Re: [RFC PATCH 2/2] livepatch: Clear relocation targets on a module removal

[Petr Mladek <pmladek@suse.com>]

On Fri 2019-07-19 14:28:40, Miroslav Benes wrote:
> Josh reported a bug:
> 
>   When the object to be patched is a module, and that module is
>   rmmod'ed and reloaded, it fails to load with:
> 
>   module: x86/modules: Skipping invalid relocation target, existing value is nonzero for type 2, loc 00000000ba0302e9, val ffffffffa03e293c
>   livepatch: failed to initialize patch 'livepatch_nfsd' for module 'nfsd' (-8)
>   livepatch: patch 'livepatch_nfsd' failed for module 'nfsd', refusing to load module 'nfsd'
> 
>   The livepatch module has a relocation which references a symbol
>   in the _previous_ loading of nfsd. When apply_relocate_add()
>   tries to replace the old relocation with a new one, it sees that
>   the previous one is nonzero and it errors out.
> 
>   On ppc64le, we have a similar issue:
> 
>   module_64: livepatch_nfsd: Expected nop after call, got e8410018 at e_show+0x60/0x548 [livepatch_nfsd]
>   livepatch: failed to initialize patch 'livepatch_nfsd' for module 'nfsd' (-8)
>   livepatch: patch 'livepatch_nfsd' failed for module 'nfsd', refusing to load module 'nfsd'
> 
> He also proposed three different solutions. We could remove the error
> check in apply_relocate_add() introduced by commit eda9cec4c9a1
> ("x86/module: Detect and skip invalid relocations"). However the check
> is useful for detecting corrupted modules.
> 
> We could also deny the patched modules to be removed. If it proved to be
> a major drawback for users, we could still implement a different
> approach. The solution would also complicate the existing code a lot.
> 
> We thus decided to reverse the relocation patching (clear all relocation
> targets on x86_64, or return back nops on powerpc). The solution is not
> universal and is too much arch-specific, but it may prove to be simpler
> in the end.
> 
> Reported-by: Josh Poimboeuf <jpoimboe@redhat.com>
> Signed-off-by: Miroslav Benes <mbenes@suse.cz>
> ---
>  arch/powerpc/kernel/Makefile    |  1 +
>  arch/powerpc/kernel/livepatch.c | 75 +++++++++++++++++++++++++++++++++
>  arch/powerpc/kernel/module.h    | 15 +++++++
>  arch/powerpc/kernel/module_64.c |  7 +--
>  arch/x86/kernel/livepatch.c     | 67 +++++++++++++++++++++++++++++
>  include/linux/livepatch.h       |  5 +++
>  kernel/livepatch/core.c         | 17 +++++---
>  7 files changed, 176 insertions(+), 11 deletions(-)
>  create mode 100644 arch/powerpc/kernel/livepatch.c
>  create mode 100644 arch/powerpc/kernel/module.h
> 
> diff --git a/arch/powerpc/kernel/Makefile b/arch/powerpc/kernel/Makefile
> index 0ea6c4aa3a20..639000f78dc3 100644
> --- a/arch/powerpc/kernel/Makefile
> +++ b/arch/powerpc/kernel/Makefile
> @@ -154,6 +154,7 @@ endif
>  
>  obj-$(CONFIG_EPAPR_PARAVIRT)	+= epapr_paravirt.o epapr_hcalls.o
>  obj-$(CONFIG_KVM_GUEST)		+= kvm.o kvm_emul.o
> +obj-$(CONFIG_LIVEPATCH)	+= livepatch.o
>  
>  # Disable GCOV, KCOV & sanitizers in odd or sensitive code
>  GCOV_PROFILE_prom_init.o := n
> diff --git a/arch/powerpc/kernel/livepatch.c b/arch/powerpc/kernel/livepatch.c
> new file mode 100644
> index 000000000000..6f2468c60695
> --- /dev/null
> +++ b/arch/powerpc/kernel/livepatch.c
> @@ -0,0 +1,75 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +/*
> + * livepatch.c - powerpc-specific Kernel Live Patching Core
> + */
> +
> +#include <linux/livepatch.h>
> +#include <asm/code-patching.h>
> +#include "module.h"
> +
> +void arch_klp_free_object_loaded(struct klp_patch *patch,
> +				 struct klp_object *obj)

If I get it correctly then this functions reverts changes done by
klp_write_object_relocations(). Therefore it should get called
klp_clear_object_relocations() or so.

There is also arch_klp_init_object_loaded() but it does different
things, for example it applies alternatives or paravirt instructions.
Do we need to revert these as well?


> +{
> +	const char *objname, *secname, *symname;
> +	char sec_objname[MODULE_NAME_LEN];
> +	struct klp_modinfo *info;
> +	Elf64_Shdr *s;
> +	Elf64_Rela *rel;
> +	Elf64_Sym *sym;
> +	void *loc;
> +	u32 *instruction;
> +	int i, cnt;
> +
> +	info = patch->mod->klp_info;
> +	objname = klp_is_module(obj) ? obj->name : "vmlinux";
> +
> +	/* See livepatch core code for BUILD_BUG_ON() explanation */
> +	BUILD_BUG_ON(MODULE_NAME_LEN < 56 || KSYM_NAME_LEN != 128);
> +
> +	/* For each klp relocation section */
> +	for (s = info->sechdrs; s < info->sechdrs + info->hdr.e_shnum; s++) {
> +		if (!(s->sh_flags & SHF_RELA_LIVEPATCH))
> +			continue;
> +
> +		/*
> +		 * Format: .klp.rela.sec_objname.section_name
> +		 */
> +		secname = info->secstrings + s->sh_name;
> +		cnt = sscanf(secname, ".klp.rela.%55[^.]", sec_objname);
> +		if (cnt != 1) {
> +			pr_err("section %s has an incorrectly formatted name\n",
> +			       secname);
> +			continue;
> +		}
> +
> +		if (strcmp(objname, sec_objname))
> +			continue;

The above code seems to be arch-independent. Please, move it into
klp_clear_object_relocations() or so.

> +		rel = (void *)s->sh_addr;
> +		for (i = 0; i < s->sh_size / sizeof(*rel); i++) {
> +			loc = (void *)info->sechdrs[s->sh_info].sh_addr
> +				+ rel[i].r_offset;
> +			sym = (Elf64_Sym *)info->sechdrs[info->symndx].sh_addr
> +				+ ELF64_R_SYM(rel[i].r_info);
> +			symname = patch->mod->core_kallsyms.strtab
> +				+ sym->st_name;
> +
> +			if (ELF64_R_TYPE(rel[i].r_info) != R_PPC_REL24)
> +				continue;
> +
> +			if (sym->st_shndx != SHN_UNDEF &&
> +			    sym->st_shndx != SHN_LIVEPATCH)
> +				continue;

The above check is livepatch-specific. But in principle, this should
revert changes done by apply_relocate_add(). I would implement
apply_relocation_clear() or apply_relocation_del() or ...
and call it from the generic klp_clear_object_relocations().

The code should be put into the same source files as
apply_relocate_add(). It will increase the chance that
any changes will be in sync.

Of course, it is possible that there was a reason for the
livepatch-specific filtering that I am not aware of.

> +
> +			instruction = (u32 *)loc;
> +			if (is_mprofile_mcount_callsite(symname, instruction))
> +				continue;
> +
> +			if (!instr_is_relative_link_branch(*instruction))
> +				continue;
> +
> +			instruction += 1;
> +			*instruction = PPC_INST_NOP;
> +		}
> +	}
> +}

Otherwise, this approach looks fine to me. I believe that this area
is pretty stable and the maintenance should be rather cheap.

Best Regards,
Petr
acceptable.