Re: [PATCH v2] x86/sgx: Avoid using iterator after loop in sgx_mmu_notifier_release()

[Jarkko Sakkinen]

On Wed, Mar 01, 2023 at 12:17:29PM +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.
> 
> 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].
> 
> Instead we'll just use a 'found' boolean to indicate if an element
> was found.
> 
> Link: https://lore.kernel.org/all/CAHk-=wgRr_D8CB-D9Kg-c=EHreAsk5SqXPwr9Y7k9sA6cWXJ6w@xxxxxxxxxxxxxx/ [1]
> Signed-off-by: Jakob Koschel <jkl820.git@xxxxxxxxx>
> ---
> Changes in v2:
> - refactor to use 'found' variable instead of moving code into the loop
> - add cover letter info into the patch
> - Link to v1: https://lore.kernel.org/r/20230206-sgx-use-after-iter-v1-1-c09fb5300b5e@xxxxxxxxx
> ---
>  arch/x86/kernel/cpu/sgx/encl.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c
> index 2a0e90fe2abc..91fa70e51004 100644
> --- a/arch/x86/kernel/cpu/sgx/encl.c
> +++ b/arch/x86/kernel/cpu/sgx/encl.c
> @@ -755,6 +755,7 @@ static void sgx_mmu_notifier_release(struct mmu_notifier *mn,
>  {
>  	struct sgx_encl_mm *encl_mm = container_of(mn, struct sgx_encl_mm, mmu_notifier);
>  	struct sgx_encl_mm *tmp = NULL;
> +	bool found = false;
>  
>  	/*
>  	 * The enclave itself can remove encl_mm.  Note, objects can't be moved
> @@ -764,12 +765,13 @@ 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);
> +			found = true;
>  			break;
>  		}
>  	}
>  	spin_unlock(&encl_mm->encl->mm_lock);
>  
> -	if (tmp == encl_mm) {
> +	if (found) {
>  		synchronize_srcu(&encl_mm->encl->srcu);
>  		mmu_notifier_put(mn);
>  	}
> 
> ---
> base-commit: d2d11f342b179f1894a901f143ec7c008caba43e
> change-id: 20230206-sgx-use-after-iter-f584c1d64c87
> 
> Best regards,
> -- 
> Jakob Koschel <jkl820.git@xxxxxxxxx>
> 

Reviewed-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx>

BR, Jarkko