On Thu, 2023-04-20 at 10:48 +0200, Roberto Sassu wrote: > 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. Actually, after studying the code better, this is what security_dentry_create_files_as() is useful for. * Compute a context for a dentry as the inode is not yet available and set * that context in passed in creds so that new files are created using that * context. Context is calculated using the passed in creds and not the creds * of the caller. And Smack is doing: if (may > 0 && (may & MAY_TRANSMUTE)) { ntsp->smk_task = isp->smk_inode; The new inode will be created with the label of the current task, that was replaced with the label of the parent directory (see above) in smac k_dentry_create_files_as(). I think the reason why Mengchi was not obtaining the desired label when replacing /data/dir1 was because /data is incorrectly labeled. To ensure that /data has label 'system' and transmute is true, I added smackfstransmute=system to the mount options. However, at the beginning, it seemed that it didn't work: # mount -t overlay overlay -o lowerdir=/data,upperdir=/home/root/data,workdir=/home/root/data_work,smackfstransmute=system /data # chsmack /data /data access="system" I found that the reason for this is that smack_inode_getsecurity() retrieves metadata from the inode only for SMACK64, and the rest from xattrs (which would not work for mount options). I just made a patch to handle SMACK64TRANSMUTE too. With the patch applied, I correctly get: # mount -t overlay overlay -o lowerdir=/data,upperdir=/home/root/data,workdir=/home/root/data_work,smackfstransmute=system /data # chsmack /data /data access="system" transmute="TRUE" With the root inode correctly labeled, I get: # mount -t overlay overlay -o lowerdir=/data,upperdir=/home/root/data,workdir=/home/root/data_work,smackfstransmute=system /data # rm -Rf /data/dir1 # mkdir /data/dir1 # chsmack /data/dir1 /data/dir1 access="system" This is partially correct, transmute="TRUE" is missing. Judging from smk_task, we cannot determine in smack_inode_init_security () if transmuting was successful in smack_dentry_create_files_as(). We need an extra information. Mengchi's solution was to add the new inode as parameter to security_dentry_create_files_as(), so that SMK_INODE_CHANGED can be set in smk_flags, and SMACK64TRANSMUTE is set in smack_d_instantiate(). One concern could be that preallocating the inode maybe is overlayfs- specific? A comment also says that we might not use that one: err = ovl_create_or_link(dentry, inode, &attr, false); /* Did we end up using the preallocated inode? */ if (inode != d_inode(dentry)) We could achieve the same goal without adding a new parameter to security_dentry_create_files_as() and, instead, by adding a new field in the task_smack structure, smk_transmuted, that is set to smk_task when transmuting is successful. Then, if smk_task == smk_transmuted, smack_inode_init_security() would set SMK_INODE_CHANGED. On top of that, I would instead just provide the second xattr SMACK64TRANSMUTE, in addition to SMACK64. Will send the patches for upstream first, and then switch to providing SMACK64TRANSMUTE in smack_inode_init_security(), in a new version of the EVM patch set. Thanks Roberto > 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
