[edk2-devel] [PATCH 2/2] NetworkPkg/HttpDxe: fix 32-bit truncation in HTTPS download

[Laszlo Ersek]

When downloading over TLS, each TLS message ("APP packet") is returned as
a (decrypted) fragment table by EFI_TLS_PROTOCOL.ProcessPacket().

The TlsProcessMessage() function in "NetworkPkg/HttpDxe/HttpsSupport.c"
linearizes the fragment table into a single contiguous data block. The
resultant flat data block contains both TLS headers and data.

The HttpsReceive() function parses the actual application data -- in this
case: decrypted HTTP data -- out of the flattened TLS data block, peeling
off the TLS headers.

The HttpResponseWorker() function in "NetworkPkg/HttpDxe/HttpImpl.c"
propagates this HTTP data outwards, implementing the
EFI_HTTP_PROTOCOL.Response() function.

Now consider the following documentation for EFI_HTTP_PROTOCOL.Response(),
quoted from "MdePkg/Include/Protocol/Http.h":

> It is the responsibility of the caller to allocate a buffer for Body and
> specify the size in BodyLength. If the remote host provides a response
> that contains a content body, up to BodyLength bytes will be copied from
> the receive buffer into Body and BodyLength will be updated with the
> amount of bytes received and copied to Body. This allows the client to
> download a large file in chunks instead of into one contiguous block of
> memory.

Note that, if the caller-allocated buffer is larger than the
server-provided chunk, then the transfer length is limited by the latter.
This is in fact the dominant case when downloading a huge file (for which
UefiBootManagerLib allocated a huge contiguous RAM Disk buffer) in small
TLS messages.

For adjusting BodyLength as described above -- i.e., to the application
data chunk that has been extracted from the TLS message --, the
HttpResponseWorker() function employs the following assignment:

    HttpMsg->BodyLength = MIN (Fragment.Len, (UINT32) HttpMsg->BodyLength);

The (UINT32) cast is motivated by the MIN() requirement -- in
"MdePkg/Include/Base.h" -- that both arguments be of the same type.

"Fragment.Len" (NET_FRAGMENT.Len) has type UINT32, and
"HttpMsg->BodyLength" (EFI_HTTP_MESSAGE.BodyLength) has type UINTN.
Therefore a cast is indeed necessary.

Unfortunately, the cast is done in the wrong direction. Consider the
following circumstances:

- "Fragment.Len" happens to be consistently 16KiB, dictated by the HTTPS
  Server's TLS stack,

- the size of the file to download is 4GiB + N*16KiB, where N is a
  positive integer.

As the download progresses, each received 16KiB application data chunk
brings the *next* input value of BodyLength closer down to 4GiB. The cast
in MIN() always masks off the high-order bits from the input value of
BodyLength, but this is no problem because the low-order bits are nonzero,
therefore the MIN() always permits progress.

However, once BodyLength reaches 4GiB exactly on input, the MIN()
invocation produces a zero value. HttpResponseWorker() adjusts the output
value of BodyLength to zero, and then passes it to HttpParseMessageBody().

HttpParseMessageBody() (in "NetworkPkg/Library/DxeHttpLib/DxeHttpLib.c")
rejects the zero BodyLength with EFI_INVALID_PARAMETER, which is fully
propagated outwards, and aborts the HTTPS download. HttpBootDxe writes the
message "Error: Unexpected network error" to the UEFI console.

For example, a file with size (4GiB + 197MiB) terminates after downloading
just 197MiB.

Invert the direction of the cast: widen "Fragment.Len" to UINTN.

Cc: Jiaxin Wu <jiaxin.wu at intel.com>
Cc: Maciej Rabeda <maciej.rabeda at linux.intel.com>
Cc: Siyuan Fu <siyuan.fu at intel.com>
Signed-off-by: Laszlo Ersek <lersek at redhat.com>
---
 NetworkPkg/HttpDxe/HttpImpl.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/NetworkPkg/HttpDxe/HttpImpl.c b/NetworkPkg/HttpDxe/HttpImpl.c
index 6b877314bd57..1acbb60d1014 100644
--- a/NetworkPkg/HttpDxe/HttpImpl.c
+++ b/NetworkPkg/HttpDxe/HttpImpl.c
@@ -1348,7 +1348,7 @@ HttpResponseWorker (
     //
     // Process the received the body packet.
     //
-    HttpMsg->BodyLength = MIN (Fragment.Len, (UINT32) HttpMsg->BodyLength);
+    HttpMsg->BodyLength = MIN ((UINTN) Fragment.Len, HttpMsg->BodyLength);
 
     CopyMem (HttpMsg->Body, Fragment.Bulk, HttpMsg->BodyLength);
 
-- 
2.19.1.3.g30247aa5d201


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