Re: [PATCH v23 6/9] x86/entry: Introduce ENDBR macro
From: Dave Hansen <hidden>
Date: 2021-03-16 15:50:07
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linux-arch, linux-doc, linux-mm, lkml
On 3/16/21 8:13 AM, Yu-cheng Yu wrote:
quoted hunk ↗ jump to hunk
--- a/arch/x86/entry/calling.h +++ b/arch/x86/entry/calling.h@@ -392,3 +392,21 @@ For 32-bit we have the following conventions - kernel is built with .endm #endif /* CONFIG_SMP */ +/* + * ENDBR is an instruction for the Indirect Branch Tracking (IBT) component + * of CET. IBT prevents attacks by ensuring that (most) indirect branches + * function calls may only land at ENDBR instructions. Branches that don't + * follow the rules will result in control flow (#CF) exceptions. + * ENDBR is a noop when IBT is unsupported or disabled. Most ENDBR + * instructions are inserted automatically by the compiler, but branch + * targets written in assembly must have ENDBR added manually. + */ +.macro ENDBR +#ifdef CONFIG_X86_CET +#ifdef __i386__ + endbr32 +#else + endbr64 +#endif +#endif +.endm
Is "#ifdef __i386__" the right thing to use here? I guess ENDBR only ends up getting used in the VDSO, but there's a lot of non-userspace-exposed stuff in calling.h. It seems a bit weird to have the normally userspace-only __i386__ in there. I don't see any existing direct use of __i386__ in arch/x86/entry/vdso.