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    SubjectRe: [PATCH 1/2] mm/mprotect: Call arch_validate_prot under mmap_lock and with length
    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

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