linux-kernel - [PATCH v2 03/12] x86/alternatives: Disable LASS when patching kernel alternatives

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Message-Id: <20230530114247.21821-4-alexander.shishkin@linux.intel.com>
Date:   Tue, 30 May 2023 14:42:38 +0300
From:   Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ravi Shankar <ravi.v.shankar@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Sohil Mehta <sohil.mehta@...el.com>,
        Paul Lai <paul.c.lai@...el.com>
Subject: [PATCH v2 03/12] x86/alternatives: Disable LASS when patching kernel alternatives

From: Sohil Mehta <sohil.mehta@...el.com>

For patching, the kernel initializes a temporary mm area in the lower
half of the address range. See commit 4fc19708b165 ("x86/alternatives:
Initialize temporary mm for patching").

Disable LASS enforcement during patching using the stac()/clac()
instructions to avoid triggering a #GP fault.

The objtool warns due to a call to a non-allowed function that exists
outside of the stac/clac guard, or references to any function with a
dynamic function pointer inside the guard. See the Objtool warnings
section #9 in the document tools/objtool/Documentation/objtool.txt.

Considering that patching is usually small, replace the memcpy and
memset functions in the text poking functions with their inline versions
respectively.

Signed-off-by: Sohil Mehta <sohil.mehta@...el.com>
---
 arch/x86/kernel/alternative.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index f615e0cb6d93..eac6a5406d39 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -1526,16 +1526,24 @@ 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;
 
+/*
+ * poking_init() initializes the text poking address from the lower half of the
+ * address space. Relax LASS enforcement when accessing the poking address.
+ */
 static void text_poke_memcpy(void *dst, const void *src, size_t len)
 {
-	memcpy(dst, src, len);
+	stac();
+	__inline_memcpy(dst, src, len);
+	clac();
 }
 
 static void text_poke_memset(void *dst, const void *src, size_t len)
 {
 	int c = *(const int *)src;
 
-	memset(dst, c, len);
+	stac();
+	__inline_memset(dst, c, len);
+	clac();
 }
 
 typedef void text_poke_f(void *dst, const void *src, size_t len);
-- 
2.39.2