3.16.55-rc1 review patch. If anyone has any objections, please let me know.
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From: Dave Martin Dave.Martin@arm.com
commit 071b6d4a5d343046f253a5a8835d477d93992002 upstream.
Currently, loading of a task's fpsimd state into the CPU registers is skipped if that task's state is already present in the registers of that CPU.
However, the code relies on the struct fpsimd_state * (and by extension struct task_struct *) to unambiguously identify a task.
There is a particular case in which this doesn't work reliably: when a task exits, its task_struct may be recycled to describe a new task.
Consider the following scenario:
1) Task P loads its fpsimd state onto cpu C. per_cpu(fpsimd_last_state, C) := P; P->thread.fpsimd_state.cpu := C;
2) Task X is scheduled onto C and loads its fpsimd state on C. per_cpu(fpsimd_last_state, C) := X; X->thread.fpsimd_state.cpu := C;
3) X exits, causing X's task_struct to be freed.
4) P forks a new child T, which obtains X's recycled task_struct. T == X. T->thread.fpsimd_state.cpu == C (inherited from P).
5) T is scheduled on C. T's fpsimd state is not loaded, because per_cpu(fpsimd_last_state, C) == T (== X) && T->thread.fpsimd_state.cpu == C.
(This is the check performed by fpsimd_thread_switch().)
So, T gets X's registers because the last registers loaded onto C were those of X, in (2).
This patch fixes the problem by ensuring that the sched-in check fails in (5): fpsimd_flush_task_state(T) is called when T is forked, so that T->thread.fpsimd_state.cpu == C cannot be true. This relies on the fact that T is not schedulable until after copy_thread() completes.
Once T's fpsimd state has been loaded on some CPU C there may still be other cpus D for which per_cpu(fpsimd_last_state, D) == &X->thread.fpsimd_state. But D is necessarily != C in this case, and the check in (5) must fail.
An alternative fix would be to do refcounting on task_struct. This would result in each CPU holding a reference to the last task whose fpsimd state was loaded there. It's not clear whether this is preferable, and it involves higher overhead than the fix proposed in this patch. It would also move all the task_struct freeing work into the context switch critical section, or otherwise some deferred cleanup mechanism would need to be introduced, neither of which seems obviously justified.
Fixes: 005f78cd8849 ("arm64: defer reloading a task's FPSIMD state to userland resume") Signed-off-by: Dave Martin Dave.Martin@arm.com Reviewed-by: Ard Biesheuvel ard.biesheuvel@linaro.org [will: word-smithed the comment so it makes more sense] Signed-off-by: Will Deacon will.deacon@arm.com Signed-off-by: Ben Hutchings ben@decadent.org.uk --- arch/arm64/kernel/process.c | 9 +++++++++ 1 file changed, 9 insertions(+)
--- a/arch/arm64/kernel/process.c +++ b/arch/arm64/kernel/process.c @@ -269,6 +269,15 @@ int copy_thread(unsigned long clone_flag
memset(&p->thread.cpu_context, 0, sizeof(struct cpu_context));
+ /* + * In case p was allocated the same task_struct pointer as some + * other recently-exited task, make sure p is disassociated from + * any cpu that may have run that now-exited task recently. + * Otherwise we could erroneously skip reloading the FPSIMD + * registers for p. + */ + fpsimd_flush_task_state(p); + if (likely(!(p->flags & PF_KTHREAD))) { *childregs = *current_pt_regs(); childregs->regs[0] = 0;