libarchive: do not extract unsafe symlinks unless $EXTRACT_UNSAFE_SYMLINKS=1

function                                             old     new   delta
unsafe_symlink_target                                  -     147    +147
unzip_main                                          2711    2732     +21
copy_file                                           1657    1678     +21
tar_main                                             999     971     -28
data_extract_all                                    1038     984     -54
------------------------------------------------------------------------------
(add/remove: 2/0 grow/shrink: 2/2 up/down: 189/-82)           Total: 107 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

3 files changed, 65 insertions, 22 deletions

diff --git a/archival/libarchive/Kbuild.src b/archival/libarchive/Kbuild.src
index 942e755b9..e1a8a7529 100644
--- a/archival/libarchive/Kbuild.src
+++ b/archival/libarchive/Kbuild.src
@@ -12,6 +12,8 @@ COMMON_FILES:= \
         data_extract_all.o \
         data_extract_to_stdout.o \
 \
+        unsafe_symlink_target.o \
+\
         filter_accept_all.o \
         filter_accept_list.o \
         filter_accept_reject_list.o \
diff --git a/archival/libarchive/data_extract_all.c b/archival/libarchive/data_extract_all.c
index 1ce927c2f..e658444e0 100644
--- a/archival/libarchive/data_extract_all.c
+++ b/archival/libarchive/data_extract_all.c
@@ -129,9 +129,7 @@ void FAST_FUNC data_extract_all(archive_handle_t *archive_handle)
                 if (res != 0 && !(archive_handle->ah_flags & ARCHIVE_EXTRACT_QUIET)) {
                         /* shared message */
                         bb_perror_msg("can't create %slink '%s' to '%s'",
-                                        "hard",
-                                        dst_name,
-                                        hard_link
+                                "hard", dst_name, hard_link
                         );
                 }
                 /* Hardlinks have no separate mode/ownership, skip chown/chmod */
@@ -181,7 +179,9 @@ void FAST_FUNC data_extract_all(archive_handle_t *archive_handle)
 //TODO: what if file_header->link_target == NULL (say, corrupted tarball?)
 
                 /* To avoid a directory traversal attack via symlinks,
-                 * for certain link targets postpone creation of symlinks.
+                 * do not restore symlinks with ".." components
+                 * or symlinks starting with "/", unless a magic
+                 * envvar is set.
                  *
                  * For example, consider a .tar created via:
                  *  $ tar cvf bug.tar anything.txt
@@ -199,24 +199,17 @@ void FAST_FUNC data_extract_all(archive_handle_t *archive_handle)
                  *
                  * Untarring bug.tar would otherwise place evil.py in '/tmp'.
                  */
-                if (file_header->link_target[0] == '/'
-                 || strstr(file_header->link_target, "..")
-                ) {
-                        llist_add_to(&archive_handle->symlink_placeholders,
-                                xasprintf("%s%c%s", file_header->name, '\0', file_header->link_target)
-                        );
-                        break;
-                }
-                res = symlink(file_header->link_target, dst_name);
-                if (res != 0
-                 && !(archive_handle->ah_flags & ARCHIVE_EXTRACT_QUIET)
-                ) {
-                        /* shared message */
-                        bb_perror_msg("can't create %slink '%s' to '%s'",
-                                "sym",
-                                dst_name,
-                                file_header->link_target
-                        );
+                if (!unsafe_symlink_target(file_header->link_target)) {
+                        res = symlink(file_header->link_target, dst_name);
+                        if (res != 0
+                         && !(archive_handle->ah_flags & ARCHIVE_EXTRACT_QUIET)
+                        ) {
+                                /* shared message */
+                                bb_perror_msg("can't create %slink '%s' to '%s'",
+                                        "sym",
+                                        dst_name, file_header->link_target
+                                );
+                        }
                 }
                 break;
         case S_IFSOCK:
diff --git a/archival/libarchive/unsafe_symlink_target.c b/archival/libarchive/unsafe_symlink_target.c
new file mode 100644
index 000000000..441ba8b24
--- /dev/null
+++ b/archival/libarchive/unsafe_symlink_target.c
@@ -0,0 +1,48 @@
+/* vi: set sw=4 ts=4: */
+/*
+ * Licensed under GPLv2 or later, see file LICENSE in this source tree.
+ */
+#include "libbb.h"
+#include "bb_archive.h"
+
+int FAST_FUNC unsafe_symlink_target(const char *target)
+{
+        const char *dot;
+
+        if (target[0] == '/') {
+                const char *var;
+ unsafe:
+                var = getenv("EXTRACT_UNSAFE_SYMLINKS");
+                if (var) {
+                        if (LONE_CHAR(var, '1'))
+                                return 0; /* pretend it's safe */
+                        return 1; /* "UNSAFE!" */
+                }
+                bb_error_msg("skipping unsafe symlink to '%s' in archive,"
+                        " set %s=1 to extract",
+                        target,
+                        "EXTRACT_UNSAFE_SYMLINKS"
+                );
+                /* Prevent further messages */
+                setenv("EXTRACT_UNSAFE_SYMLINKS", "0", 0);
+                return 1; /* "UNSAFE!" */
+        }
+
+        dot = target;
+        for (;;) {
+                dot = strchr(dot, '.');
+                if (!dot)
+                        return 0; /* safe target */
+
+                /* Is it a path component starting with ".."? */
+                if ((dot[1] == '.')
+                 && (dot == target || dot[-1] == '/')
+                    /* Is it exactly ".."? */
+                 && (dot[2] == '/' || dot[2] == '\0')
+                ) {
+                        goto unsafe;
+                }
+                /* NB: it can even be trailing ".", should only add 1 */
+                dot += 1;
+        }
+}