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

[Dave Hansen <dave.hansen@intel.com>]

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On 2/6/23 02:39, 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.
> 
> 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@gmail.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].

I think there's a big difference between "undefined behavior" and
"someone wants to flip a switch to *make* this undefined behavior".  My
understanding is that this patch avoids behavior which _is_ defined today.

Is there some effort to change this behavior across the tree that I missed?

In any case, this patch also kinda breaks the rule that you're supposed
to make the common path through the code at the lowest nesting level.
It makes the common case look like some kind of error handling.  Would
something like the attached patch work?

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diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c
index 68f8b18d2278..e1bd2a5790a7 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 mm_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);
+			mm_found = true;
 			break;
 		}
 	}
 	spin_unlock(&encl_mm->encl->mm_lock);
 
-	if (tmp == encl_mm) {
+	if (mm_found) {
 		synchronize_srcu(&encl_mm->encl->srcu);
 		mmu_notifier_put(mn);
 	}