CVE-2020-27350: tarfile: integer overflow: Limit tar items to 128 GiB

The integer overflow was detected by DonKult who added a check like this:

(std::numeric_limits<decltype(Itm.Size)>::max() - (2 * sizeof(Block)))

Which deals with the code as is, but also still is a fairly big limit,
and could become fragile if we change the code. Let's limit our file
sizes to 128 GiB, which should be sufficient for everyone.

Original comment by DonKult:

The code assumes that it can add sizeof(Block)-1 to the size of the item
later on, but if we are close to a 64bit overflow this is not possible.
Fixing this seems too complex compared to just ensuring there is enough
room left given that we will have a lot more problems the moment we will
be acting on files that large as if the item is that large, the (valid)
tar including it probably doesn't fit in 64bit either.

1 files changed, 10 insertions, 0 deletions

diff --git a/apt-pkg/contrib/extracttar.cc b/apt-pkg/contrib/extracttar.cc
index f653e227e..dc2437528 100644
--- a/apt-pkg/contrib/extracttar.cc
+++ b/apt-pkg/contrib/extracttar.cc
@@ -61,6 +61,11 @@ struct ExtractTar::TarHeader
 // us with large streams.
 static const unsigned long long APT_LONGNAME_LIMIT = 1048576llu;
 
+// A file size limit that we allow extracting. Currently, that's 128 GB.
+// We also should leave some wiggle room for code adding files to it, and
+// possibly conversion for signed, so this should not be larger than like 2**62.
+static const unsigned long long APT_FILESIZE_LIMIT = 1llu << 37;
+
 // ExtractTar::ExtractTar - Constructor					/*{{{*/
 // ---------------------------------------------------------------------
 /* */
@@ -171,6 +176,11 @@ bool ExtractTar::Go(pkgDirStream &Stream)
 	  StrToNum(Tar->Minor,Itm.Minor,sizeof(Tar->Minor),8) == false)
 	 return _error->Error(_("Corrupted archive"));
 
+      // Security check. Prevents overflows below the code when rounding up in skip/copy code,
+      // and provides modest protection against decompression bombs.
+      if (Itm.Size > APT_FILESIZE_LIMIT)
+	 return _error->Error("Tar member too large: %llu > %llu bytes", Itm.Size, APT_FILESIZE_LIMIT);
+
       // Grab the filename and link target: use last long name if one was
       // set, otherwise use the header value as-is, but remember that it may
       // fill the entire 100-byte block and needs to be zero-terminated.