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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: Dave 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: Will Deacon <will.deacon@arm.com>
This commit is contained in:
Dave Martin 2017-12-06 16:45:47 +00:00 committed by Will Deacon
parent d96cc49bff
commit cb968afc78
1 changed files with 16 additions and 12 deletions
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28
arch/arm64/kernel/fpsimd.c
View File

@ -114,7 +114,12 @@
* returned from the 2nd syscall yet, TIF_FOREIGN_FPSTATE is still set so * 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. * 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: */ /* Default VL for tasks that don't set it explicitly: */
static int sve_default_vl = -1; 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; 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()) && st->cpu == smp_processor_id())
clear_tsk_thread_flag(next, TIF_FOREIGN_FPSTATE); clear_tsk_thread_flag(next, TIF_FOREIGN_FPSTATE);
else else
@ -997,9 +1002,12 @@ void fpsimd_signal_preserve_current_state(void)
*/ */
static void fpsimd_bind_to_cpu(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; 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(); 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) 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 #ifdef CONFIG_ARM64_SVE
void sve_flush_cpu_state(void) void sve_flush_cpu_state(void)
{ {
struct fpsimd_state *const fpstate = __this_cpu_read(fpsimd_last_state); struct fpsimd_last_state_struct const *last =
struct task_struct *tsk; this_cpu_ptr(&fpsimd_last_state);
if (!fpstate) if (last->st && last->sve_in_use)
return;
tsk = container_of(fpstate, struct task_struct, thread.fpsimd_state);
if (test_tsk_thread_flag(tsk, TIF_SVE))
fpsimd_flush_cpu_state(); fpsimd_flush_cpu_state();
} }
#endif /* CONFIG_ARM64_SVE */ #endif /* CONFIG_ARM64_SVE */
@ -1272,7 +1276,7 @@ static inline void fpsimd_pm_init(void) { }
#ifdef CONFIG_HOTPLUG_CPU #ifdef CONFIG_HOTPLUG_CPU
static int fpsimd_cpu_dead(unsigned int 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; return 0;
} }