linux-kernel - [PATCH v2] x86/sgx: Avoid using iterator after loop in sgx_mmu_notifier

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Date:   Wed, 01 Mar 2023 12:17:29 +0100
From:   Jakob Koschel <jkl820.git@...il.com>
To:     Jarkko Sakkinen <jarkko@...nel.org>,
        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>
Cc:     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>, Jakob Koschel <jkl820.git@...il.com>
Subject: [PATCH v2] x86/sgx: Avoid using iterator after loop in
 sgx_mmu_notifier_release()

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@mail.gmail.com/ [1]
Signed-off-by: Jakob Koschel <jkl820.git@...il.com>
---
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@gmail.com
---
 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@...il.com>