Re: [PATCH v4] xen/arm64: Place a speculation barrier following an ret instruction

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Mon, 19 Apr 2021, Bertrand Marquis wrote:
> Hi Julien,
> 
> > On 18 Apr 2021, at 19:03, Julien Grall <julien@xxxxxxx> wrote:
> > 
> > From: Julien Grall <jgrall@xxxxxxxxxx>
> > 
> > Some CPUs can speculate past a RET instruction and potentially perform
> > speculative accesses to memory before processing the return.
> > 
> > There is no known gadget available after the RET instruction today.
> > However some of the registers (such as in check_pending_guest_serror())
> > may contain a value provided by the guest.
> > 
> > In order to harden the code, it would be better to add a speculation
> > barrier after each RET instruction. The performance impact is meant to
> > be negligeable as the speculation barrier is not meant to be
> > architecturally executed.
> > 
> > Rather than manually inserting a speculation barrier, use a macro
> > which overrides the mnemonic RET and replace with RET + SB. We need to
> > use the opcode for RET to prevent any macro recursion.
> > 
> > This patch is only covering the assembly code. C code would need to be
> > covered separately using the compiler support.
> > 
> > Note that the definition of the macros sb needs to be moved earlier in
> > asm-arm/macros.h so it can be used by the new macro.
> > 
> > This is part of the work to mitigate straight-line speculation.
> > 
> > Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
> Reviewed-by: Bertrand Marquis <bertrand.marquis@xxxxxxx>

Acked-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>