Re: [PATCH v3 3/4] arm64: implement live patching
From: Jessica Yu
Date: Fri Oct 19 2018 - 08:18:57 EST
+++ Miroslav Benes [19/10/18 13:59 +0200]:
On Thu, 18 Oct 2018, Jessica Yu wrote:
+++ Miroslav Benes [17/10/18 15:39 +0200]:
>On Mon, 1 Oct 2018, Torsten Duwe wrote:
>
>Ad relocations. I checked that everything in struct mod_arch_specific
>stays after the module is load. Both core and init get SHF_ALLOC set
>(mod->arch.core.plt->sh_flags in module_frob_arch_sections(). It is
>important because apply_relocate_add() may use those sections
>through module_emit_plt_entry() call.
Yes, it looks like the needed .plt sections will remain in module
memory. However, I think I found a slight issue... :/
In module_frob_arch_sections(), mod->arch.core.plt is set to an offset
within info->sechdrs:
if (!strcmp(secstrings + sechdrs[i].sh_name, ".plt"))
mod->arch.core.plt = sechdrs + i;
sechdrs is from info->sechdrs, which is freed at the end of
load_module() via free_copy(). So although the relevant plt section(s)
are in module memory, mod->arch.core.plt will point to invalid memory
after info is freed. In other words, the section (.plt) *is* in memory
but the section header (struct elf64_shdr) containing section metadata
like sh_addr, sh_size etc., is not.
But we have mod->klp_info->sechdrs (which is a copy of the section
headers) for this very reason. It is initialized only at the very end
of load_module(). I don't think copy_module_elf() is dependent on
anything, so it can just be moved earlier.
Note that we cannot set mod->arch.core.plt to mod->klp_info->sechdrs + i
during module_frob_arch_sections() because it is still too early, none
of the module sections have been copied and none of their sh_addr's
have been set to their final locations as this is all happening before
move_module() is called. So we can use a module_finalize() function
for arm64 to switch the mod->arch.core.plt to use the klp_info section
headers.
Thanks for the analysis. You seem to be right.
Maybe this will be more clear in the example fix below (completely
untested and unreviewed!):
diff --git a/arch/arm64/include/asm/module.h b/arch/arm64/include/asm/module.h
index 97d0ef12e2ff..150afc29171b 100644
--- a/arch/arm64/include/asm/module.h
+++ b/arch/arm64/include/asm/module.h
@@ -25,6 +25,7 @@ struct mod_plt_sec {
struct elf64_shdr *plt;
int plt_num_entries;
int plt_max_entries;
+ int plt_shndx;
};
struct mod_arch_specific {
diff --git a/arch/arm64/kernel/module-plts.c b/arch/arm64/kernel/module-plts.c
index f0690c2ca3e0..c23cef8f0165 100644
--- a/arch/arm64/kernel/module-plts.c
+++ b/arch/arm64/kernel/module-plts.c
@@ -196,6 +196,21 @@ static unsigned int count_plts(Elf64_Sym *syms,
Elf64_Rela *rela, int num,
return ret;
}
+int module_finalize(const Elf_Ehdr *hdr, const Elf_Shdr *sechdrs,
+ struct module *mod)
+{
+ /*
+ * Livepatch modules need to keep access to section headers to apply
+ * relocations. Note mod->klp_info should have already been
initialized
+ * and all section sh_addr's should have their final addresses by the
+ * time module_finalize() is called.
+ */
+ if (is_livepatch_module(mod))
+ mod->arch.core.plt = mod->klp_info->sechdrs +
mod->arch.core.plt_shndx;
+
+ return 0;
+}
There is already module_finalize() in arch/arm64/kernel/module.c, so this
should probably go there.
Ah yeah, I missed that :) Yep, that would be where it'd go.
If I am not mistaken, we do not care for arch.init.plt in livepatch. Is
that correct?
I do not believe patching of __init functions is supported (right?) So
we do not need to keep arch.init.plt alive post-module-load.
int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
char *secstrings, struct module *mod)
{
@@ -210,11 +225,13 @@ int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr
*sechdrs,
* entries. Record the symtab address as well.
*/
for (i = 0; i < ehdr->e_shnum; i++) {
- if (!strcmp(secstrings + sechdrs[i].sh_name, ".plt"))
+ if (!strcmp(secstrings + sechdrs[i].sh_name, ".plt")) {
mod->arch.core.plt = sechdrs + i;
- else if (!strcmp(secstrings + sechdrs[i].sh_name,
".init.plt"))
+ mod->arch.core.plt_shndx = i;
+ } else if (!strcmp(secstrings + sechdrs[i].sh_name,
".init.plt")) {
mod->arch.init.plt = sechdrs + i;
- else if (IS_ENABLED(CONFIG_DYNAMIC_FTRACE) &&
+ mod->arch.init.plt_shndx = i;
It is initialized here, but that's not necessarily a bad thing.
I think I added this line for consistency, but I actually don't think
it is needed. We only would need to keep the section index for
arch.core.plt then.
+ } else if (IS_ENABLED(CONFIG_DYNAMIC_FTRACE) &&
!strcmp(secstrings + sechdrs[i].sh_name,
".text.ftrace_trampoline"))
tramp = sechdrs + i;
diff --git a/kernel/module.c b/kernel/module.c
index 6746c85511fe..2fc4d74288dd 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -3363,6 +3363,12 @@ static int post_relocation(struct module *mod, const
struct load_info *info)
/* Setup kallsyms-specific fields. */
add_kallsyms(mod, info);
+ if (is_livepatch_module(mod)) {
+ err = copy_module_elf(mod, info);
+ if (err < 0)
+ return err;
+ }
+
/* Arch-specific module finalizing. */
return module_finalize(info->hdr, info->sechdrs, mod);
}
@@ -3775,12 +3781,6 @@ static int load_module(struct load_info *info, const
char __user *uargs,
if (err < 0)
goto coming_cleanup;
- if (is_livepatch_module(mod)) {
- err = copy_module_elf(mod, info);
- if (err < 0)
- goto sysfs_cleanup;
- }
-
/* Get rid of temporary copy. */
free_copy(info);
Hm, this is hopefully all right. We'd need to change the error handling a
bit. free_module_elf() is now called in free_module() and it should be
moved somewhere to load_module(). Probably under free_arch_cleanup label.
Although it would be much better to rework post_relocation() and handle it
right there.
Something like
- return module_finalize(info->hdr, info->sechdrs, mod);
+ err = module_finalize(info->hdr, info->sechdrs, mod);
+ if (err < 0)
+ free_module_elf();
+
+ return err;
Ah yeah, I'll clean up the error handling. I'll re-submit this as a
real patch after cleaning it up a bit and it could be added to this
series after review.
Thoughts? Does the fix make sense?
I think so.
Thanks Miroslav for the impromptu review :-)
Jessica