linux-kernel - Re: [PATCH v4 09/14] selftests: harness: Move teardown conditional into test metadata

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Message-ID: <aEoUhPYIAizTLADq@nvidia.com>
Date: Wed, 11 Jun 2025 16:43:00 -0700
From: Nicolin Chen <nicolinc@...dia.com>
To: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
CC: Jason Gunthorpe <jgg@...dia.com>, Shuah Khan <shuah@...nel.org>, "Shuah
 Khan" <skhan@...uxfoundation.org>, Willy Tarreau <w@....eu>, Thomas
 Weißschuh <linux@...ssschuh.net>, Kees Cook
	<kees@...nel.org>, Andy Lutomirski <luto@...capital.net>, Will Drewry
	<wad@...omium.org>, Mark Brown <broonie@...nel.org>, Muhammad Usama Anjum
	<usama.anjum@...labora.com>, <linux-kernel@...r.kernel.org>,
	<linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH v4 09/14] selftests: harness: Move teardown conditional
 into test metadata

On Wed, Jun 11, 2025 at 02:48:16PM -0700, Nicolin Chen wrote:
> On Wed, Jun 11, 2025 at 10:19:56AM -0700, Nicolin Chen wrote:
> > On Wed, Jun 11, 2025 at 10:04:35AM +0200, Thomas Weißschuh wrote:
> > > On Wed, Jun 11, 2025 at 12:05:25AM -0700, Nicolin Chen wrote:
> > > > 2) parent doesn't seem to wait for the setup() to complete..
> > > 
> > > setup() is called in the child (L431) right before the testcase itself is
> > > called (L436). The parent waits for the child to exit (L439) before unmapping.
> > > 
> > > > 3) when parent runs faster than the child that is still running
> > > >    setup(), the parent unmaps the no_teardown and set it to NULL,
> > > >    then UAF in the child, i.e. signal 11?
> > > 
> > > That should never happen as the waitpid() will block until the child running
> > > setup() and the testcase itself have exited.
> > 
> > Ah, maybe I was wrong about these narratives. But the results show
> > that iommufd_dirty_tracking_teardown() was not called in the failed
> > cases:
> 
> Here is a new finding...
> 
> As you replied that I was wrong about the race between the parent
> and the child processes, the parent does wait for the completion
> of the child. But the child exited with status=139 i.e. signal 11
> due to UAF, which however is resulted from the iommufd test code:
> 
> FIXTURE_SETUP(iommufd_dirty_tracking)
> {
> 	....
> 	vrc = mmap(self->buffer, variant->buffer_size, PROT_READ | PROT_WRITE,
> 	^
> 	|
>         after this line, the _metadata->no_teardown is set to NULL.
> 
> So, the child process accessing this NULL pointer crashed with the
> signal 11..
> 
> And I did a further experiment by turning "bool *no_teardown" to a
> "bool no_teardown". Then, the mmap() in iommufd_dirty_tracking will
> set _metadata->teardown_fn function pointer to NULL..

So, the test case sets an alignment with HUGEPAGE_SIZE=512MB while
allocating buffer_size=64MB:
	rc = posix_memalign(&self->buffer, HUGEPAGE_SIZE, variant->buffer_size);
	vrc = mmap(self->buffer, variant->buffer_size, PROT_READ | PROT_WRITE,
this gives the self->buffer a location that is 512MB aligned, but
only mmap part of one 512MB huge page.

On the other hand, _metadata->no_teardown was mmap() outside the
range of the [self->buffer, self->buffer + 64MB), but within the
range of [self->buffer, self->buffer + 512MB).

E.g.
   _metadata->no_teardown = 0xfffbfc610000 // inside range2 below
   buffer=[0xfffbe0000000, fffbe4000000) // range1
   buffer=[0xfffbe0000000, fffc00000000) // range2

Then ,the "vrc = mmap(..." overwrites the _metadata->no_teardown
location to NULL..

The following change can fix, though it feels odd that the buffer
has to be preserved with the entire huge page:
---------------------------------------------------------------
@@ -2024,3 +2027,4 @@ FIXTURE_SETUP(iommufd_dirty_tracking)

-       rc = posix_memalign(&self->buffer, HUGEPAGE_SIZE, variant->buffer_size);
+       rc = posix_memalign(&self->buffer, HUGEPAGE_SIZE,
+                           __ALIGN_KERNEL(variant->buffer_size, HUGEPAGE_SIZE));
        if (rc || !self->buffer) {
---------------------------------------------------------------

Any thought?

Thanks
Nicolin