on 2022/4/13 16:59, Christian Brauner wrote:
> On Tue, Apr 12, 2022 at 07:33:45PM +0800, Yang Xu wrote:
>> The current_umask() is stripped from the mode directly in the vfs if the
>> filesystem either doesn't support acls or the filesystem has been
>> mounted without posic acl support.
>>
>> If the filesystem does support acls then current_umask() stripping is
>> deferred to posix_acl_create(). So when the filesystem calls
>> posix_acl_create() and there are no acls set or not supported then
>> current_umask() will be stripped.
>>
>> Here we only use umask(S_IXGRP) to check whether inode strip
>> S_ISGID works correctly.
>>
>> Signed-off-by: Yang Xu<xuyang2018.jy@xxxxxxxxxxx>
>> ---
>> src/idmapped-mounts/idmapped-mounts.c | 505 +++++++++++++++++++++++++-
>> tests/generic/680 | 26 ++
>> tests/generic/680.out | 2 +
>> 3 files changed, 532 insertions(+), 1 deletion(-)
>> create mode 100755 tests/generic/680
>> create mode 100644 tests/generic/680.out
>>
>> diff --git a/src/idmapped-mounts/idmapped-mounts.c b/src/idmapped-mounts/idmapped-mounts.c
>> index 02f91558..e6c14586 100644
>> --- a/src/idmapped-mounts/idmapped-mounts.c
>> +++ b/src/idmapped-mounts/idmapped-mounts.c
>> @@ -14146,6 +14146,494 @@ out:
>> return fret;
>> }
>>
>> +/* The following tests are concerned with setgid inheritance. These can be
>> + * filesystem type specific. For xfs, if a new file or directory or node is
>> + * created within a setgid directory and irix_sgid_inhiert is set then inheritthe
>> + * setgid bit if the caller is in the group of the directory.
>> + *
>> + * The current_umask() is stripped from the mode directly in the vfs if the
>> + * filesystem either doesn't support acls or the filesystem has been
>> + * mounted without posic acl support.
>> + *
>> + * If the filesystem does support acls then current_umask() stripping is
>> + * deferred to posix_acl_create(). So when the filesystem calls
>> + * posix_acl_create() and there are no acls set or not supported then
>> + * current_umask() will be stripped.
>> + *
>> + * Use umask(S_IXGRP) to check whether inode strip S_ISGID works correctly.
>> + */
>> +static int setgid_create_umask(void)
>> +{
>> + int fret = -1;
>> + int file1_fd = -EBADF;
>> + int tmpfile_fd = -EBADF;
>> + pid_t pid;
>> + bool supported = false;
>> + char path[PATH_MAX];
>> + mode_t mode;
>> +
>> + if (!caps_supported())
>> + return 0;
>> +
>> + if (fchmod(t_dir1_fd, S_IRUSR |
>> + S_IWUSR |
>> + S_IRGRP |
>> + S_IWGRP |
>> + S_IROTH |
>> + S_IWOTH |
>> + S_IXUSR |
>> + S_IXGRP |
>> + S_IXOTH |
>> + S_ISGID), 0) {
>> + log_stderr("failure: fchmod");
>> + goto out;
>> + }
>> +
>> + /* Verify that the setgid bit got raised. */
>> + if (!is_setgid(t_dir1_fd, "", AT_EMPTY_PATH)) {
>> + log_stderr("failure: is_setgid");
>> + goto out;
>> + }
>> +
>> + supported = openat_tmpfile_supported(t_dir1_fd);
>> +
>> + /* Only umask with S_IXGRP because inode strip S_ISGID will check mode
>> + * whether has group execute or search permission.
>> + */
>> + umask(S_IXGRP);
>> + mode = umask(S_IXGRP);
>> + if (!(mode& S_IXGRP))
>> + die("failure: umask");
>> +
>> + pid = fork();
>> + if (pid< 0) {
>> + log_stderr("failure: fork");
>> + goto out;
>> + }
>> + if (pid == 0) {
>> + if (!switch_ids(0, 10000))
>> + die("failure: switch_ids");
>> +
>> + if (!caps_down_fsetid())
>> + die("failure: caps_down_fsetid");
>> +
>> + /* create regular file via open() */
>> + file1_fd = openat(t_dir1_fd, FILE1, O_CREAT | O_EXCL | O_CLOEXEC, S_IXGRP | S_ISGID);
>> + if (file1_fd< 0)
>> + die("failure: create");
>> +
>> + /* Neither in_group_p() nor capable_wrt_inode_uidgid() so setgid
>> + * bit needs to be stripped.
>> + */
>> + if (is_setgid(t_dir1_fd, FILE1, 0))
>> + die("failure: is_setgid");
>
> This test is wrong. Specifically, it is a false positive. I have
> explained this in more detail on v2 of this patchset.
>
> You want to test that umask(S_IXGRP) + setgid inheritance work together
> correctly. First, we need to establish what that means because from your
> patch series it at least seems to me as you're not completely clear
> about what you want to test just yet.
>
> A requested setgid bit for a non-directory object is only considered for
> stripping if S_IXGRP is simultaneously requested. In other words, both
> S_SISGID and S_IXGRP must be requested for the new file in order for
> additional checks such as CAP_FSETID to become relevant.
Yes, only keep S_IXGRP, then we can run into the next judgement for
group and cap_fsetid.
>
> We need to distinguish two cases afaict:
>
> 1. The filesystem does support ACLs and has an applicable ACL
> -------------------------------------------------------------
>
> umask(S_IXGRP) is not used by the kernel and thus S_IXGRP is not
> stripped (unless the ACL does make it so) and so when e.g.
> inode_init_owner() is called we do not expect the file to inherit the
> setgid bit.
>
> This is what your test above is handling correctly. But it is a false
> positive because what you're trying to test is the behavior of
> umask(S_IXGRP) but it is made irrelevant by ACL support of the
> underlying filesystem.
I test this situation in the next patch as below:
umask(S_IXGRP);
snprintf(t_buf, sizeof(t_buf), "setfacl -d -m u::rwx,g::rwx,o::rwx,m:rw
%s/%s", t_mountpoint, T_DIR1)
and
umask(S_IXGRP);
snprintf(t_buf, sizeof(t_buf), "setfacl -d -m u::rwx,g::rw,o::rwx,
%s/%s", t_mountpoint, T_DIR1
>
> 2. The filesystem does not support ACLs, has been mounted without
> -----------------------------------------------------------------
> support for it, or has no applicable ACL
> ----------------------------------------
>
> umask(S_IXGRP) is used by the kernel and will be stripped from the mode.
> So when inode_init_owner() is called we expect the file to inherit the
> setgid bit.
This is why my kernel patch put strip setgid code into vfs.
xfs will inherit the setgid bit but ext4 not because the new_inode
function and posix_acl_create function order(S_IXGRP mode has been
stripped before pass to inode_init_owner).
>
> This means the test for this case needs to be:
>
> file1_fd = openat(t_dir1_fd, FILE1, O_CREAT | O_EXCL | O_CLOEXEC, S_IXGRP | S_ISGID);
> if (file1_fd< 0)
> die("failure: create");
>
> /*
> * S_IXGRP will have been removed due to umask(S_IXGRP) so we expect the
> * new file to be S_ISGID.
No No No, see the kernel patch url
https://patchwork.kernel.org/project/linux-fsdevel/patch/1648461389-2225-2-git-send-email-xuyang2018.jy@xxxxxxxxxxx/
> */
> if (!is_setgid(t_dir1_fd, FILE1, 0))
> die("failure: is_setgid");
>
> And additionally you might want to also add a new helper is_ixgrp() to
> add an additional check:
>
> /*
> * S_IXGRP will have been removed due to umask(S_IXGRP) so we expect the
> * new file to not be S_IXGRP.
> */
> if (is_ixgrp(t_dir1_fd, FILE1, 0))
> die("failure: is_ixgrp");
This sound reasonable.
>
>
> ---
>
> So for the umask() tests you need to first check whether the underlying
> filesystem does support ACLs and remember that somewhere. If it does
> support ACLs, then you should first remove any ACLs set on the
> directories you're performing the tests on. Then you can run your
> umask() tests.
Yes, I can just add a remove step in the beginning.
ps: When I writing my v2 kernel patch, I found I still miss the GRPID
testcase in fstests.
Best Regrads
Yang Xu
