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

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Message-ID: <CACT4Y+Zyfgr6KzFJ3r3df1EKyfXiYf9LHac4_MgLmD-B9uGhAQ@mail.gmail.com>
Date:   Fri, 27 May 2022 14:35:46 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        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 at 20:33, 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

This bug doesn't have a reproducer, so unfortunately it can't be tested.
But it would also need kernel repo/branch to apply the patch, e.g.
something like #syz  test
git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
master

> 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>
> ---
>  mm/filemap.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 9a1eef6c5d35..61dd39990fda 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2991,11 +2991,12 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
>         struct address_space *mapping = file->f_mapping;
>         DEFINE_READAHEAD(ractl, file, ra, mapping, vmf->pgoff);
>         struct file *fpin = NULL;
> +       unsigned long vm_flags = vmf->vma->vm_flags;
>         unsigned int mmap_miss;
>
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>         /* Use the readahead code, even if readahead is disabled */
> -       if (vmf->vma->vm_flags & VM_HUGEPAGE) {
> +       if (vm_flags & VM_HUGEPAGE) {
>                 fpin = maybe_unlock_mmap_for_io(vmf, fpin);
>                 ractl._index &= ~((unsigned long)HPAGE_PMD_NR - 1);
>                 ra->size = HPAGE_PMD_NR;
> @@ -3003,7 +3004,7 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
>                  * Fetch two PMD folios, so we get the chance to actually
>                  * readahead, unless we've been told not to.
>                  */
> -               if (!(vmf->vma->vm_flags & VM_RAND_READ))
> +               if (!(vm_flags & VM_RAND_READ))
>                         ra->size *= 2;
>                 ra->async_size = HPAGE_PMD_NR;
>                 page_cache_ra_order(&ractl, ra, HPAGE_PMD_ORDER);
> @@ -3012,12 +3013,12 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
>  #endif
>
>         /* If we don't want any read-ahead, don't bother */
> -       if (vmf->vma->vm_flags & VM_RAND_READ)
> +       if (vm_flags & VM_RAND_READ)
>                 return fpin;
>         if (!ra->ra_pages)
>                 return fpin;
>
> -       if (vmf->vma->vm_flags & VM_SEQ_READ) {
> +       if (vm_flags & VM_SEQ_READ) {
>                 fpin = maybe_unlock_mmap_for_io(vmf, fpin);
>                 page_cache_sync_ra(&ractl, ra->ra_pages);
>                 return fpin;
> --
> 2.34.1
>
> --
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