Re: [PATCH] regmap: Fix possible double-free in regcache_rbtree_exit()

[Mark Brown]

On Mon, Oct 11, 2021 at 09:55:26PM +0800, Yang Yingliang wrote:
> In regcache_rbtree_insert_to_block(), when 'present' realloc failed,
> the 'blk' which is supposed to assign to 'rbnode->block' will be freed,
> so 'rbnode->block' points a freed memory, in the error handling path of
> regcache_rbtree_init(), 'rbnode->block' will be freed again in
> regcache_rbtree_exit(), KASAN will report double-free as follows:
> 
> BUG: KASAN: double-free or invalid-free in kfree+0xce/0x390
> Call Trace:
>  dump_stack_lvl+0xe2/0x152
>  print_address_description.constprop.7+0x21/0x150
>  kasan_report_invalid_free+0x6f/0xa0
>  __kasan_slab_free+0x125/0x140

Please think hard before including complete backtraces in upstream
reports, they are very large and contain almost no useful information
relative to their size so often obscure the relevant content in your
message. If part of the backtrace is usefully illustrative (it often is
for search engines if nothing else) then it's usually better to pull out
the relevant sections.

> Set rbnode->block to NULL when the 'present' realloc failed to fix this.

This is not a good fix, it will both leak block and corrupt the data
structure since now there's a NULL pointer where there should be a data
block.  We should instead be moving the assignment of rbnode->block up
to immediately after the reallocation has succeeded so that the data
structure stays valid even if the second reallocation fails.

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