linux-kernel - [PATCH] powerpc/code-patching: Relax verification of patchability

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Message-Id: <68d7d57675e0963fe5e2c4b84b0cb2390c78638c.1637912333.git.christophe.leroy@csgroup.eu>
Date:   Fri, 26 Nov 2021 08:39:23 +0100
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     Christophe Leroy <christophe.leroy@...roup.eu>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Sachin Sant <sachinp@...ux.vnet.ibm.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Nicholas Piggin <npiggin@...il.com>
Subject: [PATCH] powerpc/code-patching: Relax verification of patchability

Commit 8b8a8f0ab3f5 ("powerpc/code-patching: Improve verification of
patchability") introduced a stricter verification of the patched
area by checking it is proper kernel text.

But as least two usages of patch_instruction() fall outside:
- Code patching selftests, which use stack and vmalloc space.
- Ftrace

So for the time being, partially revert commit 8b8a8f0ab3f5 and add
a onetime warning:

  Running code patching self-tests ...
  patch_instruction() called on invalid text address 0xe1011e58 from test_code_patching+0x34/0xd6c

Reported-by: Sachin Sant <sachinp@...ux.vnet.ibm.com>
Reported-by: Stephen Rothwell <sfr@...b.auug.org.au>
Cc: Nicholas Piggin <npiggin@...il.com>
Fixes: 8b8a8f0ab3f5 ("powerpc/code-patching: Improve verification of patchability")
Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
---
 arch/powerpc/lib/code-patching.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/lib/code-patching.c b/arch/powerpc/lib/code-patching.c
index 1dd636a85cc1..c87eea773930 100644
--- a/arch/powerpc/lib/code-patching.c
+++ b/arch/powerpc/lib/code-patching.c
@@ -190,9 +190,13 @@ static int do_patch_instruction(u32 *addr, struct ppc_inst instr)
 int patch_instruction(u32 *addr, struct ppc_inst instr)
 {
 	/* Make sure we aren't patching a freed init section */
-	if (!kernel_text_address((unsigned long)addr))
+	if (system_state >= SYSTEM_FREEING_INITMEM && init_section_contains(addr, 4))
 		return 0;
 
+	if (!kernel_text_address((unsigned long)addr))
+		pr_warn_once("%s() called on invalid text address 0x%p from %pS\n",
+			     __func__, addr, __builtin_return_address(0));
+
 	return do_patch_instruction(addr, instr);
 }
 NOKPROBE_SYMBOL(patch_instruction);
-- 
2.33.1