[Intel-gfx] [PATCH] drm/i915: Protect debugfs per_file_stats with RCU lock

[David Weinehall]

On Tue, Sep 03, 2019 at 07:21:33AM +0100, Chris Wilson wrote:
> If we make sure we grab a strong reference to each object as we dump it,
> we can reduce the locks outside of our iterators to an rcu_read_lock.
> 
> This should prevent errors like:
> [ 2138.371911] BUG: KASAN: use-after-free in per_file_stats+0x43/0x380 [i915]
> [ 2138.371924] Read of size 8 at addr ffff888223651000 by task cat/8293
> 
> [ 2138.371947] CPU: 0 PID: 8293 Comm: cat Not tainted 5.3.0-rc6-CI-Custom_4352+ #1
> [ 2138.371953] Hardware name: To Be Filled By O.E.M. To Be Filled By O.E.M./J4205-ITX, BIOS P1.40 07/14/2017
> [ 2138.371959] Call Trace:
> [ 2138.371974]  dump_stack+0x7c/0xbb
> [ 2138.372099]  ? per_file_stats+0x43/0x380 [i915]
> [ 2138.372108]  print_address_description+0x73/0x3a0
> [ 2138.372231]  ? per_file_stats+0x43/0x380 [i915]
> [ 2138.372352]  ? per_file_stats+0x43/0x380 [i915]
> [ 2138.372362]  __kasan_report+0x14e/0x192
> [ 2138.372489]  ? per_file_stats+0x43/0x380 [i915]
> [ 2138.372502]  kasan_report+0xe/0x20
> [ 2138.372625]  per_file_stats+0x43/0x380 [i915]
> [ 2138.372751]  ? i915_panel_show+0x110/0x110 [i915]
> [ 2138.372761]  idr_for_each+0xa7/0x160
> [ 2138.372773]  ? idr_get_next_ul+0x110/0x110
> [ 2138.372782]  ? do_raw_spin_lock+0x10a/0x1d0
> [ 2138.372923]  print_context_stats+0x264/0x510 [i915]
> 
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>

Tested-by: David Weinehall <david.weinehall at linux.intel.com>

> ---
>  drivers/gpu/drm/i915/i915_debugfs.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_debugfs.c b/drivers/gpu/drm/i915/i915_debugfs.c
> index 9798f27a697a..708855e051b5 100644
> --- a/drivers/gpu/drm/i915/i915_debugfs.c
> +++ b/drivers/gpu/drm/i915/i915_debugfs.c
> @@ -237,6 +237,9 @@ static int per_file_stats(int id, void *ptr, void *data)
>  	struct file_stats *stats = data;
>  	struct i915_vma *vma;
>  
> +	if (!kref_get_unless_zero(&obj->base.refcount))
> +		return 0;
> +
>  	stats->count++;
>  	stats->total += obj->base.size;
>  	if (!atomic_read(&obj->bind_count))
> @@ -284,6 +287,7 @@ static int per_file_stats(int id, void *ptr, void *data)
>  	}
>  	spin_unlock(&obj->vma.lock);
>  
> +	i915_gem_object_put(obj);
>  	return 0;
>  }
>  
> @@ -313,10 +317,12 @@ static void print_context_stats(struct seq_file *m,
>  				    i915_gem_context_lock_engines(ctx), it) {
>  			intel_context_lock_pinned(ce);
>  			if (intel_context_is_pinned(ce)) {
> +				rcu_read_lock();
>  				if (ce->state)
>  					per_file_stats(0,
>  						       ce->state->obj, &kstats);
>  				per_file_stats(0, ce->ring->vma->obj, &kstats);
> +				rcu_read_unlock();
>  			}
>  			intel_context_unlock_pinned(ce);
>  		}
> @@ -328,9 +334,9 @@ static void print_context_stats(struct seq_file *m,
>  			struct task_struct *task;
>  			char name[80];
>  
> -			spin_lock(&file->table_lock);
> +			rcu_read_lock();
>  			idr_for_each(&file->object_idr, per_file_stats, &stats);
> -			spin_unlock(&file->table_lock);
> +			rcu_read_unlock();
>  
>  			rcu_read_lock();
>  			task = pid_task(ctx->pid ?: file->pid, PIDTYPE_PID);
> -- 
> 2.23.0
> 
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