Re: [PATCH 3/6] io_uring: add support for IORING_OP_OPENAT

[Stefan Metzmacher <metze@samba.org>]

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>> I'm sorry, but I'm still unsure we're talking about the same thing
>> (or maybe I'm missing some basics here).
>>
>> My understanding of the io_uring_enter() is that it will execute as much
>> non-blocking calls as it can without switching to any other kernel thread.
> 
> Correct, any SQE that we can do without switching, we will.
> 
>> And my fear is that openat will use get_current_cred() instead of
>> ctx->creds.
> 
> OK, I think I follow your concern. So you'd like to setup the rings from
> a _different_ user, and then later on use it for submission for SQEs that
> a specific user. So sort of the same as our initial discussion, except
> the mapping would be static. The difference being that you might setup
> the ring from a different user than the user that would be submitting IO
> on it?

Our current (much simplified here) flow is this:

  # we start as root
  seteuid(0);setegid(0);setgroups()...
  ...
  # we become the user555 and
  # create our desired credential token
  seteuid(555); seteguid(555); setgroups()...
  # Start an openat2 on behalf of user555
  openat2()
  # we unbecome the user again and run as root
  seteuid(0);setegid(0); setgroups()...
  ...
  # we become the user444 and
  # create our desired credential token
  seteuid(444); seteguid(444); setgroups()...
  # Start an openat2 on behalf of user444
  openat2()
  # we unbecome the user again and run as root
  seteuid(0);setegid(0); setgroups()...
  ...
  # we become the user555 and
  # create our desired credential token
  seteuid(555); seteguid(555); setgroups()...
  # Start an openat2 on behalf of user555
  openat2()
  # we unbecome the user again and run as root
  seteuid(0);setegid(0); setgroups()...

It means we have to do about 7 syscalls in order
to open a file on behalf of a user.
(In reality we cache things and avoid set*id()
calls most of the time, but I want to demonstrate the
simplified design here)

With io_uring I'd like to use a flow like this:

  # we start as root
  seteuid(0);setegid(0);setgroups()...
  ...
  # we become the user444 and
  # create our desired credential token
  seteuid(444); seteguid(444); setgroups()...
  # we snapshot the credentials to the new ring for user444
  ring444 = io_uring_setup()
  # we unbecome the user again and run as root
  seteuid(0);setegid(0);setgroups()...
  ...
  # we become the user555 and
  # create our desired credential token
  seteuid(555); seteguid(555); setgroups()...
  # we snapshot the credentials to the new ring for user555
  ring555 = io_uring_setup()
  # we unbecome the user again and run as root
  seteuid(0);setegid(0);setgroups()...
  ...
  # Start an openat2 on behalf of user555
  io_uring_enter(ring555, OP_OPENAT2...)
  ...
  # Start an openat2 on behalf of user444
  io_uring_enter(ring444, OP_OPENAT2...)
  ...
  # Start an openat2 on behalf of user555
  io_uring_enter(ring555, OP_OPENAT2...)

So instead of constantly doing 7 syscalls per open,
we would be down to just at most one. And I would assume
that io_uring_enter() would do the temporary credential switch
for me also in the non-blocking case.

> If so, then we do need something to support that, probably an
> IORING_REGISTER_CREDS or similar. This would allow you to replace the
> creds you currently have in ctx->creds with whatever new one.

I don't want to change ctx->creds, but I want it to be used consistently.

What I think is missing is something like this:

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 32aee149f652..55dbb154915a 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -6359,10 +6359,27 @@ SYSCALL_DEFINE6(io_uring_enter, unsigned int,
fd, u32, to_submit,
                struct mm_struct *cur_mm;

                mutex_lock(&ctx->uring_lock);
+               if (current->mm != ctx->sqo_mm) {
+                       // TODO: somthing like this...
+                       restore_mm = current->mm;
+                       use_mm(ctx->sqo_mm);
+               }
                /* already have mm, so io_submit_sqes() won't try to
grab it */
                cur_mm = ctx->sqo_mm;
+               if (current_cred() != ctx->creds) {
+                       // TODO: somthing like this...
+                       restore_cred = override_creds(ctx->creds);
+               }
                submitted = io_submit_sqes(ctx, to_submit, f.file, fd,
                                           &cur_mm, false);
+               if (restore_cred != NULL) {
+                       revert_creds(restore_cred);
+               }
+               if (restore_mm != NULL) {
+                       // TODO: something like this...
+                       unuse_mm(ctx->sqo_mm);
+                       use_mm(restore_mm);
+               }
                mutex_unlock(&ctx->uring_lock);

                if (submitted != to_submit)

I'm not sure if current->mm is needed, I just added it for completeness
and as hint that io_op_defs[req->opcode].needs_mm is there and a
needs_creds could also be added (if it helps with performance)

Is it possible to trigger a change of current->mm from userspace?

An IORING_REGISTER_CREDS would only be useful if it's possible to
register a set of credentials and then use per io_uring_sqe credentials.
That would also be fine for me, but I'm not sure it's needed for now.

Apart from IORING_REGISTER_CREDS I think a change like the one
above is needed in order to avoid potential security problems.

>> I'm I missing something?
> 
> I think we're talking about the same thing, just different views of it :-)

I hope it's clear from my side now :-)

Thanks!
metze


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