Received-SPF: pass (google.com: domain of linux-kernel+bounces-145968-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:45d1:ec00::1 as permitted sender) client-ip=2604:1380:45d1:ec00::1;
Date: Mon, 15 Apr 2024 23:15:03 +0100
From: Matthew Wilcox <willy@infradead.org>
To: Vishal Moola <vishal.moola@gmail.com>
Cc: syzbot <syzbot+ad1b592fc4483655438b@syzkaller.appspotmail.com>,
	akpm@linux-foundation.org, linux-kernel@vger.kernel.org,
	linux-mm@kvack.org, muchun.song@linux.dev,
	syzkaller-bugs@googlegroups.com
Subject: Re: [syzbot] [mm?] KASAN: slab-use-after-free Read in
 __vma_reservation_common
Message-ID: <Zh2m5_MfZ45Uk-vD@casper.infradead.org>
References: <000000000000daf1e10615e64dcb@google.com>
 <000000000000ae5d410615fea3bf@google.com>
 <Zh2kuFX9BWOGN1Mo@fedora>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <Zh2kuFX9BWOGN1Mo@fedora>

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.