In the Linux kernel, the following vulnerability has been resolved: bpf: track changespktdata 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: attribute((noinline)) long skbpulldata(struct _skbuff *sk, _u32 len) { return bpfskbpulldata(sk, len); } SEC("tc") int testinvalidatechecks(struct _skbuff *sk) { int *p = (void *)(long)sk->data; if ((void *)(p + 1) > (void *)(long)sk->dataend) return TCXDROP; skbpulldata(sk, 0); *p = 42; return TCXPASS; } After a call to bpfskbpulldata() the pointer 'p' can't be used safely. See function filter.c:bpfhelperchangespktdata() 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 bpfsubproginfo->changespktdata for each sub-program before main verification pass. changespktdata should be set if: - subprogram calls helper for which bpfhelperchangespktdata returns true; - subprogram calls a global function, for which bpfsubproginfo->changespktdata should be set. The verifier.c:checkcfg() pass is modified to compute this information. The commit relies on depth first instruction traversal done by checkcfg() and absence of recursive function calls: - checkcfg() 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 changespktdata is set if needed); - every call to subprogram S1 called by S was visited with S1 fully explored (and thus S inherits changespktdata 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 changespkt_data computation.
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