[PATCH v2] powerpc: fix alignment bug whithin the init sections

This is a bug that causes early crashes in builds 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 might become 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, make _einittext page aligned and also align .exit.text
to make sure .init.data is always seperated from the text segments.

Fixes: 060ef9d89d18 ("powerpc32: PAGE_EXEC required for inittext")
Signed-off-by: Ariel Marcovitch <redacted>
---
 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 6db90cdf11da..b6c765d8e7ee 100644
--- a/arch/powerpc/kernel/vmlinux.lds.S
+++ b/arch/powerpc/kernel/vmlinux.lds.S

@@ -187,6 +187,11 @@ SECTIONS
 	.init.text : AT(ADDR(.init.text) - LOAD_OFFSET) {
 		_sinittext = .;
 		INIT_TEXT
+
+		/* .init.text might be RO so we must
+		* ensure this section ends in a page boundary.
+		*/
+		. = ALIGN(PAGE_SIZE);
 		_einittext = .;
 #ifdef CONFIG_PPC64
 		*(.tramp.ftrace.init);

@@ -200,6 +205,8 @@ SECTIONS
 		EXIT_TEXT
 	}
 
+	. = ALIGN(PAGE_SIZE);
+
 	.init.data : AT(ADDR(.init.data) - LOAD_OFFSET) {
 		INIT_DATA
 	}

base-commit: 2c85ebc57b3e1817b6ce1a6b703928e113a90442
-- 
2.17.1

Le 02/01/2021 à 21:11, Ariel Marcovitch a écrit :

This is a bug that causes early crashes in builds 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 might become 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, make _einittext page aligned and also align .exit.text
to make sure .init.data is always seperated from the text segments.

Fixes: 060ef9d89d18 ("powerpc32: PAGE_EXEC required for inittext")
Signed-off-by: Ariel Marcovitch <redacted>

Reviewed-by: Christophe Leroy <redacted>

On Sat, 2 Jan 2021 22:11:56 +0200, Ariel Marcovitch wrote:

This is a bug that causes early crashes in builds 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

[...]

Applied to powerpc/fixes.

[1/1] powerpc: Fix alignment bug within the init sections
      https://git.kernel.org/powerpc/c/2225a8dda263edc35a0e8b858fe2945cf6240fde

cheers