shmem: fix recovery on rename failures

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: shmem: fix recovery on rename failures maple_tree insertions can fail if we are seriously short on memory; simple_offset_rename() does not recover well if it runs into that. The same goes for simple_offset_rename_exchange(). Moreover, shmem_whiteout() expects that if it succeeds, the caller will progress to d_move(), i.e. that shmem_rename2() won't fail past the successful call of shmem_whiteout(). Not hard to fix, fortunately - mtree_store() can't fail if the index we are trying to store into is already present in the tree as a singleton. For simple_offset_rename_exchange() that's enough - we just need to be careful about the order of operations. For simple_offset_rename() solution is to preinsert the target into the tree for new_dir; the rest can be done without any potentially failing operations. That preinsertion has to be done in shmem_rename2() rather than in simple_offset_rename() itself - otherwise we'd need to deal with the possibility of failure after successful shmem_whiteout().

References

• https://git.kernel.org/stable/c/4b0fe71fb3965d0db83cdfc2f4fe0b3227d70113
• https://git.kernel.org/stable/c/4642686699a46718d7f2fb5acd1e9d866a9d9cca
• https://git.kernel.org/stable/c/e1b4c6a58304fd490124cc2b454d80edc786665c