SECURITY UPDATE: Fix out of bounds read in .ar and .tar implementation (CVE-2020-3810)

When normalizing ar member names by removing trailing whitespace
and slashes, an out-out-bound read can be caused if the ar member
name consists only of such characters, because the code did not
stop at 0, but would wrap around and continue reading from the
stack, without any limit.

Add a check to abort if we reached the first character in the
name, effectively rejecting the use of names consisting just
of slashes and spaces.

Furthermore, certain error cases in arfile.cc and extracttar.cc have
included member names in the output that were not checked at all and
might hence not be nul terminated, leading to further out of bound reads.

Fixes Debian/apt#111
LP: #1878177

2 files changed, 10 insertions, 3 deletions

diff --git a/apt-inst/contrib/arfile.cc b/apt-inst/contrib/arfile.cc
index 3fc3afedb..5cb43c690 100644
--- a/apt-inst/contrib/arfile.cc
+++ b/apt-inst/contrib/arfile.cc
@@ -92,7 +92,7 @@ bool ARArchive::LoadHeaders()
 	  StrToNum(Head.Size,Memb->Size,sizeof(Head.Size)) == false)
       {
 	 delete Memb;
-	 return _error->Error(_("Invalid archive member header %s"), Head.Name);
+	 return _error->Error(_("Invalid archive member header"));
       }
 	 
       // Check for an extra long name string
@@ -119,7 +119,14 @@ bool ARArchive::LoadHeaders()
       else
       {
 	 unsigned int I = sizeof(Head.Name) - 1;
-	 for (; Head.Name[I] == ' ' || Head.Name[I] == '/'; I--);
+	 for (; Head.Name[I] == ' ' || Head.Name[I] == '/'; I--)
+	 {
+	    if (I == 0)
+	    {
+	       delete Memb;
+	       return _error->Error(_("Invalid archive member header"));
+	    }
+	 }
 	 Memb->Name = std::string(Head.Name,I+1);
       }
 
diff --git a/apt-inst/contrib/extracttar.cc b/apt-inst/contrib/extracttar.cc
index 42032ad13..19ac1c8b1 100644
--- a/apt-inst/contrib/extracttar.cc
+++ b/apt-inst/contrib/extracttar.cc
@@ -258,7 +258,7 @@ bool ExtractTar::Go(pkgDirStream &Stream)
 	 
 	 default:
 	 BadRecord = true;
-	 _error->Warning(_("Unknown TAR header type %u, member %s"),(unsigned)Tar->LinkFlag,Tar->Name);
+	 _error->Warning(_("Unknown TAR header type %u"), (unsigned)Tar->LinkFlag);
 	 break;
       }