On 3/1/24 10:52, Amir Goldstein wrote:
On Thu, Feb 29, 2024 at 7:53 PM Mete Durlu <meted@xxxxxxxxxxxxx> wrote:
In current state do_fanotify_mark() does path permission and security
checking before doing the event configuration checks. In the case
where user configures mount and sb marks with kernel internal pseudo
fs, security_path_notify() yields an EACESS and causes an earlier
exit. Instead, this particular case should have been handled by
fanotify_events_supported() and exited with an EINVAL.
What makes you say that this is the expected outcome?
I'd say that the expected outcome is undefined and we have no reason
to commit to either EACCESS or EINVAL outcome.
TLDR; I saw the failing ltp test(fanotify14) started investigating, read
the comments on the related commits and noticed that the fanotify
documentation does not mention any EACESS as an errno. For these reasons
I made an attempt to provide a fix. The placement of the checks aim
minimal change, I just tried not to alter the logic more than needed.
Thanks for the feedback, will apply suggestions.
The main reason is the following commit;
* linux: 69562eb0bd3e ("fanotify: disallow mount/sb marks on kernel
internal pseudo fs")
fanotify_user: fanotify_events_supported()
/*
* mount and sb marks are not allowed on kernel internal pseudo
* fs, like pipe_mnt, because that would subscribe to events on
* all the anonynous pipes in the system.
*/
if (mark_type != FAN_MARK_INODE &&
path->mnt->mnt_sb->s_flags & SB_NOUSER)
return -EINVAL;
It looks to me as, when configuring fanotify_mark with pipes and
FAN_MARK_MOUNT or FAN_MARK_FILESYSTEM, the path above should be taken
instead of an early return with EACCES.
Also the following commit on linux test project(ltp) expects EINVAL as
expected errno.
* ltp: 8e897008c ("fanotify14: Test disallow sb/mount mark on anonymous
pipe")
To be honest, the test added on above commit is the main reason why I
started investigating this.
I don't really mind the change of outcome, but to me it seems
nicer that those tests are inside fanotify_find_path(), so I will
want to get a good reason for moving them out.
I agree, when those tests are inside fanotify_find_path() it looks much
cleaner but then the check for psuedo fs in fanotify_events_supported()
is not made. And I believe when configuring fanotify an EINVAL makes
more sense than EACCES, it just seems more informative(at least to me).
Would it maybe make sense to put them in a separate helper function,
sth like:
static int fanotify_path_security(struct path *path,
__u64 mask,
unsigned int obj_type) {
int ret;
ret = path_permission(path, MAY_READ);
if (ret)
return ret;
ret = security_path_notify(path, mask, obj_type);
return ret;
}
...
ret = fanotify_path_security(...)
if (ret)
goto path_put_and_out;
diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
index fbdc63cc10d9..14121ad0e10d 100644
--- a/fs/notify/fanotify/fanotify_user.c
+++ b/fs/notify/fanotify/fanotify_user.c
@@ -1015,7 +1015,7 @@ static int fanotify_find_path(int dfd, const char __user *filename,
fdput(f);
goto out;
}
-
+ ret = 0;
Better convert all gotos in this helper to return.
There is nothing in the out label.
Good point, will do!
*path = f.file->f_path;
path_get(path);
fdput(f);
@@ -1028,21 +1028,7 @@ static int fanotify_find_path(int dfd, const char __user *filename,
lookup_flags |= LOOKUP_DIRECTORY;
ret = user_path_at(dfd, filename, lookup_flags, path);
- if (ret)
- goto out;
}
-
- /* you can only watch an inode if you have read permissions on it */
- ret = path_permission(path, MAY_READ);
- if (ret) {
- path_put(path);
- goto out;
- }
-
- ret = security_path_notify(path, mask, obj_type);
- if (ret)
- path_put(path);
-
out:
return ret;
}
@@ -1894,6 +1880,14 @@ static int do_fanotify_mark(int fanotify_fd, unsigned int flags, __u64 mask,
if (ret)
goto path_put_and_out;
}
+ /* you can only watch an inode if you have read permissions on it */
+ ret = path_permission(&path, MAY_READ);
+ if (ret)
+ goto path_put_and_out;
+
+ ret = security_path_notify(&path, mask, obj_type);
+ if (ret)
+ goto path_put_and_out;
if (fid_mode) {
ret = fanotify_test_fsid(path.dentry, flags, &__fsid);
If we do accept your argument that security_path_notify() should be
after fanotify_events_supported(). Why not also after fanotify_test_fsid()
and fanotify_test_fid()?
I tried to place the checks as close as possible to their original
position, that is why I placed them right after
fanotify_events_supported(). I wanted to keep the ordering as close as
possible to original to not break any other check. I am open to
suggestions regarding this.
Thank you
-Mete Durlu
