Re: [PATCH v7] livepatch: Clear relocation targets on a module removal

[Petr Mladek <pmladek@suse.com>]

On Wed 2022-12-14 09:40:35, Song Liu wrote:
> From: Miroslav Benes <mbenes@suse.cz>
> 
> 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). The solution is not
> universal and is too much arch-specific, but it may prove to be simpler
> in the end.
> 
> --- a/arch/x86/kernel/module.c
> +++ b/arch/x86/kernel/module.c
> @@ -163,40 +165,60 @@ static int __apply_relocate_add(Elf64_Shdr *sechdrs,
>  		case R_X86_64_NONE:
>  			break;
>  		case R_X86_64_64:
> -			if (*(u64 *)loc != 0)
> -				goto invalid_relocation;
> -			write(loc, &val, 8);
> +			if (apply) {
> +				if (*(u64 *)loc != 0)
> +					goto invalid_relocation;
> +				write(loc, &val, 8);
> +			} else {
> +				write(loc, &zero, 8);

It might make sense to check if the cleared value is the
expected one.

				if (*(u64 *)loc != (u64)val)
					goto invalid_relocation;
				write(loc, &zero, 8);

Maybe, we could put this into a helper function or macro that
would do this operation

#define check_and_write(loc, orig_val, new_val, type)	\
({							\
	int err = 0;					\
							\
	if (*(type)loc == (type)old_val)		\
		write(loc, &new_val, sizeof(type));	\
	else						\
		err = -EINVAL;				\
							\
	err;						\
})


It would make it more robust. The relocation might be different
when it it redirected somewhere, for example, by ftrace.
Something might go wrong in this case.

On the other hand. I wonder if the relocation might actually
by redirected intentionally, for example, by apply_alternatives()
or apply_retpolines(). These would be hard to check.

Best Regards,
Petr