linux-kernel - Re: [PATCH 4.9 65/93] sparc64: Prevent perf from running during super critical sections

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Date:   Thu, 10 Aug 2017 09:20:03 -0700
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     stable@...r.kernel.org,
        Dave Aldridge <david.j.aldridge@...cle.com>,
        Rob Gardner <rob.gardner@...cle.com>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH 4.9 65/93] sparc64: Prevent perf from running during
 super critical sections

On Wed, Aug 09, 2017 at 11:13:58AM -0700, Greg Kroah-Hartman wrote:
> 4.9-stable review patch.  If anyone has any objections, please let me know.
> 
> ------------------
> 
> From: Rob Gardner <rob.gardner@...cle.com>
> 
> 
> [ Upstream commit fc290a114fc6034b0f6a5a46e2fb7d54976cf87a ]
> 
> This fixes another cause of random segfaults and bus errors that may
> occur while running perf with the callgraph option.
> 
> Critical sections beginning with spin_lock_irqsave() raise the interrupt
> level to PIL_NORMAL_MAX (14) and intentionally do not block performance
> counter interrupts, which arrive at PIL_NMI (15).
> 
> But some sections of code are "super critical" with respect to perf
> because the perf_callchain_user() path accesses user space and may cause
> TLB activity as well as faults as it unwinds the user stack.
> 
> One particular critical section occurs in switch_mm:
> 
>         spin_lock_irqsave(&mm->context.lock, flags);
>         ...
>         load_secondary_context(mm);
>         tsb_context_switch(mm);
>         ...
>         spin_unlock_irqrestore(&mm->context.lock, flags);
> 
> If a perf interrupt arrives in between load_secondary_context() and
> tsb_context_switch(), then perf_callchain_user() could execute with
> the context ID of one process, but with an active TSB for a different
> process. When the user stack is accessed, it is very likely to
> incur a TLB miss, since the h/w context ID has been changed. The TLB
> will then be reloaded with a translation from the TSB for one process,
> but using a context ID for another process. This exposes memory from
> one process to another, and since it is a mapping for stack memory,
> this usually causes the new process to crash quickly.
> 
> This super critical section needs more protection than is provided
> by spin_lock_irqsave() since perf interrupts must not be allowed in.
> 
> Since __tsb_context_switch already goes through the trouble of
> disabling interrupts completely, we fix this by moving the secondary
> context load down into this better protected region.
> 
> Orabug: 25577560
> 
> Signed-off-by: Dave Aldridge <david.j.aldridge@...cle.com>
> Signed-off-by: Rob Gardner <rob.gardner@...cle.com>
> Signed-off-by: David S. Miller <davem@...emloft.net>
> Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> ---
>  arch/sparc/include/asm/mmu_context_64.h |   12 +++++++-----
>  arch/sparc/kernel/tsb.S                 |   12 ++++++++++++
>  arch/sparc/power/hibernate.c            |    3 +--
>  3 files changed, 20 insertions(+), 7 deletions(-)
> 
> --- a/arch/sparc/include/asm/mmu_context_64.h
> +++ b/arch/sparc/include/asm/mmu_context_64.h
> @@ -25,9 +25,11 @@ void destroy_context(struct mm_struct *m
>  void __tsb_context_switch(unsigned long pgd_pa,
>  			  struct tsb_config *tsb_base,
>  			  struct tsb_config *tsb_huge,
> -			  unsigned long tsb_descr_pa);
> +			  unsigned long tsb_descr_pa,
> +			  unsigned long secondary_ctx);
>  
> -static inline void tsb_context_switch(struct mm_struct *mm)
> +static inline void tsb_context_switch_ctx(struct mm_struct *mm,
> +					  unsigned long ctx)
>  {
>  	__tsb_context_switch(__pa(mm->pgd),
>  			     &mm->context.tsb_block[0],
> @@ -38,7 +40,8 @@ static inline void tsb_context_switch(st
>  #else
>  			     NULL
>  #endif
> -			     , __pa(&mm->context.tsb_descr[0]));
> +			     , __pa(&mm->context.tsb_descr[0]),
> +			     ctx);
>  }
>  
>  void tsb_grow(struct mm_struct *mm,
> @@ -110,8 +113,7 @@ static inline void switch_mm(struct mm_s
>  	 * cpu0 to update it's TSB because at that point the cpu_vm_mask
>  	 * only had cpu1 set in it.
>  	 */
> -	load_secondary_context(mm);
> -	tsb_context_switch(mm);
> +	tsb_context_switch_ctx(mm, CTX_HWBITS(mm->context));
>  
>  	/* Any time a processor runs a context on an address space
>  	 * for the first time, we must flush that context out of the
> --- a/arch/sparc/kernel/tsb.S
> +++ b/arch/sparc/kernel/tsb.S
> @@ -375,6 +375,7 @@ tsb_flush:
>  	 * %o1:	TSB base config pointer
>  	 * %o2:	TSB huge config pointer, or NULL if none
>  	 * %o3:	Hypervisor TSB descriptor physical address
> +	 * %o4: Secondary context to load, if non-zero
>  	 *
>  	 * We have to run this whole thing with interrupts
>  	 * disabled so that the current cpu doesn't change
> @@ -387,6 +388,17 @@ __tsb_context_switch:
>  	rdpr	%pstate, %g1
>  	wrpr	%g1, PSTATE_IE, %pstate
>  
> +	brz,pn	%o4, 1f
> +	 mov	SECONDARY_CONTEXT, %o5
> +
> +661:	stxa	%o4, [%o5] ASI_DMMU
> +	.section .sun4v_1insn_patch, "ax"
> +	.word	661b
> +	stxa	%o4, [%o5] ASI_MMU
> +	.previous
> +	flush	%g6
> +
> +1:
>  	TRAP_LOAD_TRAP_BLOCK(%g2, %g3)
>  
>  	stx	%o0, [%g2 + TRAP_PER_CPU_PGD_PADDR]
> --- a/arch/sparc/power/hibernate.c
> +++ b/arch/sparc/power/hibernate.c
> @@ -35,6 +35,5 @@ void restore_processor_state(void)
>  {
>  	struct mm_struct *mm = current->active_mm;
>  
> -	load_secondary_context(mm);
> -	tsb_context_switch(mm);
> +	tsb_context_switch_ctx(mm, CTX_HWBITS(mm->context));
>  }
> 

This seems to break the build, so I'm dropping it from the patch set.

thanks,

greg k-h