> 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 HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063 Managing Director: Li Peng, Li Jian, Shi Yanli
