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module: switch to execmem API for remapping as RW and restoring ROX
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Instead of using writable copy for module text sections, temporarily remap
the memory allocated from execmem's ROX cache as writable and restore its
ROX permissions after the module is formed.

This will allow removing nasty games with writable copy in alternatives
patching on x86.

Signed-off-by: "Mike Rapoport (Microsoft)" <rppt@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250126074733.1384926-7-rppt@kernel.org
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Mike Rapoport (Microsoft) authored and Peter Zijlstra committed Feb 3, 2025
1 parent 05e555b commit c287c07
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Showing 4 changed files with 27 additions and 72 deletions.
8 changes: 1 addition & 7 deletions include/linux/module.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -367,7 +367,6 @@ enum mod_mem_type {

struct module_memory {
void *base;
void *rw_copy;
bool is_rox;
unsigned int size;

Expand Down Expand Up @@ -769,14 +768,9 @@ static inline bool is_livepatch_module(struct module *mod)

void set_module_sig_enforced(void);

void *__module_writable_address(struct module *mod, void *loc);

static inline void *module_writable_address(struct module *mod, void *loc)
{
if (!IS_ENABLED(CONFIG_ARCH_HAS_EXECMEM_ROX) || !mod ||
mod->state != MODULE_STATE_UNFORMED)
return loc;
return __module_writable_address(mod, loc);
return loc;
}

#else /* !CONFIG_MODULES... */
Expand Down
4 changes: 0 additions & 4 deletions include/linux/moduleloader.h
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Original file line number Diff line number Diff line change
Expand Up @@ -108,10 +108,6 @@ int module_finalize(const Elf_Ehdr *hdr,
const Elf_Shdr *sechdrs,
struct module *mod);

int module_post_finalize(const Elf_Ehdr *hdr,
const Elf_Shdr *sechdrs,
struct module *mod);

#ifdef CONFIG_MODULES
void flush_module_init_free_work(void);
#else
Expand Down
78 changes: 21 additions & 57 deletions kernel/module/main.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1221,18 +1221,6 @@ void __weak module_arch_freeing_init(struct module *mod)
{
}

void *__module_writable_address(struct module *mod, void *loc)
{
for_class_mod_mem_type(type, text) {
struct module_memory *mem = &mod->mem[type];

if (loc >= mem->base && loc < mem->base + mem->size)
return loc + (mem->rw_copy - mem->base);
}

return loc;
}

static int module_memory_alloc(struct module *mod, enum mod_mem_type type)
{
unsigned int size = PAGE_ALIGN(mod->mem[type].size);
Expand All @@ -1250,21 +1238,15 @@ static int module_memory_alloc(struct module *mod, enum mod_mem_type type)
if (!ptr)
return -ENOMEM;

mod->mem[type].base = ptr;

if (execmem_is_rox(execmem_type)) {
ptr = vzalloc(size);
int err = execmem_make_temp_rw(ptr, size);

if (!ptr) {
execmem_free(mod->mem[type].base);
if (err) {
execmem_free(ptr);
return -ENOMEM;
}

mod->mem[type].rw_copy = ptr;
mod->mem[type].is_rox = true;
} else {
mod->mem[type].rw_copy = mod->mem[type].base;
memset(mod->mem[type].base, 0, size);
}

/*
Expand All @@ -1280,16 +1262,26 @@ static int module_memory_alloc(struct module *mod, enum mod_mem_type type)
*/
kmemleak_not_leak(ptr);

memset(ptr, 0, size);
mod->mem[type].base = ptr;

return 0;
}

static void module_memory_restore_rox(struct module *mod)
{
for_class_mod_mem_type(type, text) {
struct module_memory *mem = &mod->mem[type];

if (mem->is_rox)
execmem_restore_rox(mem->base, mem->size);
}
}

static void module_memory_free(struct module *mod, enum mod_mem_type type)
{
struct module_memory *mem = &mod->mem[type];

if (mem->is_rox)
vfree(mem->rw_copy);

execmem_free(mem->base);
}

Expand Down Expand Up @@ -2642,7 +2634,6 @@ static int move_module(struct module *mod, struct load_info *info)
for_each_mod_mem_type(type) {
if (!mod->mem[type].size) {
mod->mem[type].base = NULL;
mod->mem[type].rw_copy = NULL;
continue;
}

Expand All @@ -2659,7 +2650,6 @@ static int move_module(struct module *mod, struct load_info *info)
void *dest;
Elf_Shdr *shdr = &info->sechdrs[i];
const char *sname;
unsigned long addr;

if (!(shdr->sh_flags & SHF_ALLOC))
continue;
Expand All @@ -2680,14 +2670,12 @@ static int move_module(struct module *mod, struct load_info *info)
ret = PTR_ERR(dest);
goto out_err;
}
addr = (unsigned long)dest;
codetag_section_found = true;
} else {
enum mod_mem_type type = shdr->sh_entsize >> SH_ENTSIZE_TYPE_SHIFT;
unsigned long offset = shdr->sh_entsize & SH_ENTSIZE_OFFSET_MASK;

addr = (unsigned long)mod->mem[type].base + offset;
dest = mod->mem[type].rw_copy + offset;
dest = mod->mem[type].base + offset;
}

if (shdr->sh_type != SHT_NOBITS) {
Expand All @@ -2710,13 +2698,14 @@ static int move_module(struct module *mod, struct load_info *info)
* users of info can keep taking advantage and using the newly
* minted official memory area.
*/
shdr->sh_addr = addr;
shdr->sh_addr = (unsigned long)dest;
pr_debug("\t0x%lx 0x%.8lx %s\n", (long)shdr->sh_addr,
(long)shdr->sh_size, info->secstrings + shdr->sh_name);
}

return 0;
out_err:
module_memory_restore_rox(mod);
for (t--; t >= 0; t--)
module_memory_free(mod, t);
if (codetag_section_found)
Expand Down Expand Up @@ -2863,17 +2852,8 @@ int __weak module_finalize(const Elf_Ehdr *hdr,
return 0;
}

int __weak module_post_finalize(const Elf_Ehdr *hdr,
const Elf_Shdr *sechdrs,
struct module *me)
{
return 0;
}

static int post_relocation(struct module *mod, const struct load_info *info)
{
int ret;

/* Sort exception table now relocations are done. */
sort_extable(mod->extable, mod->extable + mod->num_exentries);

Expand All @@ -2885,24 +2865,7 @@ static int post_relocation(struct module *mod, const struct load_info *info)
add_kallsyms(mod, info);

/* Arch-specific module finalizing. */
ret = module_finalize(info->hdr, info->sechdrs, mod);
if (ret)
return ret;

for_each_mod_mem_type(type) {
struct module_memory *mem = &mod->mem[type];

if (mem->is_rox) {
if (!execmem_update_copy(mem->base, mem->rw_copy,
mem->size))
return -ENOMEM;

vfree(mem->rw_copy);
mem->rw_copy = NULL;
}
}

return module_post_finalize(info->hdr, info->sechdrs, mod);
return module_finalize(info->hdr, info->sechdrs, mod);
}

/* Call module constructors. */
Expand Down Expand Up @@ -3499,6 +3462,7 @@ static int load_module(struct load_info *info, const char __user *uargs,
mod->mem[type].size);
}

module_memory_restore_rox(mod);
module_deallocate(mod, info);
free_copy:
/*
Expand Down
9 changes: 5 additions & 4 deletions kernel/module/strict_rwx.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -9,6 +9,7 @@
#include <linux/mm.h>
#include <linux/vmalloc.h>
#include <linux/set_memory.h>
#include <linux/execmem.h>
#include "internal.h"

static int module_set_memory(const struct module *mod, enum mod_mem_type type,
Expand All @@ -32,12 +33,12 @@ static int module_set_memory(const struct module *mod, enum mod_mem_type type,
int module_enable_text_rox(const struct module *mod)
{
for_class_mod_mem_type(type, text) {
const struct module_memory *mem = &mod->mem[type];
int ret;

if (mod->mem[type].is_rox)
continue;

if (IS_ENABLED(CONFIG_STRICT_MODULE_RWX))
if (mem->is_rox)
ret = execmem_restore_rox(mem->base, mem->size);
else if (IS_ENABLED(CONFIG_STRICT_MODULE_RWX))
ret = module_set_memory(mod, type, set_memory_rox);
else
ret = module_set_memory(mod, type, set_memory_x);
Expand Down

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