On 19/02/2018 15:47, Denys Vlasenko wrote:
Let's also brainstorm option 3:
Allow symlinks which
(a) start with one or more "../";
(b) never end up on a higher level of directory tree:
"../dir/.." is ok,
"../../usr/bin/.." is ok,
"../file" is not ok,
"../../dir/file" is not ok;
and (c) they also should not leave tarred tree:
symlink "sbin/mkswap" to "../bin/busybox" is ok, but
symlink "mkswap" to "../bin/busybox" is not ok (it hops "above" the tree).

This sounds a bit complicated, but it can be achieved just by looking
at names, no multiple lstat() calls for every open() needed.

With only these symlinks allowed, we know that if we untar to an empty
directory or to a formerly empty directory with another tarball
untarred, any pathname we open, whilst it can contain components which
are symlinks, they nevertheless can not allow new opens to "leave" the
tree and create files elsewhere.

This wouldn't be safe, I think. Consider a tarball containing

   a/a -> ../b
   a/a/a -> ../../c

The link to "../../c" is not allowed - it fails criteria (b).

You wrote "it can be achieved just by looking at names", but that's not enough here: you have to know that a/a/a is actually b/a, so only one level deep in the -C directory, to know that ../../c points outside the -C directory. Since the a/a -> ../b symlink may not even be in this archive, the only way to determine that is by lstat().

Harald van Dijk
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