linux-kernel - [PATCH] x86/alternative: Consistently patch SMP locks in vmlinux and modules

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Date:   Tue, 30 Aug 2022 09:42:37 +0200
From:   Julian Pidancet <julian.pidancet@...cle.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org
Subject: [PATCH] x86/alternative: Consistently patch SMP locks in vmlinux and
 modules

The alternatives_smp_module_add() function restricts patching of SMP
lock prefixes to the text address range passed as an argument.

For vmlinux, patching all the instructions located between the _text and
_etext symbols is allowed. That includes the .text section but also
other sections such as .text.hot and .text.unlikely.

As per the comment inside the 'struct smp_alt_module' definition, the
original purpose of this restriction is to avoid patching the init code
which may have been deallocated when the alternatives code run.

For modules, the current code only allows patching instructions located
inside the .text segment, excluding other sections such as .text.hot or
.text.unlikely, which may need patching.

This change aims to make patching of the kernel core and modules more
consistent, by allowing all text sections of modules except .init.text
to be patched in module_finalize().

For that we use mod->core_layout.base/mod->core_layout.text_size as the
address range allowed to be patched, which include all the code sections
except the init code.

Signed-off-by: Julian Pidancet <julian.pidancet@...cle.com>
---
Public tests: https://gist.github.com/jpidancet/1ee457623426f3e3902a28edaf2c80d0 
Related thread: https://marc.info/?t=130864398400006

 arch/x86/kernel/module.c | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kernel/module.c b/arch/x86/kernel/module.c
index b1abf663417c..da22193eb5e0 100644
--- a/arch/x86/kernel/module.c
+++ b/arch/x86/kernel/module.c
@@ -251,14 +251,12 @@ int module_finalize(const Elf_Ehdr *hdr,
 		    const Elf_Shdr *sechdrs,
 		    struct module *me)
 {
-	const Elf_Shdr *s, *text = NULL, *alt = NULL, *locks = NULL,
-		*para = NULL, *orc = NULL, *orc_ip = NULL,
-		*retpolines = NULL, *returns = NULL, *ibt_endbr = NULL;
+	const Elf_Shdr *s, *alt = NULL, *locks = NULL, *para = NULL,
+		*orc = NULL, *orc_ip = NULL, *retpolines = NULL,
+		*returns = NULL, *ibt_endbr = NULL;
 	char *secstrings = (void *)hdr + sechdrs[hdr->e_shstrndx].sh_offset;

 	for (s = sechdrs; s < sechdrs + hdr->e_shnum; s++) {
-		if (!strcmp(".text", secstrings + s->sh_name))
-			text = s;
 		if (!strcmp(".altinstructions", secstrings + s->sh_name))
 			alt = s;
 		if (!strcmp(".smp_locks", secstrings + s->sh_name))
@@ -302,12 +300,13 @@ int module_finalize(const Elf_Ehdr *hdr,
 		void *iseg = (void *)ibt_endbr->sh_addr;
 		apply_ibt_endbr(iseg, iseg + ibt_endbr->sh_size);
 	}
-	if (locks && text) {
+	if (locks) {
 		void *lseg = (void *)locks->sh_addr;
-		void *tseg = (void *)text->sh_addr;
+		void *text = me->core_layout.base;
+		void *text_end = text + me->core_layout.text_size;
 		alternatives_smp_module_add(me, me->name,
 					    lseg, lseg + locks->sh_size,
-					    tseg, tseg + text->sh_size);
+					    text, text_end);
 	}

 	if (orc && orc_ip)
-- 
2.37.1