Vulnerability Database

In the Linux kernel, the following vulnerability has been resolved:

bpf: track changes_pkt_data property for global functions

When processing calls to certain helpers, verifier invalidates all packet pointers in a current state. For example, consider the following program:

After a call to bpf_skb_pull_data() the pointer 'p' can't be used safely. See function filter.c:bpf_helper_changes_pkt_data() for a list of such helpers.

At the moment verifier invalidates packet pointers when processing helper function calls, and does not traverse global sub-programs when processing calls to global sub-programs. This means that calls to helpers done from global sub-programs do not invalidate pointers in the caller state. E.g. the program above is unsafe, but is not rejected by verifier.

This commit fixes the omission by computing field bpf_subprog_info->changes_pkt_data for each sub-program before main verification pass. changes_pkt_data should be set if:

• subprogram calls helper for which bpf_helper_changes_pkt_data returns true;
• subprogram calls a global function, for which bpf_subprog_info->changes_pkt_data should be set.

The verifier.c:check_cfg() pass is modified to compute this information. The commit relies on depth first instruction traversal done by check_cfg() and absence of recursive function calls:

• check_cfg() would eventually visit every call to subprogram S in a state when S is fully explored;
• when S is fully explored:
  • every direct helper call within S is explored (and thus changes_pkt_data is set if needed);
  • every call to subprogram S1 called by S was visited with S1 fully explored (and thus S inherits changes_pkt_data from S1).

The downside of such approach is that dead code elimination is not taken into account: if a helper call inside global function is dead because of current configuration, verifier would conservatively assume that the call occurs for the purpose of the changes_pkt_data computation.