Re: [PATCH RFC v2 1/6] x86: Skip PTI when disable indication is set

Andy Lutomirski
2018-02-15T19:52:13Z

On Thu, Feb 15, 2018 at 4:35 PM, Nadav Amit <namit@vmware.com> wrote:
> If PTI is disabled, we do not want to switch page-tables. On entry to
> the kernel, this is done based on CR3 value. On return, do it according
> to per core indication.
>
> To be on the safe side, avoid speculative skipping of page-tables
> switching when returning the userspace. This can be avoided if the CPU
> cannot execute speculatively code without the proper permissions. When
> switching to the kernel page-tables, this is anyhow not an issue: if PTI
> is enabled and page-tables were not switched, the kernel part of the
> user page-tables would not be set.
>
> Signed-off-by: Nadav Amit <namit@vmware.com>
> ---
> arch/x86/entry/calling.h | 33 +++++++++++++++++++++++++++++++++
> arch/x86/include/asm/tlbflush.h | 17 +++++++++++++++--
> arch/x86/kernel/asm-offsets.c | 1 +
> 3 files changed, 49 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
> index 3f48f695d5e6..5e9895f44d11 100644
> --- a/arch/x86/entry/calling.h
> +++ b/arch/x86/entry/calling.h
> @@ -216,7 +216,14 @@ For 32-bit we have the following conventions - kernel is built with
>
> .macro SWITCH_TO_KERNEL_CR3 scratch_reg:req
> ALTERNATIVE "jmp .Lend_\@", "", X86_FEATURE_PTI
> +
> + /*
> + * Do not switch on compatibility mode.
> + */

That comment should just say "if we're already using kernel CR3, don't
switch" or something like that.

> mov %cr3, \scratch_reg
> + testq $PTI_USER_PGTABLE_MASK, \scratch_reg
> + jz .Lend_\@
> +
> ADJUST_KERNEL_CR3 \scratch_reg
> mov \scratch_reg, %cr3
> .Lend_\@:
> @@ -225,8 +232,20 @@ For 32-bit we have the following conventions - kernel is built with
> #define THIS_CPU_user_pcid_flush_mask \
> PER_CPU_VAR(cpu_tlbstate) + TLB_STATE_user_pcid_flush_mask
>
> +#define THIS_CPU_pti_disable \
> + PER_CPU_VAR(cpu_tlbstate) + TLB_STATE_pti_disable
> +
> .macro SWITCH_TO_USER_CR3_NOSTACK scratch_reg:req scratch_reg2:req
> ALTERNATIVE "jmp .Lend_\@", "", X86_FEATURE_PTI
> +
> + /*
> + * Do not switch on compatibility mode. If there is no need for a
> + * flush, run lfence to avoid speculative execution returning to user
> + * with the wrong CR3.
> + */

Nix the "compatibility mode" stuff please. Also, can someone confirm
whether the affected CPUs actually speculate through SYSRET? Because
your LFENCE might be so expensive that it negates a decent chunk of
the benefit.

> + /*
> + * Cached value of mm.pti_enable to simplify and speed up kernel entry
> + * code.
> + */
> + unsigned short pti_disable;

Why unsigned short?

IIRC a lot of CPUs use a slow path when decoding instructions with
16-bit operands like cmpw, so u8 or u32 could be waaaay faster than
u16.

> +/* Return whether page-table isolation is disabled on this CPU */
> +static inline unsigned short cpu_pti_disable(void)
> +{
> + return this_cpu_read(cpu_tlbstate.pti_disable);
> +}

This should return bool regardless of what type lives in the struct.

> - invalidate_user_asid(this_cpu_read(cpu_tlbstate.loaded_mm_asid));
> + if (!cpu_pti_disable())
> + invalidate_user_asid(this_cpu_read(cpu_tlbstate.loaded_mm_asid));

This will go badly wrong if pti_disable becomes dynamic. Can you just
leave the code as it was?

>
> /* If current->mm == NULL then the read_cr3() "borrows" an mm */
> native_write_cr3(__native_read_cr3());
> @@ -404,7 +417,7 @@ static inline void __native_flush_tlb_single(unsigned long addr)
>
> asm volatile("invlpg (%0)" ::"r" (addr) : "memory");
>
> - if (!static_cpu_has(X86_FEATURE_PTI))
> + if (!static_cpu_has(X86_FEATURE_PTI) || cpu_pti_disable())
> return;

Ditto.