On Thu, May 17, 2018 at 8:03 AM, Amir Goldstein <amir73il@xxxxxxxxx> wrote: > On Tue, May 15, 2018 at 4:37 PM, Amir Goldstein <amir73il@xxxxxxxxx> wrote: >> On Tue, May 15, 2018 at 4:23 PM, Vivek Goyal <vgoyal@xxxxxxxxxx> wrote: >>> On Tue, May 15, 2018 at 01:26:09PM +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 patch fixes the race, by using insert_inode_locked4() to add >>>> a newly created inode to icache. >>>> >>>> If the newly created inode apears to already exist in icache (hashed >>>> by the same real upper inode), we export this error to user instead >>>> of silently not hashing the new inode. >>> >>> So we might return an error to user saying operation failed, but still >>> create file on upper. Does that sound little odd? >>> >> >> Yes, but I don't see a better solution. >> >>> I am wondering why can't we call ovl_get_inode() in object creation >>> path. That should take care of race between creation path and file >>> handle decode and only one of the paths will get to instantiate and >>> initialize ovl_inode and other path will wait. >>> >> >> I don't even want to think if what you wrote makes sense. >> Remember that the use case we are talking about is quite imaginary. >> Ensuring internal structures consistency in our code and returning >> error to user is the right thing to do for imaginary use cases IMO. >> > > Having being forced to think about it ;-), I think using ovl_get_inode() > in create code does make a weird sort of sense. Going through the same code-path very much makes sense. > The reason it is weird is because we will always be throwing away > the new inode that we allocated in ovl_create_object(). > AFAICS, if only reason we need to allocate new inode in > ovl_create_object() is to calculate i_mode with inode_init_owner() > and that calculation can be factored out to not need an inode. Not the only reason: we don't want inode allocation to fail after successful creation. Solution: add a preallocated inode argument to ovl_get_inode() and deal with allocation failure there. > The reason it does makes sense to use ovl_get_inode(), is because > the alternative that Miklos suggested is to insert the new inode on > the very very likely use case and throw away the new inode in the > very very unlikely use case of collision. That alternative creates a > somewhat complicated code path that is rarely to never executed - > not the best practice. Agreed completely. Thanks, Miklos
