linux-kernel - Re: [syzbot] KASAN: use-after-free Read in do_sync_mmap

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Date:   Wed, 25 May 2022 11:59:45 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     syzbot <syzbot+5b96d55e5b54924c77ad@...kaller.appspotmail.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] KASAN: use-after-free Read in do_sync_mmap_readahead

On Wed, 25 May 2022 19:33:39 +0100 Matthew Wilcox <willy@...radead.org> wrote:

> On Wed, May 25, 2022 at 06:57:55PM +0100, Matthew Wilcox wrote:
> > 
> > Ohh, that makes sense.  We unlocked the mmap_sem, so the file is
> > pinned, but the VMA isn't.  I'll whip up a patch.
> 
> #syz test
> 
> >From 01a4917c4cfe400eb310eba4f2fa466d381623c1 Mon Sep 17 00:00:00 2001
> From: "Matthew Wilcox (Oracle)" <willy@...radead.org>
> Date: Wed, 25 May 2022 14:23:45 -0400
> Subject: [PATCH] mm/filemap: Cache the value of vm_flags
> 
> After we have unlocked the mmap_lock for I/O, the file is pinned, but
> the VMA is not.  Checking this flag after that can be a use-after-free.
> It's not a terribly interesting use-after-free as it can only read one
> bit, and it's used to decide whether to read 2MB or 4MB.  But it
> upsets the automated tools and it's generally bad practice anyway,
> so let's fix it.
> 
> Reported-by: syzbot+5b96d55e5b54924c77ad@...kaller.appspotmail.com
> Fixes: 4687fdbb805a ("mm/filemap: Support VM_HUGEPAGE for file mappings")
> Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>

cc:stable also, please.