SPARC version of arch_validate_prot() looks broken (UAF read)

[Jann Horn]

>From what I can tell from looking at the code:

SPARC's arch_validate_prot() looks up the VMA and peeks at it; that's
not permitted though. do_mprotect_pkey() calls arch_validate_prot()
before taking the mmap lock, so we can hit use-after-free reads if
someone concurrently deletes a VMA we're looking at.

Additionally, arch_validate_prot() currently only accepts the start
address as a parameter, but the SPARC code probably should be checking
the entire given range, which might consist of multiple VMAs?

I'm not sure what the best fix is here; it kinda seems like what SPARC
really wants is a separate hook that is called from inside the loop in
do_mprotect_pkey() that iterates over the VMAs? So maybe commit
9035cf9a97e4 ("mm: Add address parameter to arch_validate_prot()")
should be reverted, and a separate hook should be created?

(Luckily the ordering of the vmacache operations works out such that
AFAICS, despite calling find_vma() without holding the mmap_sem, we
can never end up establishing a vmacache entry with a dangling pointer
that might be considered valid on a subsequent call. So this should be
limited to a rather boring UAF data read, and not be exploitable for a
UAF write or UAF function pointer read.)