> > > if (mnt_idmapped(path->mnt)) {
> > > uid = kuid_from_mnt(user_ns, uid);
> > > gid = kgid_from_mnt(user_ns, gid);
> > > }
> > >
> > > could race.
> >
> > Actually, isn't a race possible now?
> >
> > kuid_from_mnt(mnt_user_ns(path->mnt) /* &init_user_ns */);
> > WRITE_ONCE(mnt->mnt.mnt_user_ns, user_ns);
> > WRITE_ONCE(m->mnt.mnt_flags, flags);
> > kgid_from_mnt(mnt_user_ns(path->mnt) /* the right user ns */);
> >
> > So maybe it should be:
> >
> > if (mnt_idmapped(path->mnt)) {
> > barrier();
> > user_ns = mnt_user_ns(path->mnt);
> > uid = kuid_from_mnt(user_ns, uid);
> > gid = kgid_from_mnt(user_ns, gid);
> > }
> >
> > since there's no data dependency between mnt_idmapped() and
> > mnt_user_ns()?
>
> I think I had something to handle this case in another branch of mine.
> The READ_ONCE() you mentioned in another patch I had originally dropped
> because I wasn't sure whether it works on pointers but after talking to
> Jann and David it seems that it handles pointers fine.
> Let me take a look and fix it in the next version. I just finished
> porting the test suite to xfstests as Christoph requested and I'm
> looking at this now.
Another way would be to just have mnt_idmapped() test
mnt_user_ns() != &init_user_ns instead of the flags; then I think you
get the data dependency and thus correct ordering for free.
Tycho
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Re: [PATCH v2 07/39] mount: attach mappings to mounts
On Tue, Nov 24, 2020 at 02:40:35PM +0100, Christian Brauner wrote:
> On Tue, Nov 24, 2020 at 08:37:40AM -0500, Tycho Andersen wrote:
> > On Tue, Nov 24, 2020 at 01:30:35PM +0100, Christian Brauner wrote:
> > > On Mon, Nov 23, 2020 at 11:24:28AM -0500, Tycho Andersen wrote:
> > > > On Mon, Nov 23, 2020 at 10:47:19AM -0500, Tycho Andersen wrote:
> > > > > On Sun, Nov 15, 2020 at 11:36:46AM +0100, Christian Brauner wrote:
> > > > > > +static inline struct user_namespace *mnt_user_ns(const struct
> > > > > > vfsmount *mnt)
> > > > > > +{
> > > > > > + return mnt->mnt_user_ns;
> > > > > > +}
> > > > >
> > > > > I think you might want a READ_ONCE() here. Right now it seems ok,
> > > > > since the
> > > > > mnt_user_ns can't change, but if we ever allow it to change (and I
> > > > > see you have
> > > > > a idmapped_mounts_wip_v2_allow_to_change_idmapping branch on your
> > > > > public tree
> > > > > :D), the pattern of,
> > > > >
> > > > > user_ns = mnt_user_ns(path->mnt);
> > > > > if (mnt_idmapped(path->mnt)) {
> > > > > uid = kuid_from_mnt(user_ns, uid);
> > > > > gid = kgid_from_mnt(user_ns, gid);
> > > > > }
> > > > >
> > > > > could race.
> > > >
> > > > Actually, isn't a race possible now?
> > > >
> > > > kuid_from_mnt(mnt_user_ns(path->mnt) /* &init_user_ns */);
> > > > WRITE_ONCE(mnt->mnt.mnt_user_ns, user_ns);
> > > > WRITE_ONCE(m->mnt.mnt_flags, flags);
> > > > kgid_from_mnt(mnt_user_ns(path->mnt) /* the right user ns */);
> > > >
> > > > So maybe it should be:
> > > >
> > > > if (mnt_idmapped(path->mnt)) {
> > > > barrier();
> > > > user_ns = mnt_user_ns(path->mnt);
> > > > uid = kuid_from_mnt(user_ns, uid);
> > > > gid = kgid_from_mnt(user_ns, gid);
> > > > }
> > > >
> > > > since there's no data dependency between mnt_idmapped() and
> > > > mnt_user_ns()?
> > >
> > > I think I had something to handle this case in another branch of mine.
> > > The READ_ONCE() you mentioned in another patch I had originally dropped
> > > because I wasn't sure whether it works on pointers but after talking to
> > > Jann and David it seems that it handles pointers fine.
> > > Let me take a look and fix it in the next version. I just finished
> > > porting the test suite to xfstests as Christoph requested and I'm
> > > looking at this now.
> >
> > Another way would be to just have mnt_idmapped() test
> > mnt_user_ns() != &init_user_ns instead of the flags; then I think you
> > get the data dependency and thus correct ordering for free.
>
> I indeed dropped mnt_idmapped() which is unnecessary. :)
It still might be a nice helper to prevent people from checking the
flags and forgetting that there's a memory ordering issue, though.
> I think we should still use smp_store_release() in mnt_user_ns() paired
> with smp_load_acquire() in do_idmap_mount() thought.
Sounds reasonable.
Tycho
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Re: [PATCH v2 07/39] mount: attach mappings to mounts
On Tue, Nov 24, 2020 at 08:37:40AM -0500, Tycho Andersen wrote:
> On Tue, Nov 24, 2020 at 01:30:35PM +0100, Christian Brauner wrote:
> > On Mon, Nov 23, 2020 at 11:24:28AM -0500, Tycho Andersen wrote:
> > > On Mon, Nov 23, 2020 at 10:47:19AM -0500, Tycho Andersen wrote:
> > > > On Sun, Nov 15, 2020 at 11:36:46AM +0100, Christian Brauner wrote:
> > > > > +static inline struct user_namespace *mnt_user_ns(const struct
> > > > > vfsmount *mnt)
> > > > > +{
> > > > > + return mnt->mnt_user_ns;
> > > > > +}
> > > >
> > > > I think you might want a READ_ONCE() here. Right now it seems ok, since
> > > > the
> > > > mnt_user_ns can't change, but if we ever allow it to change (and I see
> > > > you have
> > > > a idmapped_mounts_wip_v2_allow_to_change_idmapping branch on your
> > > > public tree
> > > > :D), the pattern of,
> > > >
> > > > user_ns = mnt_user_ns(path->mnt);
> > > > if (mnt_idmapped(path->mnt)) {
> > > > uid = kuid_from_mnt(user_ns, uid);
> > > > gid = kgid_from_mnt(user_ns, gid);
> > > > }
> > > >
> > > > could race.
> > >
> > > Actually, isn't a race possible now?
> > >
> > > kuid_from_mnt(mnt_user_ns(path->mnt) /* &init_user_ns */);
> > > WRITE_ONCE(mnt->mnt.mnt_user_ns, user_ns);
> > > WRITE_ONCE(m->mnt.mnt_flags, flags);
> > > kgid_from_mnt(mnt_user_ns(path->mnt) /* the right user ns */);
> > >
> > > So maybe it should be:
> > >
> > > if (mnt_idmapped(path->mnt)) {
> > > barrier();
> > > user_ns = mnt_user_ns(path->mnt);
> > > uid = kuid_from_mnt(user_ns, uid);
> > > gid = kgid_from_mnt(user_ns, gid);
> > > }
> > >
> > > since there's no data dependency between mnt_idmapped() and
> > > mnt_user_ns()?
> >
> > I think I had something to handle this case in another branch of mine.
> > The READ_ONCE() you mentioned in another patch I had originally dropped
> > because I wasn't sure whether it works on pointers but after talking to
> > Jann and David it seems that it handles pointers fine.
> > Let me take a look and fix it in the next version. I just finished
> > porting the test suite to xfstests as Christoph requested and I'm
> > looking at this now.
>
> Another way would be to just have mnt_idmapped() test
> mnt_user_ns() != &init_user_ns instead of the flags; then I think you
> get the data dependency and thus correct ordering for free.
I indeed dropped mnt_idmapped() which is unnecessary. :)
I think we should still use smp_store_release() in mnt_user_ns() paired
with smp_load_acquire() in do_idmap_mount() thought.
Christian
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Re: [PATCH v2 07/39] mount: attach mappings to mounts
On Mon, Nov 23, 2020 at 11:24:28AM -0500, Tycho Andersen wrote:
> On Mon, Nov 23, 2020 at 10:47:19AM -0500, Tycho Andersen wrote:
> > On Sun, Nov 15, 2020 at 11:36:46AM +0100, Christian Brauner wrote:
> > > +static inline struct user_namespace *mnt_user_ns(const struct vfsmount
> > > *mnt)
> > > +{
> > > + return mnt->mnt_user_ns;
> > > +}
> >
> > I think you might want a READ_ONCE() here. Right now it seems ok, since the
> > mnt_user_ns can't change, but if we ever allow it to change (and I see you
> > have
> > a idmapped_mounts_wip_v2_allow_to_change_idmapping branch on your public
> > tree
> > :D), the pattern of,
> >
> > user_ns = mnt_user_ns(path->mnt);
> > if (mnt_idmapped(path->mnt)) {
> > uid = kuid_from_mnt(user_ns, uid);
> > gid = kgid_from_mnt(user_ns, gid);
> > }
> >
> > could race.
>
> Actually, isn't a race possible now?
>
> kuid_from_mnt(mnt_user_ns(path->mnt) /* &init_user_ns */);
> WRITE_ONCE(mnt->mnt.mnt_user_ns, user_ns);
> WRITE_ONCE(m->mnt.mnt_flags, flags);
> kgid_from_mnt(mnt_user_ns(path->mnt) /* the right user ns */);
>
> So maybe it should be:
>
> if (mnt_idmapped(path->mnt)) {
> barrier();
> user_ns = mnt_user_ns(path->mnt);
> uid = kuid_from_mnt(user_ns, uid);
> gid = kgid_from_mnt(user_ns, gid);
> }
>
> since there's no data dependency between mnt_idmapped() and
> mnt_user_ns()?
I think I had something to handle this case in another branch of mine.
The READ_ONCE() you mentioned in another patch I had originally dropped
because I wasn't sure whether it works on pointers but after talking to
Jann and David it seems that it handles pointers fine.
Let me take a look and fix it in the next version. I just finished
porting the test suite to xfstests as Christoph requested and I'm
looking at this now.
Thanks!
Christian
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Re: [PATCH v2 07/39] mount: attach mappings to mounts
On Tue, Nov 24, 2020 at 08:44:59AM -0500, Tycho Andersen wrote:
> On Tue, Nov 24, 2020 at 02:40:35PM +0100, Christian Brauner wrote:
> > On Tue, Nov 24, 2020 at 08:37:40AM -0500, Tycho Andersen wrote:
> > > On Tue, Nov 24, 2020 at 01:30:35PM +0100, Christian Brauner wrote:
> > > > On Mon, Nov 23, 2020 at 11:24:28AM -0500, Tycho Andersen wrote:
> > > > > On Mon, Nov 23, 2020 at 10:47:19AM -0500, Tycho Andersen wrote:
> > > > > > On Sun, Nov 15, 2020 at 11:36:46AM +0100, Christian Brauner wrote:
> > > > > > > +static inline struct user_namespace *mnt_user_ns(const struct
> > > > > > > vfsmount *mnt)
> > > > > > > +{
> > > > > > > + return mnt->mnt_user_ns;
> > > > > > > +}
> > > > > >
> > > > > > I think you might want a READ_ONCE() here. Right now it seems ok,
> > > > > > since the
> > > > > > mnt_user_ns can't change, but if we ever allow it to change (and I
> > > > > > see you have
> > > > > > a idmapped_mounts_wip_v2_allow_to_change_idmapping branch on your
> > > > > > public tree
> > > > > > :D), the pattern of,
> > > > > >
> > > > > > user_ns = mnt_user_ns(path->mnt);
> > > > > > if (mnt_idmapped(path->mnt)) {
> > > > > > uid = kuid_from_mnt(user_ns, uid);
> > > > > > gid = kgid_from_mnt(user_ns, gid);
> > > > > > }
> > > > > >
> > > > > > could race.
> > > > >
> > > > > Actually, isn't a race possible now?
> > > > >
> > > > > kuid_from_mnt(mnt_user_ns(path->mnt) /* &init_user_ns */);
> > > > > WRITE_ONCE(mnt->mnt.mnt_user_ns, user_ns);
> > > > > WRITE_ONCE(m->mnt.mnt_flags, flags);
> > > > > kgid_from_mnt(mnt_user_ns(path->mnt) /* the right user ns */);
> > > > >
> > > > > So maybe it should be:
> > > > >
> > > > > if (mnt_idmapped(path->mnt)) {
> > > > > barrier();
> > > > > user_ns = mnt_user_ns(path->mnt);
> > > > > uid = kuid_from_mnt(user_ns, uid);
> > > > > gid = kgid_from_mnt(user_ns, gid);
> > > > > }
> > > > >
> > > > > since there's no data dependency between mnt_idmapped() and
> > > > > mnt_user_ns()?
> > > >
> > > > I think I had something to handle this case in another branch of mine.
> > > > The READ_ONCE() you mentioned in another patch I had originally dropped
> > > > because I wasn't sure whether it works on pointers but after talking to
> > > > Jann and David it seems that it handles pointers fine.
> > > > Let me take a look and fix it in the next version. I just finished
> > > > porting the test suite to xfstests as Christoph requested and I'm
> > > > looking at this now.
> > >
> > > Another way would be to just have mnt_idmapped() test
> > > mnt_user_ns() != &init_user_ns instead of the flags; then I think you
> > > get the data dependency and thus correct ordering for free.
> >
> > I indeed dropped mnt_idmapped() which is unnecessary. :)
>
> It still might be a nice helper to prevent people from checking the
> flags and forgetting that there's a memory ordering issue, though.
I just mentioned this offline but for the record: the flag is gone since
we can rely on the pointer alone. :)
Christian
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Re: [PATCH v2 07/39] mount: attach mappings to mounts
On Mon, Nov 23, 2020 at 10:47:19AM -0500, Tycho Andersen wrote:
> On Sun, Nov 15, 2020 at 11:36:46AM +0100, Christian Brauner wrote:
> > +static inline struct user_namespace *mnt_user_ns(const struct vfsmount
> > *mnt)
> > +{
> > + return mnt->mnt_user_ns;
> > +}
>
> I think you might want a READ_ONCE() here. Right now it seems ok, since the
> mnt_user_ns can't change, but if we ever allow it to change (and I see you
> have
> a idmapped_mounts_wip_v2_allow_to_change_idmapping branch on your public tree
> :D), the pattern of,
>
> user_ns = mnt_user_ns(path->mnt);
> if (mnt_idmapped(path->mnt)) {
> uid = kuid_from_mnt(user_ns, uid);
> gid = kgid_from_mnt(user_ns, gid);
> }
>
> could race.
Actually, isn't a race possible now?
kuid_from_mnt(mnt_user_ns(path->mnt) /* &init_user_ns */);
WRITE_ONCE(mnt->mnt.mnt_user_ns, user_ns);
WRITE_ONCE(m->mnt.mnt_flags, flags);
kgid_from_mnt(mnt_user_ns(path->mnt) /* the right user ns */);
So maybe it should be:
if (mnt_idmapped(path->mnt)) {
barrier();
user_ns = mnt_user_ns(path->mnt);
uid = kuid_from_mnt(user_ns, uid);
gid = kgid_from_mnt(user_ns, gid);
}
since there's no data dependency between mnt_idmapped() and
mnt_user_ns()?
Tycho
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Re: [PATCH v2 07/39] mount: attach mappings to mounts
On Sun, Nov 15, 2020 at 11:36:46AM +0100, Christian Brauner wrote:
> +static inline struct user_namespace *mnt_user_ns(const struct vfsmount *mnt)
> +{
> + return mnt->mnt_user_ns;
> +}
I think you might want a READ_ONCE() here. Right now it seems ok, since the
mnt_user_ns can't change, but if we ever allow it to change (and I see you have
a idmapped_mounts_wip_v2_allow_to_change_idmapping branch on your public tree
:D), the pattern of,
user_ns = mnt_user_ns(path->mnt);
if (mnt_idmapped(path->mnt)) {
uid = kuid_from_mnt(user_ns, uid);
gid = kgid_from_mnt(user_ns, gid);
}
could race.
Tycho
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