ovl_link() did not create a new inode after commit
51f7e52dc943 ("ovl: share inode for hard link"), so
in ovl_create_or_link() we should not override cred's
fsuid and fsgid when called by ovl_link().
Signed-off-by: Zhang Tianci <[email protected]>
Signed-off-by: Jiachen Zhang <[email protected]>
---
fs/overlayfs/dir.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
index 6b03457f72bb..568d338032db 100644
--- a/fs/overlayfs/dir.c
+++ b/fs/overlayfs/dir.c
@@ -595,9 +595,9 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
err = -ENOMEM;
override_cred = prepare_creds();
if (override_cred) {
- override_cred->fsuid = inode->i_uid;
- override_cred->fsgid = inode->i_gid;
if (!attr->hardlink) {
+ override_cred->fsuid = inode->i_uid;
+ override_cred->fsgid = inode->i_gid;
err = security_dentry_create_files_as(dentry,
attr->mode, &dentry->d_name, old_cred,
override_cred);
--
2.32.1 (Apple Git-133)
On Wed, Aug 17, 2022 at 6:49 AM Zhang Tianci
<[email protected]> wrote:
>
> ovl_link() did not create a new inode after commit
> 51f7e52dc943 ("ovl: share inode for hard link"), so
> in ovl_create_or_link() we should not override cred's
> fsuid and fsgid when called by ovl_link().
>
> Signed-off-by: Zhang Tianci <[email protected]>
> Signed-off-by: Jiachen Zhang <[email protected]>
> ---
> fs/overlayfs/dir.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> index 6b03457f72bb..568d338032db 100644
> --- a/fs/overlayfs/dir.c
> +++ b/fs/overlayfs/dir.c
> @@ -595,9 +595,9 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
> err = -ENOMEM;
> override_cred = prepare_creds();
> if (override_cred) {
> - override_cred->fsuid = inode->i_uid;
> - override_cred->fsgid = inode->i_gid;
> if (!attr->hardlink) {
> + override_cred->fsuid = inode->i_uid;
> + override_cred->fsgid = inode->i_gid;
> err = security_dentry_create_files_as(dentry,
> attr->mode, &dentry->d_name, old_cred,
> override_cred);
> --
This change looks incorrect.
Unless I am missing something, fsuid/fsgid still need to
be overridden for calling link() on underlying fs.
What made you do this change?
Thanks,
Amir.
On Wed, Aug 17, 2022 at 3:36 PM Amir Goldstein <[email protected]> wrote:
>
> On Wed, Aug 17, 2022 at 6:49 AM Zhang Tianci
> <[email protected]> wrote:
> >
> > ovl_link() did not create a new inode after commit
> > 51f7e52dc943 ("ovl: share inode for hard link"), so
> > in ovl_create_or_link() we should not override cred's
> > fsuid and fsgid when called by ovl_link().
> >
> > Signed-off-by: Zhang Tianci <[email protected]>
> > Signed-off-by: Jiachen Zhang <[email protected]>
> > ---
> > fs/overlayfs/dir.c | 4 ++--
> > 1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> > index 6b03457f72bb..568d338032db 100644
> > --- a/fs/overlayfs/dir.c
> > +++ b/fs/overlayfs/dir.c
> > @@ -595,9 +595,9 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
> > err = -ENOMEM;
> > override_cred = prepare_creds();
> > if (override_cred) {
> > - override_cred->fsuid = inode->i_uid;
> > - override_cred->fsgid = inode->i_gid;
> > if (!attr->hardlink) {
> > + override_cred->fsuid = inode->i_uid;
> > + override_cred->fsgid = inode->i_gid;
> > err = security_dentry_create_files_as(dentry,
> > attr->mode, &dentry->d_name, old_cred,
> > override_cred);
> > --
>
> This change looks incorrect.
> Unless I am missing something, fsuid/fsgid still need to
> be overridden for calling link() on underlying fs.
> What made you do this change?
>
> Thanks,
> Amir.
Hi Amir,
I ran into an error when I tested overlay on fuse:
$ mkdir /lower /fuse /merge
$ mount -t fuse /fuse
$ mkdir /fuse/upper /fuse/work
$ mount -t overlay /merge -o lowerdir=/lower,upperdir=/fuse/upper,workdir=work
$ touch /merge/file
$ chown bin.bin /merge/file // the file's caller becomes "bin"
$ ln /merge/file /merge/lnkfile
Then I got an error(EACCES) because fuse daemon checks the link()'s
caller is "bin", it denied this request.
I browsed the changing history of ovl_link(). There are two key commits:
The first is commit bb0d2b8ad296 ("ovl: fix sgid on directory") which
overrides the cred's fsuid/fsgid using the new inode. The new inode's
owner is initialized by inode_init_owner(), and inode->fsuid is
assigned to the current user. So the override fsuid becomes the
current user. We know link() is actually modifying the directory, so
the caller must have the MAY_WRITE permission on the directory. The
current caller may should have this permission. I think this is
acceptable to use the caller's fsuid(But I still feel a little
conflicted with the overlay's design).
The second is commit 51f7e52dc943 ("ovl: share inode for hard link")
which removed the inode creation in ovl_link(). This commit move
inode_init_owner() into ovl_create_object(), so the ovl_link() just
give the old inode to ovl_create_or_link(). Then the override fsuid
becomes the old inode's fsuid, neither the caller nor the overlay's
creator! So I think this is incorrect.
I think the link() should be like unlink(), overlay fs should just use
the creator cred to do underlying fs's operations.
Thanks,
Tianci.
On Wed, Aug 17, 2022 at 12:11 PM 天赐张 <[email protected]> wrote:
>
> On Wed, Aug 17, 2022 at 3:36 PM Amir Goldstein <[email protected]> wrote:
> >
> > On Wed, Aug 17, 2022 at 6:49 AM Zhang Tianci
> > <[email protected]> wrote:
> > >
> > > ovl_link() did not create a new inode after commit
> > > 51f7e52dc943 ("ovl: share inode for hard link"), so
> > > in ovl_create_or_link() we should not override cred's
> > > fsuid and fsgid when called by ovl_link().
> > >
> > > Signed-off-by: Zhang Tianci <[email protected]>
> > > Signed-off-by: Jiachen Zhang <[email protected]>
> > > ---
> > > fs/overlayfs/dir.c | 4 ++--
> > > 1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> > > index 6b03457f72bb..568d338032db 100644
> > > --- a/fs/overlayfs/dir.c
> > > +++ b/fs/overlayfs/dir.c
> > > @@ -595,9 +595,9 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
> > > err = -ENOMEM;
> > > override_cred = prepare_creds();
> > > if (override_cred) {
> > > - override_cred->fsuid = inode->i_uid;
> > > - override_cred->fsgid = inode->i_gid;
> > > if (!attr->hardlink) {
> > > + override_cred->fsuid = inode->i_uid;
> > > + override_cred->fsgid = inode->i_gid;
> > > err = security_dentry_create_files_as(dentry,
> > > attr->mode, &dentry->d_name, old_cred,
> > > override_cred);
> > > --
> >
> > This change looks incorrect.
> > Unless I am missing something, fsuid/fsgid still need to
> > be overridden for calling link() on underlying fs.
> > What made you do this change?
> >
> > Thanks,
> > Amir.
>
> Hi Amir,
>
> I ran into an error when I tested overlay on fuse:
> $ mkdir /lower /fuse /merge
> $ mount -t fuse /fuse
> $ mkdir /fuse/upper /fuse/work
> $ mount -t overlay /merge -o lowerdir=/lower,upperdir=/fuse/upper,workdir=work
> $ touch /merge/file
> $ chown bin.bin /merge/file // the file's caller becomes "bin"
> $ ln /merge/file /merge/lnkfile
>
> Then I got an error(EACCES) because fuse daemon checks the link()'s
> caller is "bin", it denied this request.
> I browsed the changing history of ovl_link(). There are two key commits:
> The first is commit bb0d2b8ad296 ("ovl: fix sgid on directory") which
> overrides the cred's fsuid/fsgid using the new inode. The new inode's
> owner is initialized by inode_init_owner(), and inode->fsuid is
> assigned to the current user. So the override fsuid becomes the
> current user. We know link() is actually modifying the directory, so
> the caller must have the MAY_WRITE permission on the directory. The
> current caller may should have this permission. I think this is
> acceptable to use the caller's fsuid(But I still feel a little
> conflicted with the overlay's design).
> The second is commit 51f7e52dc943 ("ovl: share inode for hard link")
> which removed the inode creation in ovl_link(). This commit move
> inode_init_owner() into ovl_create_object(), so the ovl_link() just
> give the old inode to ovl_create_or_link(). Then the override fsuid
> becomes the old inode's fsuid, neither the caller nor the overlay's
> creator! So I think this is incorrect.
> I think the link() should be like unlink(), overlay fs should just use
> the creator cred to do underlying fs's operations.
>
I see. The reproducer and explanation belong in the commit message.
Your argument makes sense to me, but CC Christian to make
sure I am not missing anything related to ACLs and what not.
Thanks,
Amir.
On Wed, Aug 17, 2022 at 12:53 PM Amir Goldstein <[email protected]> wrote:
>
> On Wed, Aug 17, 2022 at 12:11 PM 天赐张 <[email protected]> wrote:
> >
> > On Wed, Aug 17, 2022 at 3:36 PM Amir Goldstein <[email protected]> wrote:
> > >
> > > On Wed, Aug 17, 2022 at 6:49 AM Zhang Tianci
> > > <[email protected]> wrote:
> > > >
> > > > ovl_link() did not create a new inode after commit
> > > > 51f7e52dc943 ("ovl: share inode for hard link"), so
> > > > in ovl_create_or_link() we should not override cred's
> > > > fsuid and fsgid when called by ovl_link().
> > > >
> > > > Signed-off-by: Zhang Tianci <[email protected]>
> > > > Signed-off-by: Jiachen Zhang <[email protected]>
> > > > ---
> > > > fs/overlayfs/dir.c | 4 ++--
> > > > 1 file changed, 2 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> > > > index 6b03457f72bb..568d338032db 100644
> > > > --- a/fs/overlayfs/dir.c
> > > > +++ b/fs/overlayfs/dir.c
> > > > @@ -595,9 +595,9 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
> > > > err = -ENOMEM;
> > > > override_cred = prepare_creds();
> > > > if (override_cred) {
> > > > - override_cred->fsuid = inode->i_uid;
> > > > - override_cred->fsgid = inode->i_gid;
> > > > if (!attr->hardlink) {
> > > > + override_cred->fsuid = inode->i_uid;
> > > > + override_cred->fsgid = inode->i_gid;
> > > > err = security_dentry_create_files_as(dentry,
> > > > attr->mode, &dentry->d_name, old_cred,
> > > > override_cred);
> > > > --
> > >
> > > This change looks incorrect.
> > > Unless I am missing something, fsuid/fsgid still need to
> > > be overridden for calling link() on underlying fs.
> > > What made you do this change?
> > >
> > > Thanks,
> > > Amir.
> >
> > Hi Amir,
> >
> > I ran into an error when I tested overlay on fuse:
> > $ mkdir /lower /fuse /merge
> > $ mount -t fuse /fuse
> > $ mkdir /fuse/upper /fuse/work
> > $ mount -t overlay /merge -o lowerdir=/lower,upperdir=/fuse/upper,workdir=work
> > $ touch /merge/file
> > $ chown bin.bin /merge/file // the file's caller becomes "bin"
> > $ ln /merge/file /merge/lnkfile
> >
> > Then I got an error(EACCES) because fuse daemon checks the link()'s
> > caller is "bin", it denied this request.
> > I browsed the changing history of ovl_link(). There are two key commits:
> > The first is commit bb0d2b8ad296 ("ovl: fix sgid on directory") which
> > overrides the cred's fsuid/fsgid using the new inode. The new inode's
> > owner is initialized by inode_init_owner(), and inode->fsuid is
> > assigned to the current user. So the override fsuid becomes the
> > current user. We know link() is actually modifying the directory, so
> > the caller must have the MAY_WRITE permission on the directory. The
> > current caller may should have this permission. I think this is
> > acceptable to use the caller's fsuid(But I still feel a little
> > conflicted with the overlay's design).
> > The second is commit 51f7e52dc943 ("ovl: share inode for hard link")
> > which removed the inode creation in ovl_link(). This commit move
> > inode_init_owner() into ovl_create_object(), so the ovl_link() just
> > give the old inode to ovl_create_or_link(). Then the override fsuid
> > becomes the old inode's fsuid, neither the caller nor the overlay's
> > creator! So I think this is incorrect.
> > I think the link() should be like unlink(), overlay fs should just use
> > the creator cred to do underlying fs's operations.
> >
>
> I see. The reproducer and explanation belong in the commit message.
>
> Your argument makes sense to me, but CC Christian to make
> sure I am not missing anything related to ACLs and what not.
Once again with correct email address...
>
> Thanks,
> Amir.
On Wed, Aug 17, 2022 at 12:55:22PM +0300, Amir Goldstein wrote:
> On Wed, Aug 17, 2022 at 12:53 PM Amir Goldstein <[email protected]> wrote:
> >
> > On Wed, Aug 17, 2022 at 12:11 PM 天赐张 <[email protected]> wrote:
> > >
> > > On Wed, Aug 17, 2022 at 3:36 PM Amir Goldstein <[email protected]> wrote:
> > > >
> > > > On Wed, Aug 17, 2022 at 6:49 AM Zhang Tianci
> > > > <[email protected]> wrote:
> > > > >
> > > > > ovl_link() did not create a new inode after commit
> > > > > 51f7e52dc943 ("ovl: share inode for hard link"), so
> > > > > in ovl_create_or_link() we should not override cred's
> > > > > fsuid and fsgid when called by ovl_link().
> > > > >
> > > > > Signed-off-by: Zhang Tianci <[email protected]>
> > > > > Signed-off-by: Jiachen Zhang <[email protected]>
> > > > > ---
> > > > > fs/overlayfs/dir.c | 4 ++--
> > > > > 1 file changed, 2 insertions(+), 2 deletions(-)
> > > > >
> > > > > diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> > > > > index 6b03457f72bb..568d338032db 100644
> > > > > --- a/fs/overlayfs/dir.c
> > > > > +++ b/fs/overlayfs/dir.c
> > > > > @@ -595,9 +595,9 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
> > > > > err = -ENOMEM;
> > > > > override_cred = prepare_creds();
> > > > > if (override_cred) {
> > > > > - override_cred->fsuid = inode->i_uid;
> > > > > - override_cred->fsgid = inode->i_gid;
> > > > > if (!attr->hardlink) {
> > > > > + override_cred->fsuid = inode->i_uid;
> > > > > + override_cred->fsgid = inode->i_gid;
> > > > > err = security_dentry_create_files_as(dentry,
> > > > > attr->mode, &dentry->d_name, old_cred,
> > > > > override_cred);
> > > > > --
> > > >
> > > > This change looks incorrect.
> > > > Unless I am missing something, fsuid/fsgid still need to
> > > > be overridden for calling link() on underlying fs.
> > > > What made you do this change?
> > > >
> > > > Thanks,
> > > > Amir.
> > >
> > > Hi Amir,
> > >
> > > I ran into an error when I tested overlay on fuse:
> > > $ mkdir /lower /fuse /merge
> > > $ mount -t fuse /fuse
> > > $ mkdir /fuse/upper /fuse/work
> > > $ mount -t overlay /merge -o lowerdir=/lower,upperdir=/fuse/upper,workdir=work
> > > $ touch /merge/file
> > > $ chown bin.bin /merge/file // the file's caller becomes "bin"
> > > $ ln /merge/file /merge/lnkfile
> > >
> > > Then I got an error(EACCES) because fuse daemon checks the link()'s
> > > caller is "bin", it denied this request.
> > > I browsed the changing history of ovl_link(). There are two key commits:
> > > The first is commit bb0d2b8ad296 ("ovl: fix sgid on directory") which
> > > overrides the cred's fsuid/fsgid using the new inode. The new inode's
> > > owner is initialized by inode_init_owner(), and inode->fsuid is
> > > assigned to the current user. So the override fsuid becomes the
> > > current user. We know link() is actually modifying the directory, so
> > > the caller must have the MAY_WRITE permission on the directory. The
> > > current caller may should have this permission. I think this is
> > > acceptable to use the caller's fsuid(But I still feel a little
> > > conflicted with the overlay's design).
> > > The second is commit 51f7e52dc943 ("ovl: share inode for hard link")
> > > which removed the inode creation in ovl_link(). This commit move
> > > inode_init_owner() into ovl_create_object(), so the ovl_link() just
> > > give the old inode to ovl_create_or_link(). Then the override fsuid
> > > becomes the old inode's fsuid, neither the caller nor the overlay's
> > > creator! So I think this is incorrect.
> > > I think the link() should be like unlink(), overlay fs should just use
> > > the creator cred to do underlying fs's operations.
> > >
> >
> > I see. The reproducer and explanation belong in the commit message.
> >
> > Your argument makes sense to me, but CC Christian to make
> > sure I am not missing anything related to ACLs and what not.
>
> Once again with correct email address...
So we have:
ovl_create_object()
-> ovl_override_creds(ovl_sb)
-> ovl_new_inode()
-> inode_init_owner()
{
inode->i_uid = current_fsuid();
inode->i_gid = current_fsgid();
}
-> ovl_create_or_link(inode, ...)
-> prepare_creds() // Copy of caller's creds
{
override_creds->fsuid = inode->i_uid;
override_creds->fsgid = inode->i_gid;
}
-> revert_creds()
which afaict means that the mounter's credentials are used apart from
the fs{g,u}id which is taken from inode->i_{g,u}id which should
correspond to current_fs{g,u}id().
The commit that is pointed out in the patch
51f7e52dc943 ("ovl: share inode for hard link")
seems to have broken that assumption.
Given that the intention was to use the creator's creds _with the
caller's fs{g,u}id_ wouldn't it make more sense to simply ensure that
the caller's fs{g,u}id are always used instead of using the full
creator's creds just for the link operation? So something like this
(untested):
diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
index 6b03457f72bb..4a3ee16a6d70 100644
--- a/fs/overlayfs/dir.c
+++ b/fs/overlayfs/dir.c
@@ -575,6 +575,9 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
const struct cred *old_cred;
struct cred *override_cred;
struct dentry *parent = dentry->d_parent;
+ /* Retrieve caller's fs{g,u}id before we override creds below. */
+ kuid_t caller_fsuid = current_fsuid();
+ kgid_t caller_fsgid = current_fsgid();
err = ovl_copy_up(parent);
if (err)
@@ -595,8 +598,8 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
err = -ENOMEM;
override_cred = prepare_creds();
if (override_cred) {
- override_cred->fsuid = inode->i_uid;
- override_cred->fsgid = inode->i_gid;
+ override_cred->fsuid = caller_fsuid;
+ override_cred->fsgid = caller_fsgid;
if (!attr->hardlink) {
err = security_dentry_create_files_as(dentry,
attr->mode, &dentry->d_name, old_cred,
On Wed, Aug 17, 2022 at 12:27:27PM +0200, Christian Brauner wrote:
> On Wed, Aug 17, 2022 at 12:55:22PM +0300, Amir Goldstein wrote:
> > On Wed, Aug 17, 2022 at 12:53 PM Amir Goldstein <[email protected]> wrote:
> > >
> > > On Wed, Aug 17, 2022 at 12:11 PM 天赐张 <[email protected]> wrote:
> > > >
> > > > On Wed, Aug 17, 2022 at 3:36 PM Amir Goldstein <[email protected]> wrote:
> > > > >
> > > > > On Wed, Aug 17, 2022 at 6:49 AM Zhang Tianci
> > > > > <[email protected]> wrote:
> > > > > >
> > > > > > ovl_link() did not create a new inode after commit
> > > > > > 51f7e52dc943 ("ovl: share inode for hard link"), so
> > > > > > in ovl_create_or_link() we should not override cred's
> > > > > > fsuid and fsgid when called by ovl_link().
> > > > > >
> > > > > > Signed-off-by: Zhang Tianci <[email protected]>
> > > > > > Signed-off-by: Jiachen Zhang <[email protected]>
> > > > > > ---
> > > > > > fs/overlayfs/dir.c | 4 ++--
> > > > > > 1 file changed, 2 insertions(+), 2 deletions(-)
> > > > > >
> > > > > > diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> > > > > > index 6b03457f72bb..568d338032db 100644
> > > > > > --- a/fs/overlayfs/dir.c
> > > > > > +++ b/fs/overlayfs/dir.c
> > > > > > @@ -595,9 +595,9 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
> > > > > > err = -ENOMEM;
> > > > > > override_cred = prepare_creds();
> > > > > > if (override_cred) {
> > > > > > - override_cred->fsuid = inode->i_uid;
> > > > > > - override_cred->fsgid = inode->i_gid;
> > > > > > if (!attr->hardlink) {
> > > > > > + override_cred->fsuid = inode->i_uid;
> > > > > > + override_cred->fsgid = inode->i_gid;
> > > > > > err = security_dentry_create_files_as(dentry,
> > > > > > attr->mode, &dentry->d_name, old_cred,
> > > > > > override_cred);
> > > > > > --
> > > > >
> > > > > This change looks incorrect.
> > > > > Unless I am missing something, fsuid/fsgid still need to
> > > > > be overridden for calling link() on underlying fs.
> > > > > What made you do this change?
> > > > >
> > > > > Thanks,
> > > > > Amir.
> > > >
> > > > Hi Amir,
> > > >
> > > > I ran into an error when I tested overlay on fuse:
> > > > $ mkdir /lower /fuse /merge
> > > > $ mount -t fuse /fuse
> > > > $ mkdir /fuse/upper /fuse/work
> > > > $ mount -t overlay /merge -o lowerdir=/lower,upperdir=/fuse/upper,workdir=work
> > > > $ touch /merge/file
> > > > $ chown bin.bin /merge/file // the file's caller becomes "bin"
> > > > $ ln /merge/file /merge/lnkfile
> > > >
> > > > Then I got an error(EACCES) because fuse daemon checks the link()'s
> > > > caller is "bin", it denied this request.
> > > > I browsed the changing history of ovl_link(). There are two key commits:
> > > > The first is commit bb0d2b8ad296 ("ovl: fix sgid on directory") which
> > > > overrides the cred's fsuid/fsgid using the new inode. The new inode's
> > > > owner is initialized by inode_init_owner(), and inode->fsuid is
> > > > assigned to the current user. So the override fsuid becomes the
> > > > current user. We know link() is actually modifying the directory, so
> > > > the caller must have the MAY_WRITE permission on the directory. The
> > > > current caller may should have this permission. I think this is
> > > > acceptable to use the caller's fsuid(But I still feel a little
> > > > conflicted with the overlay's design).
> > > > The second is commit 51f7e52dc943 ("ovl: share inode for hard link")
> > > > which removed the inode creation in ovl_link(). This commit move
> > > > inode_init_owner() into ovl_create_object(), so the ovl_link() just
> > > > give the old inode to ovl_create_or_link(). Then the override fsuid
> > > > becomes the old inode's fsuid, neither the caller nor the overlay's
> > > > creator! So I think this is incorrect.
> > > > I think the link() should be like unlink(), overlay fs should just use
> > > > the creator cred to do underlying fs's operations.
> > > >
> > >
> > > I see. The reproducer and explanation belong in the commit message.
> > >
> > > Your argument makes sense to me, but CC Christian to make
> > > sure I am not missing anything related to ACLs and what not.
> >
> > Once again with correct email address...
>
> So we have:
>
> ovl_create_object()
> -> ovl_override_creds(ovl_sb)
> -> ovl_new_inode()
> -> inode_init_owner()
> {
> inode->i_uid = current_fsuid();
> inode->i_gid = current_fsgid();
> }
> -> ovl_create_or_link(inode, ...)
> -> prepare_creds() // Copy of caller's creds
s/caller's/creator's/
> {
> override_creds->fsuid = inode->i_uid;
> override_creds->fsgid = inode->i_gid;
> }
> -> revert_creds()
>
> which afaict means that the mounter's credentials are used apart from
> the fs{g,u}id which is taken from inode->i_{g,u}id which should
> correspond to current_fs{g,u}id().
>
> The commit that is pointed out in the patch
> 51f7e52dc943 ("ovl: share inode for hard link")
> seems to have broken that assumption.
>
> Given that the intention was to use the creator's creds _with the
> caller's fs{g,u}id_ wouldn't it make more sense to simply ensure that
> the caller's fs{g,u}id are always used instead of using the full
> creator's creds just for the link operation? So something like this
> (untested):
>
> diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> index 6b03457f72bb..4a3ee16a6d70 100644
> --- a/fs/overlayfs/dir.c
> +++ b/fs/overlayfs/dir.c
> @@ -575,6 +575,9 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
> const struct cred *old_cred;
> struct cred *override_cred;
> struct dentry *parent = dentry->d_parent;
> + /* Retrieve caller's fs{g,u}id before we override creds below. */
> + kuid_t caller_fsuid = current_fsuid();
> + kgid_t caller_fsgid = current_fsgid();
>
> err = ovl_copy_up(parent);
> if (err)
> @@ -595,8 +598,8 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
> err = -ENOMEM;
> override_cred = prepare_creds();
> if (override_cred) {
> - override_cred->fsuid = inode->i_uid;
> - override_cred->fsgid = inode->i_gid;
> + override_cred->fsuid = caller_fsuid;
> + override_cred->fsgid = caller_fsgid;
> if (!attr->hardlink) {
> err = security_dentry_create_files_as(dentry,
> attr->mode, &dentry->d_name, old_cred,
On Wed, Aug 17, 2022 at 6:29 PM Christian Brauner <[email protected]> wrote:
>
> On Wed, Aug 17, 2022 at 12:27:27PM +0200, Christian Brauner wrote:
> > On Wed, Aug 17, 2022 at 12:55:22PM +0300, Amir Goldstein wrote:
> > > On Wed, Aug 17, 2022 at 12:53 PM Amir Goldstein <[email protected]> wrote:
> > > >
> > > > On Wed, Aug 17, 2022 at 12:11 PM 天赐张 <[email protected]> wrote:
> > > > >
> > > > > On Wed, Aug 17, 2022 at 3:36 PM Amir Goldstein <[email protected]> wrote:
> > > > > >
> > > > > > On Wed, Aug 17, 2022 at 6:49 AM Zhang Tianci
> > > > > > <[email protected]> wrote:
> > > > > > >
> > > > > > > ovl_link() did not create a new inode after commit
> > > > > > > 51f7e52dc943 ("ovl: share inode for hard link"), so
> > > > > > > in ovl_create_or_link() we should not override cred's
> > > > > > > fsuid and fsgid when called by ovl_link().
> > > > > > >
> > > > > > > Signed-off-by: Zhang Tianci <[email protected]>
> > > > > > > Signed-off-by: Jiachen Zhang <[email protected]>
> > > > > > > ---
> > > > > > > fs/overlayfs/dir.c | 4 ++--
> > > > > > > 1 file changed, 2 insertions(+), 2 deletions(-)
> > > > > > >
> > > > > > > diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> > > > > > > index 6b03457f72bb..568d338032db 100644
> > > > > > > --- a/fs/overlayfs/dir.c
> > > > > > > +++ b/fs/overlayfs/dir.c
> > > > > > > @@ -595,9 +595,9 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
> > > > > > > err = -ENOMEM;
> > > > > > > override_cred = prepare_creds();
> > > > > > > if (override_cred) {
> > > > > > > - override_cred->fsuid = inode->i_uid;
> > > > > > > - override_cred->fsgid = inode->i_gid;
> > > > > > > if (!attr->hardlink) {
> > > > > > > + override_cred->fsuid = inode->i_uid;
> > > > > > > + override_cred->fsgid = inode->i_gid;
> > > > > > > err = security_dentry_create_files_as(dentry,
> > > > > > > attr->mode, &dentry->d_name, old_cred,
> > > > > > > override_cred);
> > > > > > > --
> > > > > >
> > > > > > This change looks incorrect.
> > > > > > Unless I am missing something, fsuid/fsgid still need to
> > > > > > be overridden for calling link() on underlying fs.
> > > > > > What made you do this change?
> > > > > >
> > > > > > Thanks,
> > > > > > Amir.
> > > > >
> > > > > Hi Amir,
> > > > >
> > > > > I ran into an error when I tested overlay on fuse:
> > > > > $ mkdir /lower /fuse /merge
> > > > > $ mount -t fuse /fuse
> > > > > $ mkdir /fuse/upper /fuse/work
> > > > > $ mount -t overlay /merge -o lowerdir=/lower,upperdir=/fuse/upper,workdir=work
> > > > > $ touch /merge/file
> > > > > $ chown bin.bin /merge/file // the file's caller becomes "bin"
> > > > > $ ln /merge/file /merge/lnkfile
> > > > >
> > > > > Then I got an error(EACCES) because fuse daemon checks the link()'s
> > > > > caller is "bin", it denied this request.
> > > > > I browsed the changing history of ovl_link(). There are two key commits:
> > > > > The first is commit bb0d2b8ad296 ("ovl: fix sgid on directory") which
> > > > > overrides the cred's fsuid/fsgid using the new inode. The new inode's
> > > > > owner is initialized by inode_init_owner(), and inode->fsuid is
> > > > > assigned to the current user. So the override fsuid becomes the
> > > > > current user. We know link() is actually modifying the directory, so
> > > > > the caller must have the MAY_WRITE permission on the directory. The
> > > > > current caller may should have this permission. I think this is
> > > > > acceptable to use the caller's fsuid(But I still feel a little
> > > > > conflicted with the overlay's design).
> > > > > The second is commit 51f7e52dc943 ("ovl: share inode for hard link")
> > > > > which removed the inode creation in ovl_link(). This commit move
> > > > > inode_init_owner() into ovl_create_object(), so the ovl_link() just
> > > > > give the old inode to ovl_create_or_link(). Then the override fsuid
> > > > > becomes the old inode's fsuid, neither the caller nor the overlay's
> > > > > creator! So I think this is incorrect.
> > > > > I think the link() should be like unlink(), overlay fs should just use
> > > > > the creator cred to do underlying fs's operations.
> > > > >
> > > >
> > > > I see. The reproducer and explanation belong in the commit message.
> > > >
> > > > Your argument makes sense to me, but CC Christian to make
> > > > sure I am not missing anything related to ACLs and what not.
> > >
> > > Once again with correct email address...
> >
> > So we have:
> >
> > ovl_create_object()
> > -> ovl_override_creds(ovl_sb)
> > -> ovl_new_inode()
> > -> inode_init_owner()
> > {
> > inode->i_uid = current_fsuid();
> > inode->i_gid = current_fsgid();
In inode_init_owner(), the inode->i_gid may inherit from parent dir.
And this is the main purpose of the commit bb0d2b8ad296 ("ovl: fix
sgid on directory").
> > }
> > -> ovl_create_or_link(inode, ...)
> > -> prepare_creds() // Copy of caller's creds
>
> s/caller's/creator's/
>
> > {
> > override_creds->fsuid = inode->i_uid;
> > override_creds->fsgid = inode->i_gid;
> > }
> > -> revert_creds()
> >
> > which afaict means that the mounter's credentials are used apart from
> > the fs{g,u}id which is taken from inode->i_{g,u}id which should
> > correspond to current_fs{g,u}id().
> >
> > The commit that is pointed out in the patch
> > 51f7e52dc943 ("ovl: share inode for hard link")
> > seems to have broken that assumption.
> >
> > Given that the intention was to use the creator's creds _with the
> > caller's fs{g,u}id_ wouldn't it make more sense to simply ensure that
> > the caller's fs{g,u}id are always used instead of using the full
> > creator's creds just for the link operation? So something like this
> > (untested):
> >
> > diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> > index 6b03457f72bb..4a3ee16a6d70 100644
> > --- a/fs/overlayfs/dir.c
> > +++ b/fs/overlayfs/dir.c
> > @@ -575,6 +575,9 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
> > const struct cred *old_cred;
> > struct cred *override_cred;
> > struct dentry *parent = dentry->d_parent;
> > + /* Retrieve caller's fs{g,u}id before we override creds below. */
> > + kuid_t caller_fsuid = current_fsuid();
> > + kgid_t caller_fsgid = current_fsgid();
> >
> > err = ovl_copy_up(parent);
> > if (err)
> > @@ -595,8 +598,8 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
> > err = -ENOMEM;
> > override_cred = prepare_creds();
> > if (override_cred) {
> > - override_cred->fsuid = inode->i_uid;
> > - override_cred->fsgid = inode->i_gid;
> > + override_cred->fsuid = caller_fsuid;
> > + override_cred->fsgid = caller_fsgid;
So the override_cred->fsgid should be inode->i_gid if the inode is a new inode.
> > if (!attr->hardlink) {
> > err = security_dentry_create_files_as(dentry,
> > attr->mode, &dentry->d_name, old_cred,
So your meaning should be like this:
diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
index 6b03457f72bb..9aead6ddc071 100644
--- a/fs/overlayfs/dir.c
+++ b/fs/overlayfs/dir.c
@@ -575,6 +575,8 @@ static int ovl_create_or_link(struct dentry
*dentry, struct inode *inode,
const struct cred *old_cred;
struct cred *override_cred;
struct dentry *parent = dentry->d_parent;
+ kuid_t caller_fsuid = current_fsuid();
+ kgid_t caller_fsgid = current_fsgid();
err = ovl_copy_up(parent);
if (err)
@@ -595,9 +597,9 @@ static int ovl_create_or_link(struct dentry
*dentry, struct inode *inode,
err = -ENOMEM;
override_cred = prepare_creds();
if (override_cred) {
- override_cred->fsuid = inode->i_uid;
- override_cred->fsgid = inode->i_gid;
if (!attr->hardlink) {
+ override_cred->fsuid = inode->i_uid;
+ override_cred->fsgid = inode->i_gid;
err = security_dentry_create_files_as(dentry,
attr->mode, &dentry->d_name, old_cred,
override_cred);
@@ -605,6 +607,9 @@ static int ovl_create_or_link(struct dentry
*dentry, struct inode *inode,
put_cred(override_cred);
goto out_revert_creds;
}
+ } else {
+ override_cred->fsuid = caller_fsuid;
+ override_cred->fsgid = caller_fsgid;
}
put_cred(override_creds(override_cred));
put_cred(override_cred);
As I said before, I think this is acceptable to use the caller's
fsuid. But I still feel a little conflicted with the overlay's design.
Because I am not sure if there should be some difference between
link() and unlink() after commit 51f7e52dc943 ("ovl: share inode for
hard link") remove the creation in ovl_link().