On 3/2/20 10:49 PM, Eric W. Biederman wrote:

Bernd Edlinger [off-list ref] writes:

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On 3/2/20 5:17 PM, Eric W. Biederman wrote:

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Bernd Edlinger [off-list ref] writes:

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On 3/2/20 4:57 PM, Eric W. Biederman wrote:

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Bernd Edlinger [off-list ref] writes:

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I tried this with s/EACCESS/EACCES/.

The test case in this patch is not fixed, but strace does not freeze,
at least with my setup where it did freeze repeatable.

Thanks, That is what I was aiming at.

So we have one method we can pursue to fix this in practice.

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That is
obviously because it bypasses the cred_guard_mutex.  But all other
process that access this file still freeze, and cannot be
interrupted except with kill -9.

However that smells like a denial of service, that this
simple test case which can be executed by guest, creates a /proc/$pid/mem
that freezes any process, even root, when it looks at it.
I mean: "ln -s README /proc/$pid/mem" would be a nice bomb.

Yes.  Your the test case in your patch a variant of the original
problem.


I have been staring at this trying to understand the fundamentals of the
original deeper problem.

The current scope of cred_guard_mutex in exec is because being ptraced
causes suid exec to act differently.  So we need to know early if we are
ptraced.

It has a second use, that it prevents two threads entering execve,
which would probably result in disaster.

Exec can fail with an error code up until de_thread.  de_thread causes
exec to fail with the error code -EAGAIN for the second thread to get
into de_thread.

So no.  The cred_guard_mutex is not needed for that case at all.

Okay, but that will reset current->in_execve, right?

Absolutely.

The error handling kicks in and exec_binprm fails with a negative
return code.  Then __do_excve_file cleans up and clears
current->in_execve.

Yes of course.  I was under the wrong impression that that value is
a kind of global, but it is a thread local.

So I think I need a new boolean see v3 of the patch, and soon v4 (with
just one comment fixed).

I'm currently executing the strace v5.5 testsuite, and every test
is passed so far.  I'll also look at gdb testsuite, before I send the
next version.


Thanks
Bernd.