linux-kernel - Re: [PATCH] fs: fix data race on mnt.mnt

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Date:	Mon, 21 Sep 2015 17:06:15 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Dmitry Vyukov <dvyukov@...gle.com>
Cc:	viro@...iv.linux.org.uk, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	kirill.shutemov@...ux.intel.com, riel@...hat.com, mhocko@...e.cz,
	oleg@...hat.com, sasha.levin@...cle.com, gang.chen.5i5j@...il.com,
	pfeiner@...gle.com, aarcange@...hat.com, vishnu.ps@...sung.com,
	linux-mm@...ck.org, glider@...gle.com, kcc@...gle.com,
	andreyknvl@...gle.com, ktsan@...glegroups.com,
	paulmck@...ux.vnet.ibm.com
Subject: Re: [PATCH] fs: fix data race on mnt.mnt_flags

On Mon, Sep 21, 2015 at 02:16:47PM +0200, Dmitry Vyukov wrote:
> do_remount() does:
> 
> mnt_flags |= mnt->mnt.mnt_flags & ~MNT_USER_SETTABLE_MASK;
> mnt->mnt.mnt_flags = mnt_flags;
> 
> This can easily be compiled as:
> 
> mnt->mnt.mnt_flags &= ~MNT_USER_SETTABLE_MASK;
> mnt->mnt.mnt_flags |= mnt_flags;
> 
> (also 2 memory accesses, less register pressure)
> The flags are being concurrently read by e.g. do_mmap_pgoff()
> which does:
> 
> if (file->f_path.mnt->mnt_flags & MNT_NOEXEC)
> 
> As the result we can allow to mmap a MNT_NOEXEC mount
> as VM_EXEC.
> 
> Use WRITE_ONCE() to set new flags.
> 
> The data race was found with KernelThreadSanitizer (KTSAN).
> 
> Signed-off-by: Dmitry Vyukov <dvyukov@...gle.com>

Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>

-- 
 Kirill A. Shutemov
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