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Commit cb968afc authored by Dave Martin's avatar Dave Martin Committed by Will Deacon
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arm64/sve: Avoid dereference of dead task_struct in KVM guest entry 



When deciding whether to invalidate FPSIMD state cached in the cpu,
the backend function sve_flush_cpu_state() attempts to dereference
__this_cpu_read(fpsimd_last_state).  However, this is not safe:
there is no guarantee that this task_struct pointer is still valid,
because the task could have exited in the meantime.

This means that we need another means to get the appropriate value
of TIF_SVE for the associated task.

This patch solves this issue by adding a cached copy of the TIF_SVE
flag in fpsimd_last_state, which we can check without dereferencing
the task pointer.

In particular, although this patch is not a KVM fix per se, this
means that this check is now done safely in the KVM world switch
path (which is currently the only user of this code).

Signed-off-by: default avatarDave Martin <Dave.Martin@arm.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Christoffer Dall <christoffer.dall@linaro.org>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: default avatarWill Deacon <will.deacon@arm.com>
parent d96cc49b

arch/arm64/kernel/fpsimd.c

+16 −12
Original line number Diff line number Diff line
@@ -114,7 +114,12 @@ 
 *   returned from the 2nd syscall yet, TIF_FOREIGN_FPSTATE is still set so

 *   whatever is in the FPSIMD registers is not saved to memory, but discarded.

 */

static DEFINE_PER_CPU(struct fpsimd_state *, fpsimd_last_state);

struct fpsimd_last_state_struct {

	struct fpsimd_state *st;

	bool sve_in_use;

};



static DEFINE_PER_CPU(struct fpsimd_last_state_struct, fpsimd_last_state);



/* Default VL for tasks that don't set it explicitly: */

static int sve_default_vl = -1;
@@ -905,7 +910,7 @@ void fpsimd_thread_switch(struct task_struct *next) 
		 */

		struct fpsimd_state *st = &next->thread.fpsimd_state;



		if (__this_cpu_read(fpsimd_last_state) == st

		if (__this_cpu_read(fpsimd_last_state.st) == st

		    && st->cpu == smp_processor_id())

			clear_tsk_thread_flag(next, TIF_FOREIGN_FPSTATE);

		else
@@ -997,9 +1002,12 @@ void fpsimd_signal_preserve_current_state(void) 
 */

static void fpsimd_bind_to_cpu(void)

{

	struct fpsimd_last_state_struct *last =

		this_cpu_ptr(&fpsimd_last_state);

	struct fpsimd_state *st = &current->thread.fpsimd_state;



	__this_cpu_write(fpsimd_last_state, st);

	last->st = st;

	last->sve_in_use = test_thread_flag(TIF_SVE);

	st->cpu = smp_processor_id();

}
@@ -1057,7 +1065,7 @@ void fpsimd_flush_task_state(struct task_struct *t) 


static inline void fpsimd_flush_cpu_state(void)

{

	__this_cpu_write(fpsimd_last_state, NULL);

	__this_cpu_write(fpsimd_last_state.st, NULL);

}



/*
@@ -1070,14 +1078,10 @@ static inline void fpsimd_flush_cpu_state(void) 
#ifdef CONFIG_ARM64_SVE

void sve_flush_cpu_state(void)

{

	struct fpsimd_state *const fpstate = __this_cpu_read(fpsimd_last_state);

	struct task_struct *tsk;



	if (!fpstate)

		return;

	struct fpsimd_last_state_struct const *last =

		this_cpu_ptr(&fpsimd_last_state);



	tsk = container_of(fpstate, struct task_struct, thread.fpsimd_state);

	if (test_tsk_thread_flag(tsk, TIF_SVE))

	if (last->st && last->sve_in_use)

		fpsimd_flush_cpu_state();

}

#endif /* CONFIG_ARM64_SVE */
@@ -1272,7 +1276,7 @@ static inline void fpsimd_pm_init(void) { } 
#ifdef CONFIG_HOTPLUG_CPU

static int fpsimd_cpu_dead(unsigned int cpu)

{

	per_cpu(fpsimd_last_state, cpu) = NULL;

	per_cpu(fpsimd_last_state.st, cpu) = NULL;

	return 0;

}