[PATCH 0/4] Relocate execve() sanity checks

John Johansen john.johansen at canonical.com
Tue May 19 22:58:31 UTC 2020

On 5/19/20 2:17 PM, Kees Cook wrote:
> On Tue, May 19, 2020 at 01:42:28PM -0500, Eric W. Biederman wrote:
>> Kees Cook <keescook at chromium.org> writes:
>>> On Tue, May 19, 2020 at 12:41:27PM -0500, Eric W. Biederman wrote:
>>>> Kees Cook <keescook at chromium.org> writes:
>>>>> and given the LSM hooks, I think the noexec check is too late as well.
>>>>> (This is especially true for the coming O_MAYEXEC series, which will
>>>>> absolutely need those tests earlier as well[1] -- the permission checking
>>>>> is then in the correct place: during open, not exec.) I think the only
>>>>> question is about leaving the redundant checks in fs/exec.c, which I
>>>>> think are a cheap way to retain a sense of robustness.
>>>> The trouble is when someone passes through changes one of the permission
>>>> checks for whatever reason (misses that they are duplicated in another
>>>> location) and things then fail in some very unexpected way.
>>> Do you think this series should drop the "late" checks in fs/exec.c?
>>> Honestly, the largest motivation for me to move the checks earlier as
>>> I've done is so that other things besides execve() can use FMODE_EXEC
>>> during open() and receive the same sanity-checking as execve() (i.e the
>>> O_MAYEXEC series -- the details are still under discussion but this
>>> cleanup will be needed regardless).
>> I think this series should drop the "late" checks in fs/exec.c  It feels
>> less error prone, and it feels like that would transform this into
>> something Linus would be eager to merge because series becomes a cleanup
>> that reduces line count.
> Yeah, that was my initial sense too. I just started to get nervous about
> removing the long-standing exec sanity checks. ;)
>> I haven't been inside of open recently enough to remember if the
>> location you are putting the check fundamentally makes sense.  But the
>> O_MAYEXEC bits make a pretty strong case that something of the sort
>> needs to happen.
> Right. I *think* it's correct place for now, based on my understanding
> of the call graph (which is why I included it in the commit logs).
>> I took a quick look but I can not see clearly where path_noexec
>> and the regular file tests should go.
>> I do see that you have code duplication with faccessat which suggests
>> that you haven't put the checks in the right place.
> Yeah, I have notes on the similar call sites (which I concluded, perhaps
> wrongly) to ignore:
> do_faccessat()
>     user_path_at(dfd, filename, lookup_flags, &path);
>     if (acc_mode & MAY_EXEC .... path_noexec()
>     inode_permission(inode, mode | MAY_ACCESS);
> This appears to be strictly advisory, and the path_noexec() test is
> there to, perhaps, avoid surprises when doing access() then fexecve()?
> I would note, however, that that path-based LSMs appear to have no hook
> in this call graph at all. I was expecting a call like:
> 	security_file_permission(..., mode | MAY_ACCESS)
> but I couldn't find one (or anything like it), so only
> inode_permission() is being tested (which means also the existing
> execve() late tests are missed, and the newly added S_ISREG() test from
> do_dentry_open() is missed).

sadly correct, its something that we intend to fix but haven't gotten
to it

> prctl_set_mm_exe_file()
>     err = -EACCESS;
>     if (!S_ISREG(inode->i_mode) || path_noexec(&exe.file->f_path))
>         goto exit;
>     err = inode_permission(inode, MAY_EXEC);
> This is similar (no path-based LSM hooks present, only inode_permission()
> used for permission checking), but it is at least gated by CAP_SYS_ADMIN.

dito here

> And this bring me to a related question from my review: does
> dentry_open() intentionally bypass security_inode_permission()? I.e. it
> calls vfs_open() not do_open():
> openat2(dfd, char * filename, open_how)
>     build_open_flags(open_how, open_flags)
>     do_filp_open(dfd, filename, open_flags)
>         path_openat(nameidata, open_flags, flags)
>             file = alloc_empty_file(open_flags, current_cred());
>             do_open(nameidata, file, open_flags)
>                 may_open(path, acc_mode, open_flag)
>                     inode_permission(inode, MAY_OPEN | acc_mode)
>                         security_inode_permission(inode, acc_mode)
>                 vfs_open(path, file)
>                     do_dentry_open(file, path->dentry->d_inode, open)
>                         if (unlikely(f->f_flags & FMODE_EXEC && !S_ISREG(inode->i_mode))) ...
>                         security_file_open(f)
>                                 /* path-based LSMs check for open here
> 				 * and use FMODE_* flags to determine how a file
>                                  * is being opened. */
>                         open()
> vs
> dentry_open(path, flags, cred)
>         f = alloc_empty_file(flags, cred);
>         vfs_open(path, f);
> I would expect dentry_open() to mostly duplicate a bunch of
> path_openat(), but it lacks the may_open() call, etc.
> I really got the feeling that there was some new conceptual split needed
> inside do_open() where the nameidata details have been finished, after
> we've gained the "file" information, but before we've lost the "path"
> information. For example, may_open(path, ...) has no sense of "file",
> though it does do the inode_permission() call.
yes that would be nice, sadly the path hooks are really a bolted on after thought

> Note also that may_open() is used in do_tmpfile() too, and has a comment
> implying it needs to be checking only a subset of the path details. So
> I'm not sure how to split things up.
/me neither anymore

> So, that's why I put the new checks just before the may_open() call in
> do_open(): it's the most central, positions itself correctly for dealing
> with O_MAYEXEC, and doesn't appear to make any existing paths worse.
>> I am wondering if we need something distinct to request the type of the
>> file being opened versus execute permissions.
> Well, this is why I wanted to centralize it -- the knowledge of how a
> file is going to be used needs to be tested both by the core VFS
> (S_ISREG, path_noexec) and the LSMs. Things were inconsistent before.


>> All I know is being careful and putting the tests in a good logical
>> place makes the code more maintainable, whereas not being careful
>> results in all kinds of sharp corners that might be exploitable.
>> So I think it is worth digging in and figuring out where those checks
>> should live.  Especially so that code like faccessat does not need
>> to duplicate them.
> I think this is the right place with respect to execve(), though I think
> there are other cases that could use to be improved (or at least made
> more consistent).
> -Kees

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