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SubjectRe: [PATCH V4 01/50] x86/entry: Add fence for kernel entry swapgs in paranoid_entry()
On Tue, Oct 26, 2021 at 10:13:31PM +0800, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@linux.alibaba.com>
>
> Commit 18ec54fdd6d18 ("x86/speculation: Prepare entry code for Spectre
> v1 swapgs mitigations") adds FENCE_SWAPGS_{KERNEL|USER}_ENTRY
> for conditional swapgs. And in paranoid_entry(), it uses only
> FENCE_SWAPGS_KERNEL_ENTRY for both branches. It is because the fence
> is required for both cases since the CR3 write is conditinal even PTI
> is enabled.
>
> But commit 96b2371413e8f ("x86/entry/64: Switch CR3 before SWAPGS in
> paranoid entry") switches the code order and changes the branches.
> And it misses the needed FENCE_SWAPGS_KERNEL_ENTRY for user gsbase case.
>
> Add it back.
>
> Fixes: Commit 96b2371413e8f ("x86/entry/64: Switch CR3 before SWAPGS in paranoid entry")
> Cc: Josh Poimboeuf <jpoimboe@redhat.com>
> Cc: Chang S. Bae <chang.seok.bae@intel.com>
> Cc: Sasha Levin <sashal@kernel.org>
> Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
> ---
> arch/x86/entry/entry_64.S | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index e38a4cf795d9..1f98188e83ef 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -898,6 +898,7 @@ SYM_CODE_START_LOCAL(paranoid_entry)
> rdmsr
> testl %edx, %edx
> jns .Lparanoid_entry_swapgs
> + FENCE_SWAPGS_KERNEL_ENTRY

Wouldn't it be cleaner to simply move the FENCE up from under the swapgs
to before the jns instruction?

This way you kill speculation for both cases and have a single fence.

--
Regards/Gruss,
Boris.

https://people.kernel.org/tglx/notes-about-netiquette

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