Re: [PATCH v15 05/17] arms64: untag user pointers passed to memory syscalls

Catalin Marinas
2019-05-23T09:04:40Z

On Wed, May 22, 2019 at 02:16:57PM -0700, Evgenii Stepanov wrote:
> On Wed, May 22, 2019 at 4:49 AM Catalin Marinas <catalin.marinas@arm.com> wrote:
> > On Mon, May 06, 2019 at 06:30:51PM +0200, Andrey Konovalov wrote:
> > > This patch is a part of a series that extends arm64 kernel ABI to allow to
> > > pass tagged user pointers (with the top byte set to something else other
> > > than 0x00) as syscall arguments.
> > >
> > > This patch allows tagged pointers to be passed to the following memory
> > > syscalls: brk, get_mempolicy, madvise, mbind, mincore, mlock, mlock2,
> > > mmap, mmap_pgoff, mprotect, mremap, msync, munlock, munmap,
> > > remap_file_pages, shmat and shmdt.
> > >
> > > This is done by untagging pointers passed to these syscalls in the
> > > prologues of their handlers.
> >
> > I'll go through them one by one to see if we can tighten the expected
> > ABI while having the MTE in mind.
> >
> > > diff --git a/arch/arm64/kernel/sys.c b/arch/arm64/kernel/sys.c
> > > index b44065fb1616..933bb9f3d6ec 100644
> > > --- a/arch/arm64/kernel/sys.c
> > > +++ b/arch/arm64/kernel/sys.c
> > > @@ -35,10 +35,33 @@ SYSCALL_DEFINE6(mmap, unsigned long, addr, unsigned long, len,
> > > {
> > > if (offset_in_page(off) != 0)
> > > return -EINVAL;
> > > -
> > > + addr = untagged_addr(addr);
> > > return ksys_mmap_pgoff(addr, len, prot, flags, fd, off >> PAGE_SHIFT);
> > > }
> >
> > If user passes a tagged pointer to mmap() and the address is honoured
> > (or MAP_FIXED is given), what is the expected return pointer? Does it
> > need to be tagged with the value from the hint?
>
> For HWASan the most convenient would be to use the tag from the hint.
> But since in the TBI (not MTE) mode the kernel has no idea what
> meaning userspace assigns to pointer tags, perhaps it should not try
> to guess, and should return raw (zero-tagged) address instead.

Then, just to relax the ABI for hwasan, shall we simply disallow tagged
pointers on mmap() arguments? We can leave them in for
mremap(old_address), madvise().

> > With MTE, we may want to use this as a request for the default colour of
> > the mapped pages (still under discussion).
>
> I like this - and in that case it would make sense to return the
> pointer that can be immediately dereferenced without crashing the
> process, i.e. with the matching tag.

This came up from the Android investigation work where large memory
allocations (using mmap) could be more efficiently pre-tagged by the
kernel on page fault. Not sure about the implementation details yet.

--
Catalin