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Date: Wed, 17 Apr 2024 14:31:13 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Oscar Salvador <osalvador@suse.de>
Cc: Matthew Wilcox <willy@infradead.org>, Vishal Moola
 <vishal.moola@gmail.com>, syzbot
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Subject: Re: [syzbot] [mm?] KASAN: slab-use-after-free Read in
 __vma_reservation_common
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On Tue, 16 Apr 2024 10:13:31 +0200 Oscar Salvador <osalvador@suse.de> wrote:

> On Mon, Apr 15, 2024 at 11:15:03PM +0100, Matthew Wilcox wrote:
> > On Mon, Apr 15, 2024 at 03:05:44PM -0700, Vishal Moola wrote:
> > > Commit 9acad7ba3e25 ("hugetlb: use vmf_anon_prepare() instead of
> > > anon_vma_prepare()") may bailout after allocating a folio if we do not
> > > hold the mmap lock. When this occurs, vmf_anon_prepare() will release the
> > > vma lock. Hugetlb then attempts to call restore_reserve_on_error(),
> > > which depends on the vma lock being held.
> > > 
> > > We can move vmf_anon_prepare() prior to the folio allocation in order to
> > > avoid calling restore_reserve_on_error() without the vma lock.
> > 
> > But now you're calling vmf_anon_prepare() in the wrong place -- before
> > we've determined that we need an anon folio.  So we'll create an
> > anon_vma even when we don't need one for this vma.
> > 
> > This is definitely a pre-existing bug which you've exposed by making it
> > happen more easily.  Needs a different fix though.
> 
> I do not think this is a pre-existing bug.
> Prior to 'commit: 7c43a553792a ("hugetlb: allow faults to be handled under
> the VMA lock"), we would just bail out if we had FAULT_FLAG_VMA_LOCK.
> So there was no danger in calling functions that fiddle with vmas like
> restore_reserve_on_error() does.
> After that, we allow it but vmf_anon_prepare() releases the lock and returns
> VM_FAULT_RETRY if we really need to allocate an anon_vma.
> The problem is that now restore_reserve_on_error() will re-adjust the
> reservations without the vma lock, completely unsafe.
> 
> I think the safest way to tackle this is just as Vishal did, call
> vmf_anon_prepare() upfront only for non VM_MAYSHARE faults.

Thanks.  I didn't apply anything at this stage, because this patch
appears to be against linux-next/mm-unstable whereas for a -stable
backportable thing it would best be against current -linus.

So can we please sort out a suitable Fixes:, redo the patch against
current mainline, add the cc:stable and await further input from
Matthew?