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module: kallsyms: Ensure preemption in add_kallsyms() with PREEMPT_RT

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The commit 08126db5ff ("module: kallsyms: Fix suspicious rcu usage")
under PREEMPT_RT=y, disabling preemption introduced an unbounded
latency since the loop is not fixed. This change caused a regression
since previously preemption was not disabled and we would dereference
RCU-protected pointers explicitly. That being said, these pointers
cannot change.

Before kallsyms-specific data is prepared/or set-up, we ensure that
the unformed module is known to be unique i.e. does not already exist
(see load_module()). Therefore, we can fix this by using the common and
more appropriate RCU flavour as this section of code can be safely
preempted.

Reported-by: Steven Rostedt <rostedt@goodmis.org>
Fixes: 08126db5ff ("module: kallsyms: Fix suspicious rcu usage")
Signed-off-by: Aaron Tomlin <atomlin@redhat.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
This commit is contained in:
Aaron Tomlin 2022-07-11 18:17:19 +01:00 committed by Luis Chamberlain
parent 2cc39179ac
commit e69a66147d
1 changed files with 11 additions and 11 deletions
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22
kernel/module/kallsyms.c
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@ -176,14 +176,14 @@ void add_kallsyms(struct module *mod, const struct load_info *info)
mod->kallsyms = (void __rcu *)mod->init_layout.base +
info->mod_kallsyms_init_off;
preempt_disable();
rcu_read_lock();
/* The following is safe since this pointer cannot change */
rcu_dereference_sched(mod->kallsyms)->symtab = (void *)symsec->sh_addr;
rcu_dereference_sched(mod->kallsyms)->num_symtab = symsec->sh_size / sizeof(Elf_Sym);
rcu_dereference(mod->kallsyms)->symtab = (void *)symsec->sh_addr;
rcu_dereference(mod->kallsyms)->num_symtab = symsec->sh_size / sizeof(Elf_Sym);
/* Make sure we get permanent strtab: don't use info->strtab. */
rcu_dereference_sched(mod->kallsyms)->strtab =
rcu_dereference(mod->kallsyms)->strtab =
(void *)info->sechdrs[info->index.str].sh_addr;
rcu_dereference_sched(mod->kallsyms)->typetab = mod->init_layout.base + info->init_typeoffs;
rcu_dereference(mod->kallsyms)->typetab = mod->init_layout.base + info->init_typeoffs;
/*
* Now populate the cut down core kallsyms for after init
@ -193,20 +193,20 @@ void add_kallsyms(struct module *mod, const struct load_info *info)
mod->core_kallsyms.strtab = s = mod->data_layout.base + info->stroffs;
mod->core_kallsyms.typetab = mod->data_layout.base + info->core_typeoffs;
strtab_size = info->core_typeoffs - info->stroffs;
src = rcu_dereference_sched(mod->kallsyms)->symtab;
for (ndst = i = 0; i < rcu_dereference_sched(mod->kallsyms)->num_symtab; i++) {
rcu_dereference_sched(mod->kallsyms)->typetab[i] = elf_type(src + i, info);
src = rcu_dereference(mod->kallsyms)->symtab;
for (ndst = i = 0; i < rcu_dereference(mod->kallsyms)->num_symtab; i++) {
rcu_dereference(mod->kallsyms)->typetab[i] = elf_type(src + i, info);
if (i == 0 || is_livepatch_module(mod) ||
is_core_symbol(src + i, info->sechdrs, info->hdr->e_shnum,
info->index.pcpu)) {
ssize_t ret;
mod->core_kallsyms.typetab[ndst] =
rcu_dereference_sched(mod->kallsyms)->typetab[i];
rcu_dereference(mod->kallsyms)->typetab[i];
dst[ndst] = src[i];
dst[ndst++].st_name = s - mod->core_kallsyms.strtab;
ret = strscpy(s,
&rcu_dereference_sched(mod->kallsyms)->strtab[src[i].st_name],
&rcu_dereference(mod->kallsyms)->strtab[src[i].st_name],
strtab_size);
if (ret < 0)
break;
@ -214,7 +214,7 @@ void add_kallsyms(struct module *mod, const struct load_info *info)
strtab_size -= ret + 1;
}
}
preempt_enable();
rcu_read_unlock();
mod->core_kallsyms.num_symtab = ndst;
}