[PATCH] powerpc/module_64: Fix livepatching for RO modules

[Russell Currey]

Livepatching a loaded module involves applying relocations through
apply_relocate_add(), which attempts to write to read-only memory when
CONFIG_STRICT_MODULE_RWX=y.  Work around this by performing these
writes through the text poke area by using patch_instruction().

R_PPC_REL24 is the only relocation type generated by the kpatch-build
userspace tool or klp-convert kernel tree that I observed applying a
relocation to a post-init module.

A more comprehensive solution is planned, but using patch_instruction()
for R_PPC_REL24 on should serve as a sufficient fix.

This does have a performance impact, I observed ~15% overhead in
module_load() on POWER8 bare metal with checksum verification off.

Fixes: c35717c71e98 ("powerpc: Set ARCH_HAS_STRICT_MODULE_RWX")
Cc: sta...@vger.kernel.org # v5.14+
Reported-by: Joe Lawrence <joe.lawre...@redhat.com>
Signed-off-by: Russell Currey <rus...@russell.cc>
---
Intended to be a minimal fix that can go to stable.

 arch/powerpc/kernel/module_64.c | 30 ++++++++++++++++++++++--------
 1 file changed, 22 insertions(+), 8 deletions(-)

diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module_64.c
index 6baa676e7cb6..c25ef36c3ef4 100644
--- a/arch/powerpc/kernel/module_64.c
+++ b/arch/powerpc/kernel/module_64.c
@@ -422,11 +422,16 @@ static inline int create_stub(const Elf64_Shdr *sechdrs,
                              const char *name)
 {
        long reladdr;
+       func_desc_t desc;
+       int i;
 
        if (is_mprofile_ftrace_call(name))
                return create_ftrace_stub(entry, addr, me);
 
-       memcpy(entry->jump, ppc64_stub_insns, sizeof(ppc64_stub_insns));
+       for (i = 0; i < sizeof(ppc64_stub_insns) / sizeof(u32); i++) {
+               patch_instruction(&entry->jump[i],
+                                 ppc_inst(ppc64_stub_insns[i]));
+       }
 
        /* Stub uses address relative to r2. */
        reladdr = (unsigned long)entry - my_r2(sechdrs, me);
@@ -437,10 +442,19 @@ static inline int create_stub(const Elf64_Shdr *sechdrs,
        }
        pr_debug("Stub %p get data from reladdr %li\n", entry, reladdr);
 
-       entry->jump[0] |= PPC_HA(reladdr);
-       entry->jump[1] |= PPC_LO(reladdr);
-       entry->funcdata = func_desc(addr);
-       entry->magic = STUB_MAGIC;
+       patch_instruction(&entry->jump[0],
+                         ppc_inst(entry->jump[0] | PPC_HA(reladdr)));
+       patch_instruction(&entry->jump[1],
+                         ppc_inst(entry->jump[1] | PPC_LO(reladdr)));
+
+       // func_desc_t is 8 bytes if ABIv2, else 16 bytes
+       desc = func_desc(addr);
+       for (i = 0; i < sizeof(func_desc_t) / sizeof(u32); i++) {
+               patch_instruction(((u32 *)&entry->funcdata) + i,
+                                 ppc_inst(((u32 *)(&desc))[i]));
+       }
+
+       patch_instruction(&entry->magic, ppc_inst(STUB_MAGIC));
 
        return 1;
 }
@@ -496,7 +510,7 @@ static int restore_r2(const char *name, u32 *instruction, 
struct module *me)
                return 0;
        }
        /* ld r2,R2_STACK_OFFSET(r1) */
-       *instruction = PPC_INST_LD_TOC;
+       patch_instruction(instruction, ppc_inst(PPC_INST_LD_TOC));
        return 1;
 }
 
@@ -636,9 +650,9 @@ int apply_relocate_add(Elf64_Shdr *sechdrs,
                        }
 
                        /* Only replace bits 2 through 26 */
-                       *(uint32_t *)location
-                               = (*(uint32_t *)location & ~0x03fffffc)
+                       value = (*(uint32_t *)location & ~0x03fffffc)
                                | (value & 0x03fffffc);
+                       patch_instruction((u32 *)location, ppc_inst(value));
                        break;
 
                case R_PPC64_REL64:
-- 
2.34.0