Messages in this thread | | | Date | Fri, 21 Feb 2014 17:15:47 -0300 | From | Henrique de Moraes Holschuh <> | Subject | Intel microcode extended signature table support in Linux/BITS |
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I am directing this question to you hoping one of you can either address it directly, or forward it internally at Intel.
We have code in the Linux kernel to support microcode extended signature tables which has never been tested as far as I know, as there are no public test vectors of microcode update containers with extended signature tables.
IMHO, untested kernel code is never a good thing, and useless untested kernel code is even worse.
So, the first question would be whether microcode extended signature tables are ever going to be used by Intel?
If that code is going to remain unused, we probably should remove support for Intel microcode extended signatures from Linux.
Anyway, there is an incompatibility between the code in Linux and also in Intel BITS, and what is documented in the SDM (Intel® 64 and IA-32 Architectures Software Developer Manual).
So, for the second question, could someone at Intel please clarify the following point?
In the Intel® 64 and IA-32 Architectures Software Developer Manual (SDM) vol 3A page 9-30, table 9-6, last row, the description of the extended signature table checksum ("Checksum[n]" in table 9-6) makes it clear one is supposed to strip out the extended signature table entirely from the microcode container when one applies the extended signature to the main microcode header.
However, this removal of the extended signature table also implies in a change of the "Total Size" field of the main microcode header (table 9-6). Therefore, the "CheckSum[n]" field of table 9-6 has to take into account this change in the "Total Size" field.
Two public implementations of extended microcode signature table support (Linux kernel and Intel BITS) do NOT take into account the required change to "Total Size" in order to strip the extended microcode signature table, and thus will not validate microcode with extended signature tables that follow the SDM.
So, which is correct: the code in Linux and BITS, or the text of the SDM?
As a sidenote, the way the SDM documents the extended signature table implies no extra padding can be added after the extended signature table.
Such padding would be required to satisfy the SDM requirement that "Total Size" be a multiple of 1024, unless the padding is done inside the microcode opaque data itself (payload). This seems suboptimal, as it ties the container strongly to the payload/opaque data... maybe the multiple-of-1024 requirement for Total Size is superfluous for variable-size microcode containers?
-- "One disk to rule them all, One disk to find them. One disk to bring them all and in the darkness grind them. In the Land of Redmond where the shadows lie." -- The Silicon Valley Tarot Henrique Holschuh -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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