Re: [Xen-devel] [PATCH for-4.9 v3 3/3] xen/livepatch: Don't crash on encountering STN_UNDEF relocations

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 22.06.17 at 20:15, <andrew.cooper3@xxxxxxxxxx> wrote:
> A symndx of STN_UNDEF is special, and means a symbol value of 0.  While
> legitimate in the ELF standard, its existance in a livepatch is questionable
> at best.  Until a plausible usecase presents itself, reject such a relocation
> with -EOPNOTSUPP.
> 
> Additionally, fix an off-by-one error while range checking symndx, and perform
> a safety check on elf->sym[symndx].sym before derefencing it, to avoid
> tripping over a NULL pointer when calculating val.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
with two remarks:

> --- a/xen/arch/x86/livepatch.c
> +++ b/xen/arch/x86/livepatch.c
> @@ -170,12 +170,24 @@ int arch_livepatch_perform_rela(struct livepatch_elf 
> *elf,
>          uint8_t *dest = base->load_addr + r->r_offset;
>          uint64_t val;
>  
> -        if ( symndx > elf->nsym )
> +        if ( symndx == STN_UNDEF )
> +        {
> +            dprintk(XENLOG_ERR, LIVEPATCH "%s: Encountered STN_UNDEF\n",
> +                    elf->name);
> +            return -EOPNOTSUPP;
> +        }
> +        else if ( symndx >= elf->nsym )
>          {
>              dprintk(XENLOG_ERR, LIVEPATCH "%s: Relative relocation wants 
> symbol@%u which is past end!\n",
>                      elf->name, symndx);
>              return -EINVAL;
>          }
> +        else if ( !elf->sym[symndx].sym )

Neither of the two "else" is really necessary, and elsewhere we've
been telling people to avoid such.

> +        {
> +            dprintk(XENLOG_ERR, LIVEPATCH "%s: No symbol@%u\n",

Symbol tables can grow large, and for large numbers I generally
find hex representation preferable of dec. Otoh the other
(pre-existing) message uses dec too ...

Jan


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