On Mon, Oct 12, 2020 at 11:03:33AM -0600, Khalid Aziz wrote: > On 10/10/20 5:09 AM, Catalin Marinas wrote: > > On Wed, Oct 07, 2020 at 02:14:09PM -0600, Khalid Aziz wrote: > >> On 10/7/20 1:39 AM, Jann Horn wrote: > >>> arch_validate_prot() is a hook that can validate whether a given set of > >>> protection flags is valid in an mprotect() operation. It is given the set > >>> of protection flags and the address being modified. > >>> > >>> However, the address being modified can currently not actually be used in > >>> a meaningful way because: > >>> > >>> 1. Only the address is given, but not the length, and the operation can > >>> span multiple VMAs. Therefore, the callee can't actually tell which > >>> virtual address range, or which VMAs, are being targeted. > >>> 2. The mmap_lock is not held, meaning that if the callee were to check > >>> the VMA at @addr, that VMA would be unrelated to the one the > >>> operation is performed on. > >>> > >>> Currently, custom arch_validate_prot() handlers are defined by > >>> arm64, powerpc and sparc. > >>> arm64 and powerpc don't care about the address range, they just check the > >>> flags against CPU support masks. > >>> sparc's arch_validate_prot() attempts to look at the VMA, but doesn't take > >>> the mmap_lock. > >>> > >>> Change the function signature to also take a length, and move the > >>> arch_validate_prot() call in mm/mprotect.c down into the locked region. > > [...] > >> As Chris pointed out, the call to arch_validate_prot() from do_mmap2() > >> is made without holding mmap_lock. Lock is not acquired until > >> vm_mmap_pgoff(). This variance is uncomfortable but I am more > >> uncomfortable forcing all implementations of validate_prot to require > >> mmap_lock be held when non-sparc implementations do not have such need > >> yet. Since do_mmap2() is in powerpc specific code, for now this patch > >> solves a current problem. > > > > I still think sparc should avoid walking the vmas in > > arch_validate_prot(). The core code already has the vmas, though not > > when calling arch_validate_prot(). That's one of the reasons I added > > arch_validate_flags() with the MTE patches. For sparc, this could be > > (untested, just copied the arch_validate_prot() code): > > I am little uncomfortable with the idea of validating protection bits > inside the VMA walk loop in do_mprotect_pkey(). When ADI is being > enabled across multiple VMAs and arch_validate_flags() fails on a VMA > later, do_mprotect_pkey() will bail out with error leaving ADI enabled > on earlier VMAs. This will apply to protection bits other than ADI as > well of course. This becomes a partial failure of mprotect() call. I > think it should be all or nothing with mprotect() - when one calls > mprotect() from userspace, either the entire address range passed in > gets its protection bits updated or none of it does. That requires > validating protection bits upfront or undoing what earlier iterations of > VMA walk loop might have done. I thought the same initially but mprotect() already does this with the VM_MAY* flag checking. If you ask it for an mprotect() that crosses multiple vmas and one of them fails, it doesn't roll back the changes to the prior ones. I considered that a similar approach is fine for MTE (it's most likely a user error). -- Catalin
