Re: [PATCH V4 01/50] x86/entry: Add fence for kernel entry swapgs in paranoid_entry()

[Borislav Petkov]

On Tue, Oct 26, 2021 at 10:13:31PM +0800, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> 
> 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@xxxxxxxxxx>
> Cc: Chang S. Bae <chang.seok.bae@xxxxxxxxx>
> Cc: Sasha Levin <sashal@xxxxxxxxxx>
> Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> ---
>  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.

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