Commit cb968afc authored by Dave Martin's avatar Dave Martin Committed by Will Deacon

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 <>
Cc: Ard Biesheuvel <>
Cc: Christoffer Dall <>
Cc: Marc Zyngier <>
Signed-off-by: default avatarWill Deacon <>
parent d96cc49b
......@@ -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( == st
&& st->cpu == smp_processor_id())
clear_tsk_thread_flag(next, TIF_FOREIGN_FPSTATE);
......@@ -997,9 +1002,12 @@ void fpsimd_signal_preserve_current_state(void)
static void fpsimd_bind_to_cpu(void)
struct fpsimd_last_state_struct *last =
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(, NULL);
......@@ -1070,14 +1078,10 @@ static inline void fpsimd_flush_cpu_state(void)
void sve_flush_cpu_state(void)
struct fpsimd_state *const fpstate = __this_cpu_read(fpsimd_last_state);
struct task_struct *tsk;
if (!fpstate)
struct fpsimd_last_state_struct const *last =
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)
#endif /* CONFIG_ARM64_SVE */
......@@ -1272,7 +1276,7 @@ static inline void fpsimd_pm_init(void) { }
static int fpsimd_cpu_dead(unsigned int cpu)
per_cpu(fpsimd_last_state, cpu) = NULL;
per_cpu(, cpu) = NULL;
return 0;
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