On 3/28/2024 12:08 PM, Christian Brauner wrote:
On Thu, Mar 28, 2024 at 12:53:40PM +0200, Roberto Sassu wrote:
On 3/26/2024 12:40 PM, Christian Brauner wrote:
we can change the parameter of security_path_post_mknod() from
dentry to inode?
If all current callers only operate on the inode then it seems the best
to only pass the inode. If there's some reason someone later needs a
dentry the hook can always be changed.
Ok, so the crash is likely caused by:
void security_path_post_mknod(struct mnt_idmap *idmap, struct dentry
*dentry)
{
if (unlikely(IS_PRIVATE(d_backing_inode(dentry))))
I guess we can also simply check if there is an inode attached to the
dentry, to minimize the changes. I can do both.
More technical question, do I need to do extra checks on the dentry before
calling security_path_post_mknod()?
Why do you need the dentry? The two users I see are ima in [1] and evm in [2].
Both of them don't care about the dentry. They only care about the
inode. So why is that hook not just:
Sure, I can definitely do that. Seems an easier fix to do an extra check
in security_path_post_mknod(), rather than changing the parameter
everywhere.
Next time, when we introduce new LSM hooks we can try to introduce more
specific parameters.
Also, consider that the pre hook security_path_mknod() has the dentry as
parameter. For symmetry, we could keep it in the post hook.
What I was also asking is if I can still call d_backing_inode() on the
dentry without extra checks, and avoiding the IS_PRIVATE() check if the
former returns NULL.
diff --git a/security/security.c b/security/security.c
index 7e118858b545..025689a7e912 100644
--- a/security/security.c
+++ b/security/security.c
@@ -1799,11 +1799,11 @@ EXPORT_SYMBOL(security_path_mknod);
*
* Update inode security field after a file has been created.
*/
-void security_path_post_mknod(struct mnt_idmap *idmap, struct dentry *dentry)
+void security_inode_post_mknod(struct mnt_idmap *idmap, struct inode *inode)
{
- if (unlikely(IS_PRIVATE(d_backing_inode(dentry))))
+ if (unlikely(IS_PRIVATE(inode)))
return;
- call_void_hook(path_post_mknod, idmap, dentry);
+ call_void_hook(path_post_mknod, idmap, inode);
}
/**
And one another thing I'd like to point out is that the security hook is
called "security_path_post_mknod()" while the evm and ima hooks are
called evm_post_path_mknod() and ima_post_path_mknod() respectively. In
other words:
git grep _path_post_mknod() doesn't show the implementers of that hook
which is rather unfortunate. It would be better if the pattern were:
<specific LSM>_$some_$ordered_$words()
I know, yes. Didn't want to change just yet since people familiar with
the IMA code know the current function name. I don't see any problem to
rename the functions.
Thanks
Roberto
[1]:
static void evm_post_path_mknod(struct mnt_idmap *idmap, struct dentry *dentry)
{
struct inode *inode = d_backing_inode(dentry);
struct evm_iint_cache *iint = evm_iint_inode(inode);
if (!S_ISREG(inode->i_mode))
return;
if (iint)
iint->flags |= EVM_NEW_FILE;
}
[2]:
static void ima_post_path_mknod(struct mnt_idmap *idmap, struct dentry *dentry)
{
struct ima_iint_cache *iint;
struct inode *inode = dentry->d_inode;
int must_appraise;
if (!ima_policy_flag || !S_ISREG(inode->i_mode))
return;
must_appraise = ima_must_appraise(idmap, inode, MAY_ACCESS,
FILE_CHECK);
if (!must_appraise)
return;
/* Nothing to do if we can't allocate memory */
iint = ima_inode_get(inode);
if (!iint)
return;
/* needed for re-opening empty files */
iint->flags |= IMA_NEW_FILE;
}
Thanks
Roberto
For bigger changes it's also worthwhile if the object that's passed down
into the hook-based LSM layer is as specific as possible. If someone
does a change that affects lifetime rules of mounts then any hook that
takes a struct path argument that's unused means going through each LSM
that implements the hook only to find out it's not actually used.
Similar for dentry vs inode imho.
