linux-kernel - [PATCH] powerpc: fix alignment bug whithin the init sections

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Message-Id: <20201213183556.16976-1-ariel.marcovitch@gmail.com>
Date:   Sun, 13 Dec 2020 20:35:56 +0200
From:   Ariel Marcovitch <arielmarcovitch@...il.com>
To:     mpe@...erman.id.au
Cc:     benh@...nel.crashing.org, paulus@...ba.org,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        Ariel Marcovitch <ariel.marcovitch@...il.com>
Subject: [PATCH] powerpc: fix alignment bug whithin the init sections

This is a bug that can cause early crashes in configurations with a
.exit.text section smaller than a page and a .init.text section that
ends in the beginning of a physical page (this is kinda random, which
might explain why this wasn't really encountered before).

The init sections are ordered like this:
	.init.text
	.exit.text
	.init.data

Currently, these sections aren't page aligned.

Because the init code is mapped read-only at runtime and because the
.init.text section can potentially reside on the same physical page as
.init.data, the beginning of .init.data might be mapped read-only along
with .init.text.

Then when the kernel tries to modify a variable in .init.data (like
kthreadd_done, used in kernel_init()) the kernel panics.

To avoid this, I made these sections page aligned.

Fixes: 060ef9d89d18 ("powerpc32: PAGE_EXEC required for inittext")
Signed-off-by: Ariel Marcovitch <ariel.marcovitch@...il.com>
---
 arch/powerpc/kernel/vmlinux.lds.S | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/powerpc/kernel/vmlinux.lds.S b/arch/powerpc/kernel/vmlinux.lds.S
index 326e113d2e45..e3a7c90c03f4 100644
--- a/arch/powerpc/kernel/vmlinux.lds.S
+++ b/arch/powerpc/kernel/vmlinux.lds.S
@@ -179,6 +179,11 @@ SECTIONS
 #endif
 	} :text

+	/* .init.text is made RO and .exit.text is not, so we must
+	 * ensure these sections reside in separate physical pages.
+	 */
+	. = ALIGN(PAGE_SIZE);
+
 	/* .exit.text is discarded at runtime, not link time,
 	 * to deal with references from __bug_table
 	 */
@@ -186,6 +191,8 @@ SECTIONS
 		EXIT_TEXT
 	}

+	. = ALIGN(PAGE_SIZE);
+
 	.init.data : AT(ADDR(.init.data) - LOAD_OFFSET) {
 		INIT_DATA
 	}

base-commit: 1398820fee515873379809a6415930ad0764b2f6
-- 
2.17.1