Re: [PATCH 4/5] evm: Use the real inode's metadata to calculate metadata hash

[Stefan Berger]

On 1/31/24 10:54, Amir Goldstein wrote:
> On Wed, Jan 31, 2024 at 4:40 PM Stefan Berger <stefanb@xxxxxxxxxxxxx> wrote:
> > On 1/31/24 08:16, Amir Goldstein wrote:
> > > On Wed, Jan 31, 2024 at 4:11 AM Stefan Berger <stefanb@xxxxxxxxxxxxx> wrote:
> > > > On 1/30/24 16:46, Stefan Berger wrote:
> > > > > Changes to the file attribute (mode bits, uid, gid) on the lower layer
> > > > > are not take into account when d_backing_inode() is used when a file is
> > > > > accessed on the overlay layer and this file has not yet been copied up.
> > > > > This is because d_backing_inode() does not return the real inode of the
> > > > > lower layer but instead returns the backing inode which holds old file
> > > > > attributes. When the old file attributes are used for calculating the
> > > > > metadata hash then the expected hash is calculated and the file then
> > > > > mistakenly passes signature verification. Therefore, use d_real_inode()
> > > > > which returns the inode of the lower layer for as long as the file has
> > > > > not been copied up and returns the upper layer's inode otherwise.
> > > > >
> > > > > Signed-off-by: Stefan Berger <stefanb@xxxxxxxxxxxxx>
> > > > > ---
> > > > >     security/integrity/evm/evm_crypto.c | 2 +-
> > > > >     1 file changed, 1 insertion(+), 1 deletion(-)
> > > > >
> > > > > diff --git a/security/integrity/evm/evm_crypto.c b/security/integrity/evm/evm_crypto.c
> > > > > index b1ffd4cc0b44..2e48fe54e899 100644
> > > > > --- a/security/integrity/evm/evm_crypto.c
> > > > > +++ b/security/integrity/evm/evm_crypto.c
> > > > > @@ -223,7 +223,7 @@ static int evm_calc_hmac_or_hash(struct dentry *dentry,
> > > > >                                  size_t req_xattr_value_len,
> > > > >                                  uint8_t type, struct evm_digest *data)
> > > > >     {
> > > > > -     struct inode *inode = d_backing_inode(dentry);
> > > > > +     struct inode *inode = d_real_inode(dentry);
> > > > >         struct xattr_list *xattr;
> > > > >         struct shash_desc *desc;
> > > > >         size_t xattr_size = 0;
> > > >
> > > > We need this patch when NOT activating CONFIG_OVERLAY_FS_METACOPY but
> > > > when setting CONFIG_OVERLAY_FS_METACOPY=y it has to be reverted...  I am
> > > > not sure what the solution is.
> > >
> > > I think d_real_inode() does not work correctly for all its current users for
> > > a metacopy file.
> > >
> > > I think the solution is to change d_real_inode() to return the data inode
> > > and add another helper to get the metadata inode if needed.
> > > I will post some patches for it.
> >
> > I thought that we may have to go through vfs_getattr() but even better
> > if we don't because we don't have the file *file anywhere 'near'.
> >
> > > However, I must say that I do not know if evm_calc_hmac_or_hash()
> > > needs the lower data inode, the upper metadata inode or both.
> >
> > What it needs are data structures with mode bits, uid, and gid that stat
> > in userspace would show.
>
> With or without metacopy enabled, an overlay inode st_uid st_gid st_mode
> are always taken from the upper most inode which is what d_real_inode()
> currently returns, so I do not understand what the problem is.

I have testcases that work fine with this series when 
CONFIG_OVERLAY_FS_METACOPY is not active. Once I activate this then a 
test case that changes a file's gid on the overlay layer from 0 to '12' 
while causing a copy-up allows a file to execute even thugh it should 
not execute. The reason is because d_real_inode(dentry)->i_guid shows 
the '0' while d_backing_dentry(dentry)->i_guid shows '12'. User space 
stat also shows '12' as expected.


Just saw your other email, will try that now ...