Re: [Xen-devel] [PATCH v5 08/16] livepatch/arm/x86: Check payload for for unwelcomed symbols.

[Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>]

On 09/21/2016 06:32 PM, Konrad Rzeszutek Wilk wrote:
> Certain platforms, such as ARM [32|64] add extra mapping symbols
> such as $x (for ARM64 instructions), or more interesting to
> this patch: $t (for Thumb instructions). These symbols are suppose
> to help the final linker to make any adjustments (such as
> add an veneer). But more importantly - we do not compile Xen
> with any Thumb instructions (which are variable length) - and
> if we find these mapping symbols we should disallow such payload.
>
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> ---
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> Cc: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
> Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> Cc: Julien Grall <julien.grall@xxxxxxx
> Cc: Jan Beulich <jbeulich@xxxxxxxx>
> Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>
> v3: New submission.
>     Use &sym[i] instead of sym (as that will always be NULL).
> v4: Use bool instead of int for return
>     Update comment in common code about ARM odd symbols.
>     s/_check/_deny/ to make it more clear.
> v5: Also check for $t.* wildcard.
>     Use Julien's variant where we roll the [2] check in the return.
> ---
>  xen/arch/arm/livepatch.c    | 16 ++++++++++++++++
>  xen/arch/x86/livepatch.c    |  7 +++++++
>  xen/common/livepatch_elf.c  |  7 +++++++
>  xen/include/xen/livepatch.h |  2 ++
>  4 files changed, 32 insertions(+)
>
> diff --git a/xen/arch/arm/livepatch.c b/xen/arch/arm/livepatch.c
> index 9959315..5a99ab5 100644
> --- a/xen/arch/arm/livepatch.c
> +++ b/xen/arch/arm/livepatch.c
> @@ -117,6 +117,22 @@ bool arch_livepatch_symbol_ok(const struct livepatch_elf 
> *elf,
>      return true;
>  }
>
> +bool arch_livepatch_symbol_deny(const struct livepatch_elf *elf,
> +                                const struct livepatch_elf_sym *sym)
> +{
> +#ifdef CONFIG_ARM_32
> +    /*
> +     * Xen does not use Thumb instructions - and we should not see any of
> +     * them. If we do, abort.
> +     */
> +    if ( sym->name && sym->name[0] == '$' && sym->name[1] == 't' )
> +    {
> +        return ( !sym->name[2] || sym->name[2] == '.' );
> +    }
> +#endif
> +    return false;
> +}
> +
>  int arch_livepatch_perform_rel(struct livepatch_elf *elf,
>                                 const struct livepatch_elf_sec *base,
>                                 const struct livepatch_elf_sec *rela)
> diff --git a/xen/arch/x86/livepatch.c b/xen/arch/x86/livepatch.c
> index 7a369a0..9663ef6 100644
> --- a/xen/arch/x86/livepatch.c
> +++ b/xen/arch/x86/livepatch.c
> @@ -131,6 +131,13 @@ bool arch_livepatch_symbol_ok(const struct livepatch_elf 
> *elf,
>      return true;
>  }
>
> +bool arch_livepatch_symbol_deny(const struct livepatch_elf *elf,
> +                                const struct livepatch_elf_sym *sym)
> +{
> +    /* No special checks on x86. */
> +    return false;
> +}
> +
>  int arch_livepatch_perform_rel(struct livepatch_elf *elf,
>                                 const struct livepatch_elf_sec *base,
>                                 const struct livepatch_elf_sec *rela)
> diff --git a/xen/common/livepatch_elf.c b/xen/common/livepatch_elf.c
> index f46990e..ec74beb 100644
> --- a/xen/common/livepatch_elf.c
> +++ b/xen/common/livepatch_elf.c
> @@ -251,6 +251,13 @@ static int elf_get_sym(struct livepatch_elf *elf, const 
> void *data)
>
>          sym[i].sym = s;
>          sym[i].name = strtab_sec->data + delta;
> +        /* e.g. On ARM we should NEVER see $t* symbols. */
I'd prefer this comment in ARM's arch_livepatch_symbol_deny.

Either way,
Reviewed-by: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>

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