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x86/srso: Explain the untraining sequences a bit more

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commit 9dbd23e42ff0b10c9b02c9e649c76e5228241a8e upstream.

The goal is to eventually have a proper documentation about all this.

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20230814164447.GFZNpZ/64H4lENIe94@fat_crate.local
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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Borislav Petkov (AMD) 2023-08-14 21:29:50 +02:00 committed by Greg Kroah-Hartman
parent 529a9f087a
commit c16d0b3baf
1 changed files with 19 additions and 0 deletions
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19
arch/x86/lib/retpoline.S
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@ -129,6 +129,25 @@ SYM_CODE_START(srso_alias_return_thunk)
ud2
SYM_CODE_END(srso_alias_return_thunk)
/*
* Some generic notes on the untraining sequences:
*
* They are interchangeable when it comes to flushing potentially wrong
* RET predictions from the BTB.
*
* The SRSO Zen1/2 (MOVABS) untraining sequence is longer than the
* Retbleed sequence because the return sequence done there
* (srso_safe_ret()) is longer and the return sequence must fully nest
* (end before) the untraining sequence. Therefore, the untraining
* sequence must fully overlap the return sequence.
*
* Regarding alignment - the instructions which need to be untrained,
* must all start at a cacheline boundary for Zen1/2 generations. That
* is, instruction sequences starting at srso_safe_ret() and
* the respective instruction sequences at retbleed_return_thunk()
* must start at a cacheline boundary.
*/
/*
* Safety details here pertain to the AMD Zen{1,2} microarchitecture:
* 1) The RET at retbleed_return_thunk must be on a 64 byte boundary, for