Re: [PATCH 3/9] SecurityPkg/DxeImageVerificationLib: fix wrong fetching dbx in IsAllowedByDb(CVE-2019-14575)

["Yao, Jiewen" <jiewen.yao@intel.com>]

Reviewed-by: Jiewen Yao <Jiewen.yao@intel.com>

> -----Original Message-----
> From: Wang, Jian J <jian.j.wang@intel.com>
> Sent: Thursday, February 6, 2020 10:19 PM
> To: devel@edk2.groups.io
> Cc: Yao, Jiewen <jiewen.yao@intel.com>; Zhang, Chao B
> <chao.b.zhang@intel.com>
> Subject: [PATCH 3/9] SecurityPkg/DxeImageVerificationLib: fix wrong fetching
> dbx in IsAllowedByDb(CVE-2019-14575)
> 
> REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1608
> 
> Normally two times of calling gRT->GetVariable() are needed to get
> the data of a variable: get the variable size by passing zero variable
> size, and then allocate enough memory and pass the correct variable size
> and buffer.
> 
> But in the inner loop in IsAllowedByDb(), the DbxDataSize was not
> initialized to zero before calling gRT->GetVariable(). It won't cause
> problem if dbx does not exist. But it will give wrong result if dbx
> exists and the DbxDataSize happens to be a small enough value. In this
> situation, EFI_BUFFER_TOO_SMALL will be returned. Then the result check
> code followed will jump to 'Done', which is not correct because it's
> actually the value expected.
> 
>             if (Status == EFI_BUFFER_TOO_SMALL) {
>               goto Done;
>             }
> 
> Cc: Jiewen Yao <jiewen.yao@intel.com>
> Cc: Chao Zhang <chao.b.zhang@intel.com>
> Signed-off-by: Jian J Wang <jian.j.wang@intel.com>
> ---
>  .../Library/DxeImageVerificationLib/DxeImageVerificationLib.c  | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git
> a/SecurityPkg/Library/DxeImageVerificationLib/DxeImageVerificationLib.c
> b/SecurityPkg/Library/DxeImageVerificationLib/DxeImageVerificationLib.c
> index 5dcd6efed5..1efb2f96cd 100644
> --- a/SecurityPkg/Library/DxeImageVerificationLib/DxeImageVerificationLib.c
> +++ b/SecurityPkg/Library/DxeImageVerificationLib/DxeImageVerificationLib.c
> @@ -1456,8 +1456,9 @@ IsAllowedByDb (
>              //
> 
>              // Here We still need to check if this RootCert's Hash is revoked
> 
>              //
> 
> +            DbxDataSize = 0;
> 
>              Status   = gRT->GetVariable (EFI_IMAGE_SECURITY_DATABASE1,
> &gEfiImageSecurityDatabaseGuid, NULL, &DbxDataSize, NULL);
> 
> -            if (Status == EFI_BUFFER_TOO_SMALL) {
> 
> +            if (Status != EFI_BUFFER_TOO_SMALL) {
> 
>                goto Done;
> 
>              }
> 
>              DbxData = (UINT8 *) AllocateZeroPool (DbxDataSize);
> 
> --
> 2.24.0.windows.2