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modules: fix longstanding /proc/kallsyms vs module insertion race.
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For CONFIG_KALLSYMS, we keep two symbol tables and two string tables.
There's one full copy, marked SHF_ALLOC and laid out at the end of the
module's init section.  There's also a cut-down version that only
contains core symbols and strings, and lives in the module's core
section.

After module init (and before we free the module memory), we switch
the mod->symtab, mod->num_symtab and mod->strtab to point to the core
versions.  We do this under the module_mutex.

However, kallsyms doesn't take the module_mutex: it uses
preempt_disable() and rcu tricks to walk through the modules, because
it's used in the oops path.  It's also used in /proc/kallsyms.
There's nothing atomic about the change of these variables, so we can
get the old (larger!) num_symtab and the new symtab pointer; in fact
this is what I saw when trying to reproduce.

By grouping these variables together, we can use a
carefully-dereferenced pointer to ensure we always get one or the
other (the free of the module init section is already done in an RCU
callback, so that's safe).  We allocate the init one at the end of the
module init section, and keep the core one inside the struct module
itself (it could also have been allocated at the end of the module
core, but that's probably overkill).

Reported-by: Weilong Chen <[email protected]>
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=111541
Cc: [email protected]
Signed-off-by: Rusty Russell <[email protected]>
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@rustyrussell
rustyrussell committed Feb 3, 2016
1 parent 2e7bac5 commit 8244062
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Showing 2 changed files with 79 additions and 52 deletions.
19 changes: 10 additions & 9 deletions include/linux/module.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -324,6 +324,12 @@ struct module_layout {
#define __module_layout_align
#endif

struct mod_kallsyms {
Elf_Sym *symtab;
unsigned int num_symtab;
char *strtab;
};

struct module {
enum module_state state;

Expand Down Expand Up @@ -405,15 +411,10 @@ struct module {
#endif

#ifdef CONFIG_KALLSYMS
/*
* We keep the symbol and string tables for kallsyms.
* The core_* fields below are temporary, loader-only (they
* could really be discarded after module init).
*/
Elf_Sym *symtab, *core_symtab;
unsigned int num_symtab, core_num_syms;
char *strtab, *core_strtab;

/* Protected by RCU and/or module_mutex: use rcu_dereference() */
struct mod_kallsyms *kallsyms;
struct mod_kallsyms core_kallsyms;

/* Section attributes */
struct module_sect_attrs *sect_attrs;

Expand Down
112 changes: 69 additions & 43 deletions kernel/module.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -303,6 +303,9 @@ struct load_info {
struct _ddebug *debug;
unsigned int num_debug;
bool sig_ok;
#ifdef CONFIG_KALLSYMS
unsigned long mod_kallsyms_init_off;
#endif
struct {
unsigned int sym, str, mod, vers, info, pcpu;
} index;
Expand Down Expand Up @@ -2480,10 +2483,21 @@ static void layout_symtab(struct module *mod, struct load_info *info)
strsect->sh_flags |= SHF_ALLOC;
strsect->sh_entsize = get_offset(mod, &mod->init_layout.size, strsect,
info->index.str) | INIT_OFFSET_MASK;
mod->init_layout.size = debug_align(mod->init_layout.size);
pr_debug("\t%s\n", info->secstrings + strsect->sh_name);

/* We'll tack temporary mod_kallsyms on the end. */
mod->init_layout.size = ALIGN(mod->init_layout.size,
__alignof__(struct mod_kallsyms));
info->mod_kallsyms_init_off = mod->init_layout.size;
mod->init_layout.size += sizeof(struct mod_kallsyms);
mod->init_layout.size = debug_align(mod->init_layout.size);
}

/*
* We use the full symtab and strtab which layout_symtab arranged to
* be appended to the init section. Later we switch to the cut-down
* core-only ones.
*/
static void add_kallsyms(struct module *mod, const struct load_info *info)
{
unsigned int i, ndst;
Expand All @@ -2492,29 +2506,34 @@ static void add_kallsyms(struct module *mod, const struct load_info *info)
char *s;
Elf_Shdr *symsec = &info->sechdrs[info->index.sym];

mod->symtab = (void *)symsec->sh_addr;
mod->num_symtab = symsec->sh_size / sizeof(Elf_Sym);
/* Set up to point into init section. */
mod->kallsyms = mod->init_layout.base + info->mod_kallsyms_init_off;

mod->kallsyms->symtab = (void *)symsec->sh_addr;
mod->kallsyms->num_symtab = symsec->sh_size / sizeof(Elf_Sym);
/* Make sure we get permanent strtab: don't use info->strtab. */
mod->strtab = (void *)info->sechdrs[info->index.str].sh_addr;
mod->kallsyms->strtab = (void *)info->sechdrs[info->index.str].sh_addr;

/* Set types up while we still have access to sections. */
for (i = 0; i < mod->num_symtab; i++)
mod->symtab[i].st_info = elf_type(&mod->symtab[i], info);

mod->core_symtab = dst = mod->core_layout.base + info->symoffs;
mod->core_strtab = s = mod->core_layout.base + info->stroffs;
src = mod->symtab;
for (ndst = i = 0; i < mod->num_symtab; i++) {
for (i = 0; i < mod->kallsyms->num_symtab; i++)
mod->kallsyms->symtab[i].st_info
= elf_type(&mod->kallsyms->symtab[i], info);

/* Now populate the cut down core kallsyms for after init. */
mod->core_kallsyms.symtab = dst = mod->core_layout.base + info->symoffs;
mod->core_kallsyms.strtab = s = mod->core_layout.base + info->stroffs;
src = mod->kallsyms->symtab;
for (ndst = i = 0; i < mod->kallsyms->num_symtab; i++) {
if (i == 0 ||
is_core_symbol(src+i, info->sechdrs, info->hdr->e_shnum,
info->index.pcpu)) {
dst[ndst] = src[i];
dst[ndst++].st_name = s - mod->core_strtab;
s += strlcpy(s, &mod->strtab[src[i].st_name],
dst[ndst++].st_name = s - mod->core_kallsyms.strtab;
s += strlcpy(s, &mod->kallsyms->strtab[src[i].st_name],
KSYM_NAME_LEN) + 1;
}
}
mod->core_num_syms = ndst;
mod->core_kallsyms.num_symtab = ndst;
}
#else
static inline void layout_symtab(struct module *mod, struct load_info *info)
Expand Down Expand Up @@ -3263,9 +3282,8 @@ static noinline int do_init_module(struct module *mod)
module_put(mod);
trim_init_extable(mod);
#ifdef CONFIG_KALLSYMS
mod->num_symtab = mod->core_num_syms;
mod->symtab = mod->core_symtab;
mod->strtab = mod->core_strtab;
/* Switch to core kallsyms now init is done: kallsyms may be walking! */
rcu_assign_pointer(mod->kallsyms, &mod->core_kallsyms);
#endif
mod_tree_remove_init(mod);
disable_ro_nx(&mod->init_layout);
Expand Down Expand Up @@ -3627,9 +3645,9 @@ static inline int is_arm_mapping_symbol(const char *str)
&& (str[2] == '\0' || str[2] == '.');
}

static const char *symname(struct module *mod, unsigned int symnum)
static const char *symname(struct mod_kallsyms *kallsyms, unsigned int symnum)
{
return mod->strtab + mod->symtab[symnum].st_name;
return kallsyms->strtab + kallsyms->symtab[symnum].st_name;
}

static const char *get_ksymbol(struct module *mod,
Expand All @@ -3639,6 +3657,7 @@ static const char *get_ksymbol(struct module *mod,
{
unsigned int i, best = 0;
unsigned long nextval;
struct mod_kallsyms *kallsyms = rcu_dereference_sched(mod->kallsyms);

/* At worse, next value is at end of module */
if (within_module_init(addr, mod))
Expand All @@ -3648,32 +3667,32 @@ static const char *get_ksymbol(struct module *mod,

/* Scan for closest preceding symbol, and next symbol. (ELF
starts real symbols at 1). */
for (i = 1; i < mod->num_symtab; i++) {
if (mod->symtab[i].st_shndx == SHN_UNDEF)
for (i = 1; i < kallsyms->num_symtab; i++) {
if (kallsyms->symtab[i].st_shndx == SHN_UNDEF)
continue;

/* We ignore unnamed symbols: they're uninformative
* and inserted at a whim. */
if (*symname(mod, i) == '\0'
|| is_arm_mapping_symbol(symname(mod, i)))
if (*symname(kallsyms, i) == '\0'
|| is_arm_mapping_symbol(symname(kallsyms, i)))
continue;

if (mod->symtab[i].st_value <= addr
&& mod->symtab[i].st_value > mod->symtab[best].st_value)
if (kallsyms->symtab[i].st_value <= addr
&& kallsyms->symtab[i].st_value > kallsyms->symtab[best].st_value)
best = i;
if (mod->symtab[i].st_value > addr
&& mod->symtab[i].st_value < nextval)
nextval = mod->symtab[i].st_value;
if (kallsyms->symtab[i].st_value > addr
&& kallsyms->symtab[i].st_value < nextval)
nextval = kallsyms->symtab[i].st_value;
}

if (!best)
return NULL;

if (size)
*size = nextval - mod->symtab[best].st_value;
*size = nextval - kallsyms->symtab[best].st_value;
if (offset)
*offset = addr - mod->symtab[best].st_value;
return symname(mod, best);
*offset = addr - kallsyms->symtab[best].st_value;
return symname(kallsyms, best);
}

/* For kallsyms to ask for address resolution. NULL means not found. Careful
Expand Down Expand Up @@ -3763,18 +3782,21 @@ int module_get_kallsym(unsigned int symnum, unsigned long *value, char *type,

preempt_disable();
list_for_each_entry_rcu(mod, &modules, list) {
struct mod_kallsyms *kallsyms;

if (mod->state == MODULE_STATE_UNFORMED)
continue;
if (symnum < mod->num_symtab) {
*value = mod->symtab[symnum].st_value;
*type = mod->symtab[symnum].st_info;
strlcpy(name, symname(mod, symnum), KSYM_NAME_LEN);
kallsyms = rcu_dereference_sched(mod->kallsyms);
if (symnum < kallsyms->num_symtab) {
*value = kallsyms->symtab[symnum].st_value;
*type = kallsyms->symtab[symnum].st_info;
strlcpy(name, symname(kallsyms, symnum), KSYM_NAME_LEN);
strlcpy(module_name, mod->name, MODULE_NAME_LEN);
*exported = is_exported(name, *value, mod);
preempt_enable();
return 0;
}
symnum -= mod->num_symtab;
symnum -= kallsyms->num_symtab;
}
preempt_enable();
return -ERANGE;
Expand All @@ -3783,11 +3805,12 @@ int module_get_kallsym(unsigned int symnum, unsigned long *value, char *type,
static unsigned long mod_find_symname(struct module *mod, const char *name)
{
unsigned int i;
struct mod_kallsyms *kallsyms = rcu_dereference_sched(mod->kallsyms);

for (i = 0; i < mod->num_symtab; i++)
if (strcmp(name, symname(mod, i)) == 0 &&
mod->symtab[i].st_info != 'U')
return mod->symtab[i].st_value;
for (i = 0; i < kallsyms->num_symtab; i++)
if (strcmp(name, symname(kallsyms, i)) == 0 &&
kallsyms->symtab[i].st_info != 'U')
return kallsyms->symtab[i].st_value;
return 0;
}

Expand Down Expand Up @@ -3826,11 +3849,14 @@ int module_kallsyms_on_each_symbol(int (*fn)(void *, const char *,
module_assert_mutex();

list_for_each_entry(mod, &modules, list) {
/* We hold module_mutex: no need for rcu_dereference_sched */
struct mod_kallsyms *kallsyms = mod->kallsyms;

if (mod->state == MODULE_STATE_UNFORMED)
continue;
for (i = 0; i < mod->num_symtab; i++) {
ret = fn(data, symname(mod, i),
mod, mod->symtab[i].st_value);
for (i = 0; i < kallsyms->num_symtab; i++) {
ret = fn(data, symname(kallsyms, i),
mod, kallsyms->symtab[i].st_value);
if (ret != 0)
return ret;
}
Expand Down

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