linux-kernel - Re: [PATCH] x86/sgx: Avoid using iterator after loop in sgx_mmu_notifier

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Message-ID: <Y+MCYYQJ0G5w1H63@kernel.org>
Date:   Wed, 8 Feb 2023 04:01:05 +0200
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Jakob Koschel <jkl820.git@...il.com>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        linux-sgx@...r.kernel.org, linux-kernel@...r.kernel.org,
        Pietro Borrello <borrello@...g.uniroma1.it>,
        Cristiano Giuffrida <c.giuffrida@...nl>,
        "Bos, H.J." <h.j.bos@...nl>
Subject: Re: [PATCH] x86/sgx: Avoid using iterator after loop in
 sgx_mmu_notifier_release()

On Mon, Feb 06, 2023 at 11:39:02AM +0100, Jakob Koschel wrote:
> If &encl_mm->encl->mm_list does not contain the searched 'encl_mm',
> 'tmp' will not point to a valid sgx_encl_mm struct.

Please explain.

Perhaps, extend with something like ", and can be turned into
transient gadget."

> 
> Since the code within the guarded block is just called when the element
> is found, it can simply be moved into the list iterator.
> Within the list iterator 'tmp' is guaranteed to point to a valid
> element.
> 
> Signed-off-by: Jakob Koschel <jkl820.git@...il.com>
> ---
> Linus proposed to avoid any use of the list iterator variable after the
> loop, in the attempt to move the list iterator variable declaration into
> the marcro to avoid any potential misuse after the loop.
> Using it in a pointer comparision after the loop is undefined behavior
> and should be omitted if possible [1].
> 
> Link: https://lore.kernel.org/all/CAHk-=wgRr_D8CB-D9Kg-c=EHreAsk5SqXPwr9Y7k9sA6cWXJ6w@mail.gmail.com/ [1]

I would move all this to the commit message. It is useful information.

> ---
>  arch/x86/kernel/cpu/sgx/encl.c | 10 ++++------
>  1 file changed, 4 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c
> index 2a0e90fe2abc..db585b780141 100644
> --- a/arch/x86/kernel/cpu/sgx/encl.c
> +++ b/arch/x86/kernel/cpu/sgx/encl.c
> @@ -764,15 +764,13 @@ static void sgx_mmu_notifier_release(struct mmu_notifier *mn,
>  	list_for_each_entry(tmp, &encl_mm->encl->mm_list, list) {
>  		if (tmp == encl_mm) {
>  			list_del_rcu(&encl_mm->list);
> -			break;
> +			spin_unlock(&encl_mm->encl->mm_lock);
> +			synchronize_srcu(&encl_mm->encl->srcu);
> +			mmu_notifier_put(mn);
> +			return;
>  		}
>  	}
>  	spin_unlock(&encl_mm->encl->mm_lock);
> -
> -	if (tmp == encl_mm) {
> -		synchronize_srcu(&encl_mm->encl->srcu);
> -		mmu_notifier_put(mn);
> -	}
>  }
>  
>  static void sgx_mmu_notifier_free(struct mmu_notifier *mn)
> 
> ---
> base-commit: d2d11f342b179f1894a901f143ec7c008caba43e
> change-id: 20230206-sgx-use-after-iter-f584c1d64c87
> 
> Best regards,
> -- 
> Jakob Koschel <jkl820.git@...il.com>
> 

BR, Jarkko