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

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Date:   Wed, 25 May 2022 18:57:55 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Andrew Morton <akpm@...ux-foundation.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, May 25, 2022 at 09:58:42AM -0700, Andrew Morton wrote:
> On Wed, 25 May 2022 07:26:22 -0700 syzbot <syzbot+5b96d55e5b54924c77ad@...kaller.appspotmail.com> wrote:
> > BUG: KASAN: use-after-free in do_sync_mmap_readahead+0x465/0x9f0 mm/filemap.c:3006
> > Read of size 8 at addr ffff88801fedb050 by task syz-executor.5/1755
> 
> A race?
> 
> #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> 	/* Use the readahead code, even if readahead is disabled */
> 	if (vmf->vma->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;
> 		/*
> 		 * 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))
> 			ra->size *= 2;
> 		ra->async_size = HPAGE_PMD_NR;
> 		page_cache_ra_order(&ractl, ra, HPAGE_PMD_ORDER);
> 		return fpin;
> 	}
> #endif
> 
> Reading from vmf->vma->vm_flags was OK, then it suddenly wasn't.

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.