Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
From:   Lee Jones <lee@kernel.org>
To:     lee@kernel.org, dave.hansen@linux.intel.com, luto@kernel.org,
        peterz@infradead.org, tglx@linutronix.de, mingo@redhat.com,
        bp@alien8.de, x86@kernel.org, hpa@zytor.com, linux-mm@kvack.org
Cc:     linux-kernel@vger.kernel.org
Subject: [PATCH 1/1] x86/mm/KASLR: Store pud_page_tramp into entry rather than page
Date:   Wed, 14 Jun 2023 16:52:10 +0100
Message-ID: <20230614155223.905222-2-lee@kernel.org>
In-Reply-To: <20230614155223.905222-1-lee@kernel.org>
References: <20230614155223.905222-1-lee@kernel.org>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

set_pgd() expects to be passed whole pages to operate on, whereas
trampoline_pgd_entry is, as the name suggests, an entry.  The
ramifications for using set_pgd() here are that the following thread of
execution will not only place the suggested value into the
trampoline_pgd_entry (8-Byte globally stored [.bss]) variable, PTI will
also attempt to replicate that value into the non-existent neighboring
user page (located +4k away), leading to the corruption of other global
[.bss] stored variables.

Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Lee Jones <lee@kernel.org>
---
 arch/x86/mm/kaslr.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/mm/kaslr.c b/arch/x86/mm/kaslr.c
index 557f0fe25dff4..ee6f22f3385b6 100644
--- a/arch/x86/mm/kaslr.c
+++ b/arch/x86/mm/kaslr.c
@@ -175,7 +175,7 @@ void __meminit init_trampoline_kaslr(void)
 		set_pgd(&trampoline_pgd_entry,
 			__pgd(_KERNPG_TABLE | __pa(p4d_page_tramp)));
 	} else {
-		set_pgd(&trampoline_pgd_entry,
-			__pgd(_KERNPG_TABLE | __pa(pud_page_tramp)));
+		trampoline_pgd_entry =
+			__pgd(_KERNPG_TABLE | __pa(pud_page_tramp));
 	}
 }
-- 
2.41.0.162.gfafddb0af9-goog