linux-kernel - Re: [PATCH] x86/kvm: fix FASTOP_SIZE when return thunks are enabled

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Message-ID: <CA+G9fYvxR8+4cajcCBbPRuhR1tnuBmrLxosSOMzPg7CjxQU35w@mail.gmail.com>
Date:   Thu, 14 Jul 2022 21:45:34 +0530
From:   Naresh Kamboju <naresh.kamboju@...aro.org>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Thadeu Lima de Souza Cascardo <cascardo@...onical.com>,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org, x86@...nel.org,
        stable@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Borislav Petkov <bp@...e.de>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Linux Kernel Functional Testing <lkft@...aro.org>
Subject: Re: [PATCH] x86/kvm: fix FASTOP_SIZE when return thunks are enabled

Hi Paolo,

On Thu, 14 Jul 2022 at 17:06, Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 7/14/22 11:52, Peter Zijlstra wrote:
> > On Wed, Jul 13, 2022 at 02:12:41PM -0300, Thadeu Lima de Souza Cascardo wrote:
> >> The return thunk call makes the fastop functions larger, just like IBT
> >> does. Consider a 16-byte FASTOP_SIZE when CONFIG_RETHUNK is enabled.
> >>
> >> Otherwise, functions will be incorrectly aligned and when computing their
> >> position for differently sized operators, they will executed in the middle
> >> or end of a function, which may as well be an int3, leading to a crash
> >> like:
> >
> > Bah.. I did the SETcc stuff, but then forgot about the FASTOP :/
> >
> >    af2e140f3420 ("x86/kvm: Fix SETcc emulation for return thunks")
> >
> >> Fixes: aa3d480315ba ("x86: Use return-thunk in asm code")
> >> Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@...onical.com>
> >> Cc: Peter Zijlstra (Intel) <peterz@...radead.org>
> >> Cc: Borislav Petkov <bp@...e.de>
> >> Cc: Josh Poimboeuf <jpoimboe@...nel.org>
> >> Cc: Paolo Bonzini <pbonzini@...hat.com>
> >> Reported-by: Linux Kernel Functional Testing <lkft@...aro.org>

Tested-by: Linux Kernel Functional Testing <lkft@...aro.org>

> >> ---
> >>   arch/x86/kvm/emulate.c | 2 +-
> >>   1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> >> index db96bf7d1122..d779eea1052e 100644
> >> --- a/arch/x86/kvm/emulate.c
> >> +++ b/arch/x86/kvm/emulate.c
> >> @@ -190,7 +190,7 @@
> >>   #define X16(x...) X8(x), X8(x)
> >>
> >>   #define NR_FASTOP (ilog2(sizeof(ulong)) + 1)
> >> -#define FASTOP_SIZE (8 * (1 + HAS_KERNEL_IBT))
> >> +#define FASTOP_SIZE (8 * (1 + (HAS_KERNEL_IBT | IS_ENABLED(CONFIG_RETHUNK))))
> >
> > Would it make sense to do something like this instead?
>
> Yes, definitely.  Applied with a small tweak to make FASTOP_LENGTH
> more similar to SETCC_LENGTH:
>
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index db96bf7d1122..0a15b0fec6d9 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -189,8 +189,12 @@
>   #define X8(x...) X4(x), X4(x)
>   #define X16(x...) X8(x), X8(x)
>
> -#define NR_FASTOP (ilog2(sizeof(ulong)) + 1)
> -#define FASTOP_SIZE (8 * (1 + HAS_KERNEL_IBT))
> +#define NR_FASTOP      (ilog2(sizeof(ulong)) + 1)
> +#define RET_LENGTH     (1 + (4 * IS_ENABLED(CONFIG_RETHUNK)) + \
> +                        IS_ENABLED(CONFIG_SLS))
> +#define FASTOP_LENGTH  (ENDBR_INSN_SIZE + 7 + RET_LENGTH)
> +#define FASTOP_SIZE    (8 << ((FASTOP_LENGTH > 8) & 1) << ((FASTOP_LENGTH > 16) & 1))
> +static_assert(FASTOP_LENGTH <= FASTOP_SIZE);
>
>   struct opcode {
>         u64 flags;
> @@ -442,8 +446,6 @@ static int fastop(struct x86_emulate_ctxt *ctxt, fastop_t fop);
>    * RET | JMP __x86_return_thunk       [1,5 bytes; CONFIG_RETHUNK]
>    * INT3                               [1 byte; CONFIG_SLS]
>    */
> -#define RET_LENGTH     (1 + (4 * IS_ENABLED(CONFIG_RETHUNK)) + \
> -                        IS_ENABLED(CONFIG_SLS))
>   #define SETCC_LENGTH  (ENDBR_INSN_SIZE + 3 + RET_LENGTH)
>   #define SETCC_ALIGN   (4 << ((SETCC_LENGTH > 4) & 1) << ((SETCC_LENGTH > 8) & 1))
>   static_assert(SETCC_LENGTH <= SETCC_ALIGN);
>
>
> Paolo

Applied your patch and tested on top of the mainline kernel and
tested kvm-unit-tests and reported kernel panic fixed.

https://lkft.validation.linaro.org/scheduler/job/5284626

- Naresh