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x86/mm: Refactor cond_ibpb() to support other use cases

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cond_ibpb() has the necessary bits required to track the previous mm in
switch_mm_irqs_off(). This can be reused for other use cases like L1D
flushing on context switch.

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Balbir Singh <sblbir@amazon.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20210108121056.21940-3-sblbir@amazon.com
This commit is contained in:
Balbir Singh 2021-01-08 23:10:53 +11:00 committed by Thomas Gleixner
parent c52787b590
commit 371b09c6fd
2 changed files with 30 additions and 25 deletions
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2
arch/x86/include/asm/tlbflush.h
View File

@ -83,7 +83,7 @@ struct tlb_state {
/* Last user mm for optimizing IBPB */
union {
struct mm_struct *last_user_mm;
unsigned long last_user_mm_ibpb;
unsigned long last_user_mm_spec;
};
u16 loaded_mm_asid;

53
arch/x86/mm/tlb.c
View File

@ -43,10 +43,14 @@
*/
/*
* Use bit 0 to mangle the TIF_SPEC_IB state into the mm pointer which is
* stored in cpu_tlb_state.last_user_mm_ibpb.
* Bits to mangle the TIF_SPEC_IB state into the mm pointer which is
* stored in cpu_tlb_state.last_user_mm_spec.
*/
#define LAST_USER_MM_IBPB 0x1UL
#define LAST_USER_MM_SPEC_MASK (LAST_USER_MM_IBPB)
/* Bits to set when tlbstate and flush is (re)initialized */
#define LAST_USER_MM_INIT LAST_USER_MM_IBPB
/*
* The x86 feature is called PCID (Process Context IDentifier). It is similar
@ -317,20 +321,29 @@ void switch_mm(struct mm_struct *prev, struct mm_struct *next,
local_irq_restore(flags);
}
static unsigned long mm_mangle_tif_spec_ib(struct task_struct *next)
static unsigned long mm_mangle_tif_spec_bits(struct task_struct *next)
{
unsigned long next_tif = task_thread_info(next)->flags;
unsigned long ibpb = (next_tif >> TIF_SPEC_IB) & LAST_USER_MM_IBPB;
unsigned long spec_bits = (next_tif >> TIF_SPEC_IB) & LAST_USER_MM_SPEC_MASK;
return (unsigned long)next->mm | ibpb;
return (unsigned long)next->mm | spec_bits;
}
static void cond_ibpb(struct task_struct *next)
static void cond_mitigation(struct task_struct *next)
{
unsigned long prev_mm, next_mm;
if (!next || !next->mm)
return;
next_mm = mm_mangle_tif_spec_bits(next);
prev_mm = this_cpu_read(cpu_tlbstate.last_user_mm_spec);
/*
* Avoid user/user BTB poisoning by flushing the branch predictor
* when switching between processes. This stops one process from
* doing Spectre-v2 attacks on another.
*
* Both, the conditional and the always IBPB mode use the mm
* pointer to avoid the IBPB when switching between tasks of the
* same process. Using the mm pointer instead of mm->context.ctx_id
@ -340,8 +353,6 @@ static void cond_ibpb(struct task_struct *next)
* exposed data is not really interesting.
*/
if (static_branch_likely(&switch_mm_cond_ibpb)) {
unsigned long prev_mm, next_mm;
/*
* This is a bit more complex than the always mode because
* it has to handle two cases:
@ -371,20 +382,14 @@ static void cond_ibpb(struct task_struct *next)
* Optimize this with reasonably small overhead for the
* above cases. Mangle the TIF_SPEC_IB bit into the mm
* pointer of the incoming task which is stored in
* cpu_tlbstate.last_user_mm_ibpb for comparison.
*/
next_mm = mm_mangle_tif_spec_ib(next);
prev_mm = this_cpu_read(cpu_tlbstate.last_user_mm_ibpb);
/*
* cpu_tlbstate.last_user_mm_spec for comparison.
*
* Issue IBPB only if the mm's are different and one or
* both have the IBPB bit set.
*/
if (next_mm != prev_mm &&
(next_mm | prev_mm) & LAST_USER_MM_IBPB)
indirect_branch_prediction_barrier();
this_cpu_write(cpu_tlbstate.last_user_mm_ibpb, next_mm);
}
if (static_branch_unlikely(&switch_mm_always_ibpb)) {
@ -393,11 +398,12 @@ static void cond_ibpb(struct task_struct *next)
* different context than the user space task which ran
* last on this CPU.
*/
if (this_cpu_read(cpu_tlbstate.last_user_mm) != next->mm) {
if ((prev_mm & ~LAST_USER_MM_SPEC_MASK) !=
(unsigned long)next->mm)
indirect_branch_prediction_barrier();
this_cpu_write(cpu_tlbstate.last_user_mm, next->mm);
}
}
this_cpu_write(cpu_tlbstate.last_user_mm_spec, next_mm);
}
#ifdef CONFIG_PERF_EVENTS
@ -531,11 +537,10 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
need_flush = true;
} else {
/*
* Avoid user/user BTB poisoning by flushing the branch
* predictor when switching between processes. This stops
* one process from doing Spectre-v2 attacks on another.
* Apply process to process speculation vulnerability
* mitigations if applicable.
*/
cond_ibpb(tsk);
cond_mitigation(tsk);
/*
* Stop remote flushes for the previous mm.
@ -643,7 +648,7 @@ void initialize_tlbstate_and_flush(void)
write_cr3(build_cr3(mm->pgd, 0));
/* Reinitialize tlbstate. */
this_cpu_write(cpu_tlbstate.last_user_mm_ibpb, LAST_USER_MM_IBPB);
this_cpu_write(cpu_tlbstate.last_user_mm_spec, LAST_USER_MM_INIT);
this_cpu_write(cpu_tlbstate.loaded_mm_asid, 0);
this_cpu_write(cpu_tlbstate.next_asid, 1);
this_cpu_write(cpu_tlbstate.ctxs[0].ctx_id, mm->context.ctx_id);