linux-hardening - [PATCH v5 3/8] x86_64: Add LKDTM accessor for patching addr

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Date:   Tue, 13 Jul 2021 00:31:08 -0500
From:   "Christopher M. Riedl" <cmr@...ux.ibm.com>
To:     linuxppc-dev@...ts.ozlabs.org
Cc:     tglx@...utronix.de, x86@...nel.org,
        linux-hardening@...r.kernel.org, keescook@...omium.org,
        npiggin@...il.com, dja@...ens.net, peterz@...radead.org
Subject: [PATCH v5 3/8] x86_64: Add LKDTM accessor for patching addr

When live patching with STRICT_KERNEL_RWX a mapping is installed at a
"patching address" with temporary write permissions. Provide a
LKDTM-only accessor function for this address in preparation for a LKDTM
test which attempts to "hijack" this mapping by writing to it from
another CPU.

Signed-off-by: Christopher M. Riedl <cmr@...ux.ibm.com>
---
 arch/x86/include/asm/text-patching.h | 4 ++++
 arch/x86/kernel/alternative.c        | 7 +++++++
 2 files changed, 11 insertions(+)

diff --git a/arch/x86/include/asm/text-patching.h b/arch/x86/include/asm/text-patching.h
index b7421780e4e92..f0caf9ee13bd8 100644
--- a/arch/x86/include/asm/text-patching.h
+++ b/arch/x86/include/asm/text-patching.h
@@ -167,4 +167,8 @@ void int3_emulate_ret(struct pt_regs *regs)
 }
 #endif /* !CONFIG_UML_X86 */
 
+#if IS_BUILTIN(CONFIG_LKDTM)
+unsigned long read_cpu_patching_addr(unsigned int cpu);
+#endif
+
 #endif /* _ASM_X86_TEXT_PATCHING_H */
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index e9da3dc712541..28bb92b695639 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -773,6 +773,13 @@ static inline void unuse_temporary_mm(temp_mm_state_t prev_state)
 __ro_after_init struct mm_struct *poking_mm;
 __ro_after_init unsigned long poking_addr;
 
+#if IS_BUILTIN(CONFIG_LKDTM)
+unsigned long read_cpu_patching_addr(unsigned int cpu)
+{
+	return poking_addr;
+}
+#endif
+
 static void *__text_poke(void *addr, const void *opcode, size_t len)
 {
 	bool cross_page_boundary = offset_in_page(addr) + len > PAGE_SIZE;
-- 
2.26.1