On Wed, 2023-04-19 at 12:25 -0700, Mengchi Cheng wrote: > > I got some errors during xattr removal, so not sure if my patch was > > working properly or not (it happened also without it, didn't > > investigate more). > > > > However, I saw another discussion related to transmute: > > > > https://lore.kernel.org/linux-security-module/20230419002338.566487-1-mengcc@xxxxxxxxxx/ > > > > I add the people in CC. > > > > The steps described were so easy to understand and executed, I tried > > without and with overlayfs. > > > > Without: > > > > # echo "_ system rwxatl" > /sys/fs/smackfs/load2 > > # mkdir /data > > # chsmack -a "system" /data > > # chsmack -t /data > > # mkdir -p /data/dir1/dir2 > > # chsmack /data/dir1 > > /data/dir1 access="system" transmute="TRUE" > > # chsmack /data/dir1/dir2 > > /data/dir1/dir2 access="system" transmute="TRUE" > > > > It seems to work, right? > > > > With overlay fs it didn't work, same result as the one Mengchi > > reported. Since Mengchi's solution was to set SMK_INODE_CHANGED, and I > > want to get rid of it, I thought to investigate more. > > > > Looking at smack_dentry_create_files_as(), I see that the label of the > > process is overwritten with the label of the transmuting directory. > > > > That causes smack_inode_init_security() to lookup the transmuting rule > > on the overridden credential, and not on the original one. > > > > In the example above, it means that, when overlayfs is creating the new > > inode, the label of the process is system, not _. So no transmute > > permission, and also the xattr will not be added, as observed by > > Mengchi. > > > > Hopefully I undertood the code, so in this particular case we would not > > need to override the label of the process in smack_dentry_create_files_ > > as(). > > > > If you see smack_inode_init_security(): > > > > struct smack_known *skp = smk_of_current(); > > struct smack_known *isp = smk_of_inode(inode); > > struct smack_known *dsp = smk_of_inode(dir); > > > > [...] > > > > if (may > 0 && ((may & MAY_TRANSMUTE) != 0) && > > smk_inode_transmutable(dir)) { > > isp = dsp; > > [...] > > > > xattr->value = kstrdup(isp->smk_known, GFP_NOFS); > > > > This code is telling, if there is a transmute rule, and the directory > > is transmuting, set the label of the new inode to the label of the > > directory. That should be already the result that we wanted to obtain. > > > > The current code should have been doing it by overriding the label of > > the process in smack_dentry_create_files_as() with the label of the > > parent directory, and letting the inode being created with the > > overridden label of the process. The transmute xattr is not set due to > > the problem described above. > > > > So, as a quick test, I kept this patch with the change to xattr2->name, > > and skipped the label override in smack_dentry_create_files_as(). It > > worked, I get the same result as without overlayfs. Wondering if the > > process label override is necessary in other cases. > > If I understand correctly, removing the if block below is what you suggested. Yes, more or less is what I did. > diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c > index cfcbb748da25..a867288e9de9 100644 > --- a/security/smack/smack_lsm.c > +++ b/security/smack/smack_lsm.c > @@ -4769,8 +4769,8 @@ static int smack_dentry_create_files_as(struct dentry *dentry, int mode, > * providing access is transmuting use the containing > * directory label instead of the process label. > */ > - if (may > 0 && (may & MAY_TRANSMUTE)) > - ntsp->smk_task = isp->smk_inode; > +// if (may > 0 && (may & MAY_TRANSMUTE)) > +// ntsp->smk_task = isp->smk_inode; > } > return 0; > } > > This way will have issue in the following situation on the vanila kernel. > data in the lowerdir has "_" label before overlay and dir1 is already > created in the lowerdir. > # chsmack /data > /data access="_" > # chsmack /data/dir1 > /data/dir1 access="system" transmute="TRUE" > Apply overlay on data directory and set the smack rule in the same way. > data has the same smack label. > # chsmack /data > /data access="system" transmute="TRUE" I'm using an older kernel, but I get _ instead of system. > After that, remove dir1 and mkdir dir1 again. dir1 did not get the correct > label. > # rm -r /data/dir1 > # mkdir -p /data/dir1 > # chsmack /data/dir1 > /data/dir1 access="_" Unfortunately, it cannot work: Thread 3 hit Breakpoint 1, smack_inode_init_security (...) at security/smack/smack_lsm.c:959 959 { (gdb) p dir->i_ino $12 = 9169116 (gdb) p dsp $13 = (struct smack_known *) 0xffffffff831fc0a0 <smack_known_floor> ls -i /home/root/data_work/ 9169116 work So, transmuting is decided on the working directory. If I do: # chsmack -a system -t /home/root/data_work/work/ # mkdir /data/dir1 # chsmack /data/dir1 /data/dir1 access="system" transmute="TRUE" I obtain the expected result. However, this problem is due to how overlayfs works: static int ovl_create_over_whiteout(struct dentry *dentry, struct inode *inode, struct ovl_cattr *cattr) { [...] newdentry = ovl_create_temp(ofs, workdir, cattr); err = PTR_ERR(newdentry); if (IS_ERR(newdentry)) goto out_dput; The good news seems to be that, once you set the label to the correct directory, transmuting works with the changes I proposed. Roberto > Since I am not very familiar your change. Could you help check with your > patch will this issue also happen? > > > Best, > Mengchi > > > > > Roberto
