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

[Stefano Stabellini <sstabellini@kernel.org>]

On Thu, 22 Jun 2017, Andrew Cooper 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@citrix.com>
> ---
> CC: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> CC: Ross Lagerwall <ross.lagerwall@citrix.com>
> CC: Jan Beulich <JBeulich@suse.com>
> CC: Stefano Stabellini <sstabellini@kernel.org>
> CC: Julien Grall <julien.grall@arm.com>
> 
> v3:
>  * Fix off-by-one error
> v2:
>  * Reject STN_UNDEF with -EOPNOTSUPP

Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>


> ---
>  xen/arch/arm/arm32/livepatch.c | 14 +++++++++++++-
>  xen/arch/arm/arm64/livepatch.c | 14 +++++++++++++-
>  xen/arch/x86/livepatch.c       | 14 +++++++++++++-
>  3 files changed, 39 insertions(+), 3 deletions(-)
> 
> diff --git a/xen/arch/arm/arm32/livepatch.c b/xen/arch/arm/arm32/livepatch.c
> index a328179..41378a5 100644
> --- a/xen/arch/arm/arm32/livepatch.c
> +++ b/xen/arch/arm/arm32/livepatch.c
> @@ -254,12 +254,24 @@ int arch_livepatch_perform(struct livepatch_elf *elf,
>              addend = get_addend(type, dest);
>          }
>  
> -        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 symbol wants symbol@%u which is past end!\n",
>                      elf->name, symndx);
>              return -EINVAL;
>          }
> +        else if ( !elf->sym[symndx].sym )
> +        {
> +            dprintk(XENLOG_ERR, LIVEPATCH "%s: No relative symbol@%u\n",
> +                    elf->name, symndx);
> +            return -EINVAL;
> +        }
>  
>          val = elf->sym[symndx].sym->st_value; /* S */
>  
> diff --git a/xen/arch/arm/arm64/livepatch.c b/xen/arch/arm/arm64/livepatch.c
> index 63929b1..2247b92 100644
> --- a/xen/arch/arm/arm64/livepatch.c
> +++ b/xen/arch/arm/arm64/livepatch.c
> @@ -252,12 +252,24 @@ int arch_livepatch_perform_rela(struct livepatch_elf *elf,
>          int ovf = 0;
>          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 )
> +        {
> +            dprintk(XENLOG_ERR, LIVEPATCH "%s: No relative symbol@%u\n",
> +                    elf->name, symndx);
> +            return -EINVAL;
> +        }
>  
>          val = elf->sym[symndx].sym->st_value +  r->r_addend; /* S+A */
>  
> diff --git a/xen/arch/x86/livepatch.c b/xen/arch/x86/livepatch.c
> index 7917610..406eb91 100644
> --- 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 )
> +        {
> +            dprintk(XENLOG_ERR, LIVEPATCH "%s: No symbol@%u\n",
> +                    elf->name, symndx);
> +            return -EINVAL;
> +        }
>  
>          val = r->r_addend + elf->sym[symndx].sym->st_value;
>  
> -- 
> 2.1.4
> 

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