On Fri, May 18, 2018 at 5:36 PM, Amir Goldstein <amir73il@xxxxxxxxx> wrote:
> On Fri, May 18, 2018 at 6:11 PM, Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
>> On Fri, May 18, 2018 at 5:03 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
>>> On Fri, May 18, 2018 at 11:29:37AM +0300, Amir Goldstein wrote:
>>>> Currently, there is a small window where ovl_obtain_alias() can
>>>> race with ovl_instantiate() and create two different overlay inodes
>>>> with the same underlying real non-dir non-hardlink inode.
>>>>
>>>> The race requires an adversary to guess the file handle of the
>>>> yet to be created upper inode and decode the guessed file handle
>>>> after ovl_creat_real(), but before ovl_instantiate().
>>>> This race does not affect overlay directory inodes, because those
>>>> are decoded via ovl_lookup_real() and not with ovl_obtain_alias().
>>>>
>>>> This patch fixes the race, by using iget5_prealloc() to add a newly
>>>> created inode to cache.
>>>
>>> Mind explaining what the hell is wrong with insert_inode_locked4()?
>>
>> That it doesn't return the old inode if found.
>>
>
> FYI, I have set a side a version I was working on before iget5_prealloc()
> that uses insert_inode_locked5 (runner up for ugliest function name):
>
> +int insert_inode_locked4(struct inode *inode, unsigned long hashval,
> + int (*test)(struct inode *, void *), void *data)
> +{
> + struct inode *old = insert_inode_locked5(inode, hashval, test, data);
>
> + if (old) {
> + iput(old);
> + return -EBUSY;
> + }
> +
> + return 0;
> +}
> +EXPORT_SYMBOL(insert_inode_locked4);
Can do exact same thing with iget5_prealloc(), just need to move
inode_sb_list_add() out to iget5_locked() (meaning, overlayfs can
continue to use new_inode()/iput() instead of having to do
alloc/destroy_inode()).
Thanks,
Miklos
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