Re: [PATCH v10 2/2] livepatch,x86: Clear relocation targets on a module removal

[Josh Poimboeuf <jpoimboe@kernel.org>]

On Tue, Jan 24, 2023 at 01:24:15PM +0100, Petr Mladek wrote:
> On Fri 2023-01-20 16:49:45, Song Liu 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.
> > 
> > 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.
> > 
> > Reported-by: Josh Poimboeuf <jpoimboe@redhat.com>
> > Originally-by: Miroslav Benes <mbenes@suse.cz>
> > Signed-off-by: Song Liu <song@kernel.org>
> > Acked-by: Miroslav Benes <mbenes@suse.cz>
> > 
> > --- a/arch/x86/kernel/module.c
> > +++ b/arch/x86/kernel/module.c
> > @@ -129,22 +129,27 @@ int apply_relocate(Elf32_Shdr *sechdrs,
> >  	return 0;
> >  }
> >  #else /*X86_64*/
> > -static int __apply_relocate_add(Elf64_Shdr *sechdrs,
> > +static int __write_relocate_add(Elf64_Shdr *sechdrs,
> >  		   const char *strtab,
> >  		   unsigned int symindex,
> >  		   unsigned int relsec,
> >  		   struct module *me,
> > -		   void *(*write)(void *dest, const void *src, size_t len))
> > +		   void *(*write)(void *dest, const void *src, size_t len),
> > +		   bool apply)
> >  {
> >  	unsigned int i;
> >  	Elf64_Rela *rel = (void *)sechdrs[relsec].sh_addr;
> >  	Elf64_Sym *sym;
> >  	void *loc;
> >  	u64 val;
> > +	u64 zero = 0ULL;
> >  
> > -	DEBUGP("Applying relocate section %u to %u\n",
> > +	DEBUGP("%s relocate section %u to %u\n",
> > +	       apply ? "Applying" : "Clearing",
> >  	       relsec, sechdrs[relsec].sh_info);
> >  	for (i = 0; i < sechdrs[relsec].sh_size / sizeof(*rel); i++) {
> > +		int size = 0;
> 
> The value 0 should never be used. It is better to do not initialize
> it at all so that the compiler would warn when the variable might be
> used uninitialized.

Yes.  Also it can be unsigned, i.e. size_t.

-- 
Josh