Description

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: __attribute__((__noinline__)) long skb_pull_data(struct __sk_buff *sk, __u32 len) { return bpf_skb_pull_data(sk, len); } SEC("tc") int test_invalidate_checks(struct __sk_buff *sk) { int *p = (void *)(long)sk->data; if ((void *)(p + 1) > (void *)(long)sk->data_end) return TCX_DROP; skb_pull_data(sk, 0); *p = 42; return TCX_PASS; } 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.

Metrics

 

CVSS Version 4.0 CVSS Version 3.x CVSS Version 2.0

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CVSS 4.0 Severity and Vector Strings:

NIST: NVD

N/A

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CVSS 3.x Severity and Vector Strings:

NIST: NVD

Base Score:  N/A

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CVSS 2.0 Severity and Vector Strings:

NIST: NVD

Base Score: N/A

NVD assessment not yet provided.

https://git.kernel.org/stable/c/79751e9227a5910c0e5a2c7186877d91821d957d

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:

    __attribute__((__noinline__))
    long skb_pull_data(struct __sk_buff *sk, __u32 len)
    {
        return bpf_skb_pull_data(sk, len);
    }

    SEC("tc")
    int test_invalidate_checks(struct __sk_buff *sk)
    {
        int *p = (void *)(long)sk->data;
        if ((void *)(p + 1) > (void *)(long)sk->data_end) return TCX_DROP;
        skb_pull_data(sk, 0);
        *p = 42;
        return TCX_PASS;
    }

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
  

https://git.kernel.org/stable/c/1d572c60488b52882b719ed273767ee3b280413d

https://git.kernel.org/stable/c/51081a3f25c742da5a659d7fc6fd77ebfdd555be