Re: [PATCH 4.19 77/79] x86/nospec: Fix i386 RSB stuffing

Ben Hutchings
2022-09-14T18:46:18Z

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On Tue, 2022-09-13 at 16:07 +0200, Greg Kroah-Hartman wrote:
> From: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
>
> From: Peter Zijlstra <peterz@infradead.org>
>
> commit 332924973725e8cdcc783c175f68cf7e162cb9e5 upstream.

This should have only "From: Peter Zijlstra".

Ben.

>
> Turns out that i386 doesn't unconditionally have LFENCE, as such the
> loop in __FILL_RETURN_BUFFER isn't actually speculation safe on such
> chips.
>
> Fixes: ba6e31af2be9 ("x86/speculation: Add LFENCE to RSB fill sequence")
> Reported-by: Ben Hutchings <ben@decadent.org.uk>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Link: https://lkml.kernel.org/r/Yv9tj9vbQ9nNlXoY@worktop.programming.kicks-ass.net
> [bwh: Backported to 4.19/5.4:
> - __FILL_RETURN_BUFFER takes an sp parameter
> - Open-code __FILL_RETURN_SLOT]
> Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
> Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> ---
> arch/x86/include/asm/nospec-branch.h | 14 ++++++++++++++
> 1 file changed, 14 insertions(+)
>
> --- a/arch/x86/include/asm/nospec-branch.h
> +++ b/arch/x86/include/asm/nospec-branch.h
> @@ -35,6 +35,7 @@
> * the optimal version — two calls, each with their own speculation
> * trap should their return address end up getting used, in a loop.
> */
> +#ifdef CONFIG_X86_64
> #define __FILL_RETURN_BUFFER(reg, nr, sp) \
> mov $(nr/2), reg; \
> 771: \
> @@ -55,6 +56,19 @@
> add $(BITS_PER_LONG/8) * nr, sp; \
> /* barrier for jnz misprediction */ \
> lfence;
> +#else
> +/*
> + * i386 doesn't unconditionally have LFENCE, as such it can't
> + * do a loop.
> + */
> +#define __FILL_RETURN_BUFFER(reg, nr, sp) \
> + .rept nr; \
> + call 772f; \
> + int3; \
> +772:; \
> + .endr; \
> + add $(BITS_PER_LONG/8) * nr, sp;
> +#endif
>
> /* Sequence to mitigate PBRSB on eIBRS CPUs */
> #define __ISSUE_UNBALANCED_RET_GUARD(sp) \
>
>

--
Ben Hutchings
It is easier to change the specification to fit the program
than vice versa.

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