Re: [RFC PATCH 1/1] mm/percpu.c: fix potential memory leakage for pcpu_embed_first_chunk()

On 2016/9/30 0:44, Tejun Heo wrote:

Hello,

On Fri, Sep 30, 2016 at 12:03:20AM +0800, zijun_hu wrote:

quoted

From: zijun_hu <redacted>

it will cause memory leakage for pcpu_embed_first_chunk() to go to
label @out_free if the chunk spans over 3/4 VMALLOC area. all memory
are allocated and recorded into array @areas for each CPU group, but
the memory allocated aren't be freed before returning after going to
label @out_free

in order to fix this bug, we check chunk spanned area immediately
after completing memory allocation for all CPU group, we go to label
@out_free_areas other than @out_free to free all memory allocated if
the checking is failed.

Signed-off-by: zijun_hu <redacted>

...

quoted

@@ -2000,6 +2001,21 @@ int __init pcpu_embed_first_chunk(size_t reserved_size, size_t dyn_size,
 		areas[group] = ptr;
 
 		base = min(ptr, base);
+		if (ptr > areas[j])
+			j = group;
+	}
+	max_distance = areas[j] - base;
+	max_distance += ai->unit_size * ai->groups[j].nr_units;
+
+	/* warn if maximum distance is further than 75% of vmalloc space */
+	if (max_distance > VMALLOC_TOTAL * 3 / 4) {
+		pr_warn("max_distance=0x%lx too large for vmalloc space 0x%lx\n",
+				max_distance, VMALLOC_TOTAL);
+#ifdef CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK
+		/* and fail if we have fallback */
+		rc = -EINVAL;
+		goto out_free_areas;
+#endif

Isn't it way simpler to make the error path jump to out_free_areas?
There's another similar case after pcpu_setup_first_chunk() failure
too.  Also, can you please explain how you tested the changes?

Thanks.

1) the simpler way don't work because it maybe free many memory block twice

let us take a CPU group as a example, after we allocate All memory
needed by a CPU group, we maybe free a unit memory block which
don't map to a available CPU, we maybe free a part of unit memory which 
we don't used too, you can refer to following code segments for detailed
info.
for (group = 0; group < ai->nr_groups; group++) {
	struct pcpu_group_info *gi = &ai->groups[group];
	void *ptr = areas[group];

	for (i = 0; i < gi->nr_units; i++, ptr += ai->unit_size) {
	if (gi->cpu_map[i] == NR_CPUS) {
				/* unused unit, free whole */
				free_fn(ptr, ai->unit_size);
				continue;
	}
	/* copy and return the unused part */
	memcpy(ptr, __per_cpu_load, ai->static_size);
	free_fn(ptr + size_sum, ai->unit_size - size_sum);
	}
}

2) as we seen, pcpu_setup_first_chunk() doesn't cause a failure, it  return 0
   always or panic by BUG_ON(), even if it fails, we can conclude the allocated
   memory based on information recorded by it, such as pcpu_base_addr and many of
   static variable, we can complete the free operations; but we can't if we
   fail in the case pointed by this patch

3) my test way is simple, i force "if (max_distance > VMALLOC_TOTAL * 3 / 4)"
   to if (1) and print which memory i allocate before the jumping, then print which memory
   i free after the jumping and before returning, then check whether i free the memory i 
   allocate in this function, the result is okay

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