Re: [PATCH] x86: entry: flush the cache if syscall error

[Andy Lutomirski]

On Thu, Oct 11, 2018 at 11:55 AM Kristen Carlson Accardi
<kristen@xxxxxxxxxxxxxxx> wrote:
>
> This patch aims to make it harder to perform cache timing attacks on data
> left behind by system calls. If we have an error returned from a syscall,
> flush the L1 cache.
>
> It's important to note that this patch is not addressing any specific
> exploit, nor is it intended to be a complete defense against anything.
> It is intended to be a low cost way of eliminating some of side effects
> of a failed system call.
>
> A performance test using sysbench on one hyperthread and a script which
> attempts to repeatedly access files it does not have permission to access
> on the other hyperthread found no significant performance impact.
>
> Suggested-by: Alan Cox <alan@xxxxxxxxxxxxxxx>
> Signed-off-by: Kristen Carlson Accardi <kristen@xxxxxxxxxxxxxxx>
> ---
>  arch/x86/Kconfig        |  9 +++++++++
>  arch/x86/entry/common.c | 18 ++++++++++++++++++
>  2 files changed, 27 insertions(+)
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 1a0be022f91d..bde978eb3b4e 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -445,6 +445,15 @@ config RETPOLINE
>           code are eliminated. Since this includes the syscall entry path,
>           it is not entirely pointless.
>
> +config SYSCALL_FLUSH
> +       bool "Clear L1 Cache on syscall errors"
> +       default n
> +       help
> +         Selecting 'y' allows the L1 cache to be cleared upon return of
> +         an error code from a syscall if the CPU supports "flush_l1d".
> +         This may reduce the likelyhood of speculative execution style
> +         attacks on syscalls.
> +
>  config INTEL_RDT
>         bool "Intel Resource Director Technology support"
>         default n
> diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
> index 3b2490b81918..26de8ea71293 100644
> --- a/arch/x86/entry/common.c
> +++ b/arch/x86/entry/common.c
> @@ -268,6 +268,20 @@ __visible inline void syscall_return_slowpath(struct pt_regs *regs)
>         prepare_exit_to_usermode(regs);
>  }
>
> +__visible inline void l1_cache_flush(struct pt_regs *regs)
> +{
> +       if (IS_ENABLED(CONFIG_SYSCALL_FLUSH) &&
> +           static_cpu_has(X86_FEATURE_FLUSH_L1D)) {
> +               if (regs->ax == 0 || regs->ax == -EAGAIN ||
> +                   regs->ax == -EEXIST || regs->ax == -ENOENT ||
> +                   regs->ax == -EXDEV || regs->ax == -ETIMEDOUT ||
> +                   regs->ax == -ENOTCONN || regs->ax == -EINPROGRESS)

What about ax > 0?  (Or more generally, any ax outside the range of -1
.. -4095 or whatever the error range is.)  As it stands, it looks like
you'll flush on successful read(), write(), recv(), etc, and that
could seriously hurt performance on real workloads.

> +                       return;
> +
> +               wrmsrl(MSR_IA32_FLUSH_CMD, L1D_FLUSH);
> +       }
> +}
> +
>  #ifdef CONFIG_X86_64
>  __visible void do_syscall_64(unsigned long nr, struct pt_regs *regs)
>  {
> @@ -290,6 +304,8 @@ __visible void do_syscall_64(unsigned long nr, struct pt_regs *regs)
>                 regs->ax = sys_call_table[nr](regs);
>         }
>
> +       l1_cache_flush(regs);
> +
>         syscall_return_slowpath(regs);
>  }
>  #endif
> @@ -338,6 +354,8 @@ static __always_inline void do_syscall_32_irqs_on(struct pt_regs *regs)
>  #endif /* CONFIG_IA32_EMULATION */
>         }
>
> +       l1_cache_flush(regs);
> +
>         syscall_return_slowpath(regs);
>  }
>
> --
> 2.14.4
>