On Mon, Oct 17, 2022 at 1:06 PM Christian Brauner <brauner@xxxxxxxxxx> wrote:
>
> Currently setgid stripping in file_remove_privs()'s should_remove_suid()
> helper is inconsistent with other parts of the vfs. Specifically, it only
> raises ATTR_KILL_SGID if the inode is S_ISGID and S_IXGRP but not if the
> inode isn't in the caller's groups and the caller isn't privileged over the
> inode although we require this already in setattr_prepare() and
> setattr_copy() and so all filesystem implement this requirement implicitly
> because they have to use setattr_{prepare,copy}() anyway.
>
> But the inconsistency shows up in setgid stripping bugs for overlayfs in
> xfstests (e.g., generic/673, generic/683, generic/685, generic/686,
> generic/687). For example, we test whether suid and setgid stripping works
> correctly when performing various write-like operations as an unprivileged
> user (fallocate, reflink, write, etc.):
>
> echo "Test 1 - qa_user, non-exec file $verb"
> setup_testfile
> chmod a+rws $junk_file
> commit_and_check "$qa_user" "$verb" 64k 64k
>
> The test basically creates a file with 6666 permissions. While the file has
> the S_ISUID and S_ISGID bits set it does not have the S_IXGRP set. On a
> regular filesystem like xfs what will happen is:
>
> sys_fallocate()
> -> vfs_fallocate()
> -> xfs_file_fallocate()
> -> file_modified()
> -> __file_remove_privs()
> -> dentry_needs_remove_privs()
> -> should_remove_suid()
> -> __remove_privs()
> newattrs.ia_valid = ATTR_FORCE | kill;
> -> notify_change()
> -> setattr_copy()
>
> In should_remove_suid() we can see that ATTR_KILL_SUID is raised
> unconditionally because the file in the test has S_ISUID set.
>
> But we also see that ATTR_KILL_SGID won't be set because while the file
> is S_ISGID it is not S_IXGRP (see above) which is a condition for
> ATTR_KILL_SGID being raised.
>
> So by the time we call notify_change() we have attr->ia_valid set to
> ATTR_KILL_SUID | ATTR_FORCE. Now notify_change() sees that
> ATTR_KILL_SUID is set and does:
>
> ia_valid = attr->ia_valid |= ATTR_MODE
> attr->ia_mode = (inode->i_mode & ~S_ISUID);
>
> which means that when we call setattr_copy() later we will definitely
> update inode->i_mode. Note that attr->ia_mode still contains S_ISGID.
>
> Now we call into the filesystem's ->setattr() inode operation which will
> end up calling setattr_copy(). Since ATTR_MODE is set we will hit:
>
> if (ia_valid & ATTR_MODE) {
> umode_t mode = attr->ia_mode;
> vfsgid_t vfsgid = i_gid_into_vfsgid(mnt_userns, inode);
> if (!vfsgid_in_group_p(vfsgid) &&
> !capable_wrt_inode_uidgid(mnt_userns, inode, CAP_FSETID))
> mode &= ~S_ISGID;
> inode->i_mode = mode;
> }
>
> and since the caller in the test is neither capable nor in the group of the
> inode the S_ISGID bit is stripped.
>
> But assume the file isn't suid then ATTR_KILL_SUID won't be raised which
> has the consequence that neither the setgid nor the suid bits are stripped
> even though it should be stripped because the inode isn't in the caller's
> groups and the caller isn't privileged over the inode.
>
> If overlayfs is in the mix things become a bit more complicated and the bug
> shows up more clearly. When e.g., ovl_setattr() is hit from
> ovl_fallocate()'s call to file_remove_privs() then ATTR_KILL_SUID and
> ATTR_KILL_SGID might be raised but because the check in notify_change() is
> questioning the ATTR_KILL_SGID flag again by requiring S_IXGRP for it to be
> stripped the S_ISGID bit isn't removed even though it should be stripped:
>
> sys_fallocate()
> -> vfs_fallocate()
> -> ovl_fallocate()
> -> file_remove_privs()
> -> dentry_needs_remove_privs()
> -> should_remove_suid()
> -> __remove_privs()
> newattrs.ia_valid = ATTR_FORCE | kill;
> -> notify_change()
> -> ovl_setattr()
> // TAKE ON MOUNTER'S CREDS
> -> ovl_do_notify_change()
> -> notify_change()
> // GIVE UP MOUNTER'S CREDS
> // TAKE ON MOUNTER'S CREDS
> -> vfs_fallocate()
> -> xfs_file_fallocate()
> -> file_modified()
> -> __file_remove_privs()
> -> dentry_needs_remove_privs()
> -> should_remove_suid()
> -> __remove_privs()
> newattrs.ia_valid = attr_force | kill;
> -> notify_change()
>
> The fix for all of this is to make file_remove_privs()'s
> should_remove_suid() helper to perform the same checks as we already
> require in setattr_prepare() and setattr_copy() and have notify_change()
> not pointlessly requiring S_IXGRP again. It doesn't make any sense in the
> first place because the caller must calculate the flags via
> should_remove_suid() anyway which would raise ATTR_KILL_SGID.
>
> While we're at it we move should_remove_suid() from inode.c to attr.c
> where it belongs with the rest of the iattr helpers. Especially since it
> returns ATTR_KILL_S{G,U}ID flags. We also rename it to
> setattr_should_drop_suidgid() to better reflect that it indicates both
> setuid and setgid bit removal and also that it returns attr flags.
>
> Running xfstests with this doesn't report any regressions. We should really
> try and use consistent checks.
>
> Signed-off-by: Christian Brauner (Microsoft) <brauner@xxxxxxxxxx>
> Reviewed-by: Amir Goldstein <amir73il@xxxxxxxxx>
> Signed-off-by: Christian Brauner (Microsoft) <brauner@xxxxxxxxxx>
Doubly signed...
> ---
>
> Notes:
> /* v2 */
> Amir Goldstein <amir73il@xxxxxxxxx>:
> - mention xfstests that failed prior to that
>
> Christian Brauner <brauner@xxxxxxxxxx>:
> - Use should_remove_sgid() in chown_common() just like we do in do_truncate().
>
> /* v3 */
> Christian Brauner <brauner@xxxxxxxxxx>:
> - Move should_remove_suid() from inode.c to attr.c where it belongs with the
> rest of the iattr helpers. Especially since it returns ATTR_KILL_S{G,U}ID
> flags. We also rename it to setattr_should_drop_suidgid() to better reflect
> that it indicates both setuid and setgid bit removal and also that it returns
> attr flags.
> - Since setattr_should_drop_suidgid() only needs the inode pass an inode,
> not a dentry.
Moving between C files and changing code in the same commit is bad
for review.
Please move between C files in a prep patch.
Thanks,
Amir.
