linux-kernel - [PATCH 5.4 064/183] x86, relocs: Ignore relocations in .notes section

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Date: Sun, 24 Mar 2024 19:44:37 -0400
From: Sasha Levin <sashal@...nel.org>
To: linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Cc: Kees Cook <keescook@...omium.org>,
	Guixiong Wei <guixiongwei@...il.com>,
	Juergen Gross <jgross@...e.com>,
	Sasha Levin <sashal@...nel.org>
Subject: [PATCH 5.4 064/183] x86, relocs: Ignore relocations in .notes section

From: Kees Cook <keescook@...omium.org>

[ Upstream commit aaa8736370db1a78f0e8434344a484f9fd20be3b ]

When building with CONFIG_XEN_PV=y, .text symbols are emitted into
the .notes section so that Xen can find the "startup_xen" entry point.
This information is used prior to booting the kernel, so relocations
are not useful. In fact, performing relocations against the .notes
section means that the KASLR base is exposed since /sys/kernel/notes
is world-readable.

To avoid leaking the KASLR base without breaking unprivileged tools that
are expecting to read /sys/kernel/notes, skip performing relocations in
the .notes section. The values readable in .notes are then identical to
those found in System.map.

Reported-by: Guixiong Wei <guixiongwei@...il.com>
Closes: https://lore.kernel.org/all/20240218073501.54555-1-guixiongwei@gmail.com/
Fixes: 5ead97c84fa7 ("xen: Core Xen implementation")
Fixes: da1a679cde9b ("Add /sys/kernel/notes")
Reviewed-by: Juergen Gross <jgross@...e.com>
Signed-off-by: Kees Cook <keescook@...omium.org>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 arch/x86/tools/relocs.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/x86/tools/relocs.c b/arch/x86/tools/relocs.c
index 1c3a1962cade6..0043fd374a62f 100644
--- a/arch/x86/tools/relocs.c
+++ b/arch/x86/tools/relocs.c
@@ -596,6 +596,14 @@ static void print_absolute_relocs(void)
 		if (!(sec_applies->shdr.sh_flags & SHF_ALLOC)) {
 			continue;
 		}
+		/*
+		 * Do not perform relocations in .notes section; any
+		 * values there are meant for pre-boot consumption (e.g.
+		 * startup_xen).
+		 */
+		if (sec_applies->shdr.sh_type == SHT_NOTE) {
+			continue;
+		}
 		sh_symtab  = sec_symtab->symtab;
 		sym_strtab = sec_symtab->link->strtab;
 		for (j = 0; j < sec->shdr.sh_size/sizeof(Elf_Rel); j++) {
-- 
2.43.0