RE: [PATCH v4 08/11] evm: Allow setxattr() and setattr() for unmodified metadata

[Roberto Sassu]

> From: Christian Brauner [mailto:christian.brauner@xxxxxxxxxx]
> Sent: Thursday, March 25, 2021 1:21 PM
> On Thu, Mar 25, 2021 at 01:13:41PM +0100, Christian Brauner wrote:
> > On Thu, Mar 25, 2021 at 10:53:43AM +0000, Roberto Sassu wrote:
> > > > From: Roberto Sassu
> > > > Sent: Friday, March 5, 2021 4:19 PM
> > > > With the patch to allow xattr/attr operations if a portable signature
> > > > verification fails, cp and tar can copy all xattrs/attrs so that at the
> > > > end of the process verification succeeds.
> > > >
> > > > However, it might happen that the xattrs/attrs are already set to the
> > > > correct value (taken at signing time) and signature verification
> succeeds
> > > > before the copy has completed. For example, an archive might
> contains files
> > > > owned by root and the archive is extracted by root.
> > > >
> > > > Then, since portable signatures are immutable, all subsequent
> operations
> > > > fail (e.g. fchown()), even if the operation is legitimate (does not alter
> > > > the current value).
> > > >
> > > > This patch avoids this problem by reporting successful operation to
> user
> > > > space when that operation does not alter the current value of
> xattrs/attrs.
> > > >
> > > > Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> > > > ---
> > > >  security/integrity/evm/evm_main.c | 96
> > > > +++++++++++++++++++++++++++++++
> > > >  1 file changed, 96 insertions(+)
> > > >
> > > > diff --git a/security/integrity/evm/evm_main.c
> > > > b/security/integrity/evm/evm_main.c
> > > > index eab536fa260f..a07516dcb920 100644
> > > > --- a/security/integrity/evm/evm_main.c
> > > > +++ b/security/integrity/evm/evm_main.c
> > > > @@ -18,6 +18,7 @@
> > > >  #include <linux/integrity.h>
> > > >  #include <linux/evm.h>
> > > >  #include <linux/magic.h>
> > > > +#include <linux/posix_acl_xattr.h>
> > > >
> > > >  #include <crypto/hash.h>
> > > >  #include <crypto/hash_info.h>
> > > > @@ -328,6 +329,79 @@ static enum integrity_status
> > > > evm_verify_current_integrity(struct dentry *dentry)
> > > >  	return evm_verify_hmac(dentry, NULL, NULL, 0, NULL);
> > > >  }
> > > >
> > > > +/*
> > > > + * evm_xattr_acl_change - check if passed ACL changes the inode
> mode
> > > > + * @dentry: pointer to the affected dentry
> > > > + * @xattr_name: requested xattr
> > > > + * @xattr_value: requested xattr value
> > > > + * @xattr_value_len: requested xattr value length
> > > > + *
> > > > + * Check if passed ACL changes the inode mode, which is protected
> by
> > > > EVM.
> > > > + *
> > > > + * Returns 1 if passed ACL causes inode mode change, 0 otherwise.
> > > > + */
> > > > +static int evm_xattr_acl_change(struct dentry *dentry, const char
> > > > *xattr_name,
> > > > +				const void *xattr_value, size_t
> > > > xattr_value_len)
> > > > +{
> > > > +	umode_t mode;
> > > > +	struct posix_acl *acl = NULL, *acl_res;
> > > > +	struct inode *inode = d_backing_inode(dentry);
> > > > +	int rc;
> > > > +
> > > > +	/* UID/GID in ACL have been already converted from user to init ns
> > > > */
> > > > +	acl = posix_acl_from_xattr(&init_user_ns, xattr_value,
> > > > xattr_value_len);
> > > > +	if (!acl)
> > >
> > > Based on Mimi's review, I will change this to:
> > >
> > > if (IS_ERR_OR_NULL(acl))
> > >
> > > > +		return 1;
> > > > +
> > > > +	acl_res = acl;
> > > > +	rc = posix_acl_update_mode(&init_user_ns, inode, &mode,
> > > > &acl_res);
> > >
> > > About this part, probably it is not correct.
> > >
> > > I'm writing a test for this patch that checks if operations
> > > that don't change the file mode succeed and those that
> > > do fail.
> > >
> > > mount-idmapped --map-mount b:3001:0:1 /mnt /mnt-idmapped
> > > pushd /mnt
> > > echo "test" > test-file
> > > chown 3001 test-file
> > > chgrp 3001 test-file
> > > chmod 2644 test-file
> > > <check enabled>
> > > setfacl --set u::rw,g::r,o::r,m:r test-file (expected to succeed, caller has
> CAP_FSETID, so S_ISGID is not dropped)
> > > setfacl --set u::rw,g::r,o::r,m:rw test-file (expected to fail)
> > > pushd /mnt-idmapped
> > > capsh --drop=cap_fsetid -- -c setfacl --set u::rw,g::r,o::r test-file
> (expected to succeed, caller is in the owning group of test-file, so S_ISGID is
> not dropped)
> > >
> > > After adding a debug line in posix_acl_update_mode():
> > > printk("%s: %d(%d) %d\n", __func__,
> in_group_p(i_gid_into_mnt(mnt_userns, inode)),
> __kgid_val(i_gid_into_mnt(mnt_userns, inode)),
> capable_wrt_inode_uidgid(mnt_userns, inode, CAP_FSETID));
> > >
> > > without passing mnt_userns:
> > > [  748.262582] setfacl --set u::rw,g::r,o::r,m:r test-file
> > > [  748.268021] posix_acl_update_mode: 0(3001) 1
> > > [  748.268035] posix_acl_update_mode: 0(3001) 1
> > > [  748.268570] setfacl --set u::rw,g::r,o::r,m:rw test-file
> > > [  748.274193] posix_acl_update_mode: 0(3001) 1
> > > [  748.279198] capsh --drop=cap_fsetid -- -c setfacl --set u::rw,g::r,o::r
> test-file
> > > [  748.287894] posix_acl_update_mode: 0(3001) 0
> > >
> > > passing mnt_userns:
> > > [   81.159766] setfacl --set u::rw,g::r,o::r,m:r test-file
> > > [   81.165207] posix_acl_update_mode: 0(3001) 1
> > > [   81.165226] posix_acl_update_mode: 0(3001) 1
> > > [   81.165732] setfacl --set u::rw,g::r,o::r,m:rw test-file
> > > [   81.170978] posix_acl_update_mode: 0(3001) 1
> > > [   81.176014] capsh --drop=cap_fsetid -- -c setfacl --set u::rw,g::r,o::r
> test-file
> > > [   81.184648] posix_acl_update_mode: 1(0) 0
> > > [   81.184663] posix_acl_update_mode: 1(0) 0
> > >
> > > The difference is that, by passing mnt_userns, the caller (root) is
> > > in the owning group of the file (3001 -> 0). Without passing mnt_userns,
> > > it is not (3001 -> 3001).
> > >
> > > Christian, Andreas, could you confirm that this is correct?
> >
> > Hey Robert,
> 
> s/Robert/Roberto/
> 
> Sorry for the typo.

No worries!

Roberto

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