Re: [PATCH 2/3] SecurityPkg/DxeImageVerificationLib: assign WinCertificate after size check

["wenyi,xie" <xiewenyi2@huawei.com>]

On 2020/9/1 17:12, Laszlo Ersek wrote:
> Currently the (SecDataDirLeft <= sizeof (WIN_CERTIFICATE)) check only
> guards the de-referencing of the "WinCertificate" pointer. It does not
> guard the calculation of the pointer itself:
> 
>   WinCertificate = (WIN_CERTIFICATE *) (mImageBase + OffSet);
> 
> This is wrong; if we don't know for sure that we have enough room for a
> WIN_CERTIFICATE, then even creating such a pointer, not just
> de-referencing it, may invoke undefined behavior.
> 
> Move the pointer calculation after the size check.
> 
> Cc: Jian J Wang <jian.j.wang@intel.com>
> Cc: Jiewen Yao <jiewen.yao@intel.com>
> Cc: Min Xu <min.m.xu@intel.com>
> Cc: Wenyi Xie <xiewenyi2@huawei.com>
> Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2215
> Signed-off-by: Laszlo Ersek <lersek@redhat.com>

Tested-by: Wenyi Xie <xiewenyi2@huawei.com>

> ---
>  SecurityPkg/Library/DxeImageVerificationLib/DxeImageVerificationLib.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/SecurityPkg/Library/DxeImageVerificationLib/DxeImageVerificationLib.c b/SecurityPkg/Library/DxeImageVerificationLib/DxeImageVerificationLib.c
> index 377feebb205a..100739eb3eb6 100644
> --- a/SecurityPkg/Library/DxeImageVerificationLib/DxeImageVerificationLib.c
> +++ b/SecurityPkg/Library/DxeImageVerificationLib/DxeImageVerificationLib.c
> @@ -1855,10 +1855,12 @@ DxeImageVerificationHandler (
>    for (OffSet = SecDataDir->VirtualAddress;
>         OffSet < SecDataDirEnd;
>         OffSet += (WinCertificate->dwLength + ALIGN_SIZE (WinCertificate->dwLength))) {
> -    WinCertificate = (WIN_CERTIFICATE *) (mImageBase + OffSet);
>      SecDataDirLeft = SecDataDirEnd - OffSet;
> -    if (SecDataDirLeft <= sizeof (WIN_CERTIFICATE) ||
> -        SecDataDirLeft < WinCertificate->dwLength) {
> +    if (SecDataDirLeft <= sizeof (WIN_CERTIFICATE)) {
> +      break;
> +    }
> +    WinCertificate = (WIN_CERTIFICATE *) (mImageBase + OffSet);
> +    if (SecDataDirLeft < WinCertificate->dwLength) {
>        break;
>      }
>  
>