Some AMD CPU's RDRAND might not return random data after a suspend/resume

Re:Some AMD CPU's RDRAND might not return random data after ... by JdeBP on Tuesday May 07, 2019 @01:44PM (#19851693)

The instruction is signalling success, but returning FFFFFFFFFFFFFFFF as the random value.

* https://github.com/systemd/systemd/issues/11810#issuecomment...

H. Peter Anvin's educated guess was that some MSR flag, that has the effect of controlling this instruction, is not being saved and restored in the processor across a suspend, and the result is a processor state where it signals that the instruction is succeeding but it is not actually returning random values.

* https://bugzilla.kernel.org/show_bug.cgi?id=85911#c4

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Re:Some AMD CPU's RDRAND might not return random data after ... by lelf on Tuesday May 07, 2019 @02:17PM (#19852014)

> The instruction is signalling success

Let’s wait for clarifications how that person has done the tests.

However: the bug is about systemd failing to get entropy, not getting nonsense entropy.

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Re:Some AMD CPU's RDRAND might not return random data after ... by JdeBP on Tuesday May 07, 2019 @02:54PM (#19852316)

How xe was doing this was in fact explained in the comment hyperlinked right at the start of this page, and H. Peter Anvin was thinking about this over 4 years ago. But for the doubters Vladislav has already reiterated the point.

* https://github.com/systemd/systemd/issues/11810#issuecomment...

There is no "however". This bug is about code that is, according to the AMD doco, using the instruction correctly; but that is, because the AMD processor has this possible state after a suspend+resume, getting all-ones as its random data, thereby causing ID collisions in a fairly wide range of possible things from freshly re-generated machine IDs to journal file header block IDs, and including unit invocation IDs.

* https://github.com/systemd/systemd/blob/717e8eda77b93ac396dc...

This indicates that a "should be fine" in another comment is not in fact true. (-:

* https://github.com/systemd/systemd/blob/717e8eda77b93ac396dc...

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Re:Some AMD CPU's RDRAND might not return random data after ... by zaroth on Wednesday May 08, 2019 @03:51PM (#19862249)

The Bugzilla report seemed to indicate that AMD was returning an error code, because OpenSSL was failing to generate a key, not generating a bad key?

Why would OpenSSL fail visibly if the API was returning success but with non-random data?

https://bugzilla.redhat.com/show_bug.cgi?id=1150286

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Re:Some AMD CPU's RDRAND might not return random data after ... by int_19h on Wednesday May 08, 2019 @11:31PM (#19865129)

If you look closely at one of the error messages, it says "too many iterations", in what looks like a random generator. I wonder if at some point it has to iterate to get a number different from another one, and it just gets an endless stream of identical values?

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Re:Some AMD CPU's RDRAND might not return random data after ... by zaroth on Wednesday May 08, 2019 @11:51PM (#19865212)

I can’t imagine the code is checking the returned value for uniqueness after seeing a SUCCESS code, and iterating in the hopes that becomes less true.

That would be quite something to read the comment on;

// Here we check to be sure the Earth is not flat

...

// Seeing that the Earth is indeed flat, we will iterate in this absurdity some more in the hopes it rounds out eventually

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Re:Some AMD CPU's RDRAND might not return random data after ... by int_19h on Thursday May 09, 2019 @04:11PM (#19871914)

Suppose you need to generate several unique 32-bit IDs for something?

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