News: 0001585037

  ARM Give a man a fire and he's warm for a day, but set fire to him and he's warm for the rest of his life (Terry Pratchett, Jingo)

Meta Uncovers RDSEED Architectural Issue In AMD Zen 5 CPUs

([AMD] 3 Hours Ago RDSEED Bug)


Over the years we have seen various [1]workarounds like disabling RDSEED for select AMD CPUs due to hardware bugs and early on in the Zen days were also [2]some RdRand issues due to [3]different problems . It turns out the newest AMD EPYC 5th Gen "Turin" processors have a new RDSEED issue.

[4]RDSEED is principally used for seeding software psuedo random number generators with additional entropy where not needing high quality RDRAND. Meta engineer Gregory Price announced on the Linux kernel mailing list the uncovering of an RDSEED bug affecting AMD [5]EPYC 9005 "Turin" processors.

Gregory Price [6]wrote :

"Under unknown architectural conditions, Zen5 chips running rdseed can produce (val=0,CF=1) as a "random" result over 10% of the time (when rdseed is successful). CF=1 indicates success, while val=0 is typically only produced when rdseed fails (CF=0).

This suggests there is an architectural issue which causes rdseed to misclassify a failure as a success under unknown conditions.

This was reproduced reliably by launching 2-threads per available core, 1-thread per for hamming on RDSEED, and 1-thread per core collectively eating and hammering on ~90% of memory."

The proposed patch is disabling RDSEED usage on AMD Zen 5 Turin processors, similar to what the Linux kernel did for Cyan Skillfish APUs.

No AMD engineer has yet commented on this matter on the Linux kernel mailing list.

Update: Price posted [7]a follow-up message now suggesting that RDSEED perhaps should be disabled for all AMD Zen 5 cores (just not EPYC / Turin) as reportedly other Zen 5 CPU models may also hit this problem.



[1] https://www.phoronix.com/news/Linux-6.16-rc6-x86-urgent

[2] https://www.phoronix.com/news/AMD-CPUs-RdRand-Suspend

[3] https://www.phoronix.com/news/Linux-5.3-Disabled-Older-AMD-Rd

[4] https://www.phoronix.com/search/RDSEED

[5] https://www.phoronix.com/search/EPYC+9005

[6] https://lore.kernel.org/lkml/20251016182107.3496116-1-gourry@gourry.net/

[7] https://lore.kernel.org/lkml/aPFDn-4Cm6n0_3_e@gourry-fedora-PF4VCD3F/


After the Children of Israel had wandered for thirty-nine years
in the wilderness, Ferdinand Feghoot arrived to make sure that they would
finally find and enter the Promised Land. With him, he brought his
favorite robot, faithful old Yewtoo Artoo, to carry his gear and do
assorted camp chores.
The Israelites soon got over their initial fear of the robot and,
as the months passed, became very fond of him. Patriarchs took to
discussing abtruse theological problems with him, and each evening the
children all gathered to hear the many stories with which he was programmed.
Therefore it came as a great shock to them when, just as their journey was
ending, he abruptly wore out. Even Feghoot couldn't console them.
"It may be true, Ferdinand Feghoot," said Moses, "that our friend
Yewtoo Artoo was soulless, but we cannot believe it. He must be properly
interred. We cannot embalm him as do the Egyptians. Nor have we wood for
a coffin. But I do have a most splendid skin from one of Pharoah's own
cattle. We shall bury him in it."
Feghoot agreed. "Yes, let this be his last rusting place."
"Rusting?" Moses cried. "Not in this dreadful dry desert!"
"Ah!" sighed Ferdinand Feghoot, shedding a tear, "I fear you do not
realize the full significance of Pharoah's oxhide!"
-- Grendel Briarton "Through Time & Space With Ferdinand
Feghoot!"