The following commit has been merged into the x86/bugs branch of tip: Commit-ID: 9d9c22cc444af01ce254872b729af26864c43a3a Gitweb: https://git.kernel.org/tip/9d9c22cc444af01ce254872b729af26864c43a3a Author: Borislav Petkov (AMD) <bp@xxxxxxxxx> AuthorDate: Fri, 20 Oct 2023 13:17:14 +02:00 Committer: Borislav Petkov (AMD) <bp@xxxxxxxxx> CommitterDate: Fri, 20 Oct 2023 13:17:14 +02:00 x86/retpoline: Document some thunk handling aspects After a lot of experimenting (see thread Link points to) document for now the issues and requirements for future improvements to the thunk handling and potential issuing of a diagnostic when the default thunk hasn't been patched out. This documentation is only temporary and that close before the merge window it is only a placeholder for those future improvements. Suggested-by: Ingo Molnar <mingo@xxxxxxxxxx> Signed-off-by: Borislav Petkov (AMD) <bp@xxxxxxxxx> Link: https://lore.kernel.org/r/20231010171020.462211-1-david.kaplan@xxxxxxx --- arch/x86/lib/retpoline.S | 15 +++++++++++++++ 1 file changed, 15 insertions(+) diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S index d410aba..a48077c 100644 --- a/arch/x86/lib/retpoline.S +++ b/arch/x86/lib/retpoline.S @@ -129,6 +129,13 @@ SYM_CODE_END(__x86_indirect_jump_thunk_array) #ifdef CONFIG_RETHUNK +/* + * Be careful here: that label cannot really be removed because in + * some configurations and toolchains, the JMP __x86_return_thunk the + * compiler issues is either a short one or the compiler doesn't use + * relocations for same-section JMPs and that breaks the returns + * detection logic in apply_returns() and in objtool. + */ .section .text..__x86.return_thunk #ifdef CONFIG_CPU_SRSO @@ -361,6 +368,14 @@ SYM_FUNC_END(call_depth_return_thunk) * This code is only used during kernel boot or module init. All * 'JMP __x86_return_thunk' sites are changed to something else by * apply_returns(). + * + * This should be converted eventually to call a warning function which + * should scream loudly when the default return thunk is called after + * alternatives have been applied. + * + * That warning function cannot BUG() because the bug splat cannot be + * displayed in all possible configurations, leading to users not really + * knowing why the machine froze. */ SYM_CODE_START(__x86_return_thunk) UNWIND_HINT_FUNC
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