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3f00c52393
Kfuncs currently support specifying the KF_TRUSTED_ARGS flag to signal
to the verifier that it should enforce that a BPF program passes it a
"safe", trusted pointer. Currently, "safe" means that the pointer is
either PTR_TO_CTX, or is refcounted. There may be cases, however, where
the kernel passes a BPF program a safe / trusted pointer to an object
that the BPF program wishes to use as a kptr, but because the object
does not yet have a ref_obj_id from the perspective of the verifier, the
program would be unable to pass it to a KF_ACQUIRE | KF_TRUSTED_ARGS
kfunc.
The solution is to expand the set of pointers that are considered
trusted according to KF_TRUSTED_ARGS, so that programs can invoke kfuncs
with these pointers without getting rejected by the verifier.
There is already a PTR_UNTRUSTED flag that is set in some scenarios,
such as when a BPF program reads a kptr directly from a map
without performing a bpf_kptr_xchg() call. These pointers of course can
and should be rejected by the verifier. Unfortunately, however,
PTR_UNTRUSTED does not cover all the cases for safety that need to
be addressed to adequately protect kfuncs. Specifically, pointers
obtained by a BPF program "walking" a struct are _not_ considered
PTR_UNTRUSTED according to BPF. For example, say that we were to add a
kfunc called bpf_task_acquire(), with KF_ACQUIRE | KF_TRUSTED_ARGS, to
acquire a struct task_struct *. If we only used PTR_UNTRUSTED to signal
that a task was unsafe to pass to a kfunc, the verifier would mistakenly
allow the following unsafe BPF program to be loaded:
SEC("tp_btf/task_newtask")
int BPF_PROG(unsafe_acquire_task,
struct task_struct *task,
u64 clone_flags)
{
struct task_struct *acquired, *nested;
nested = task->last_wakee;
/* Would not be rejected by the verifier. */
acquired = bpf_task_acquire(nested);
if (!acquired)
return 0;
bpf_task_release(acquired);
return 0;
}
To address this, this patch defines a new type flag called PTR_TRUSTED
which tracks whether a PTR_TO_BTF_ID pointer is safe to pass to a
KF_TRUSTED_ARGS kfunc or a BPF helper function. PTR_TRUSTED pointers are
passed directly from the kernel as a tracepoint or struct_ops callback
argument. Any nested pointer that is obtained from walking a PTR_TRUSTED
pointer is no longer PTR_TRUSTED. From the example above, the struct
task_struct *task argument is PTR_TRUSTED, but the 'nested' pointer
obtained from 'task->last_wakee' is not PTR_TRUSTED.
A subsequent patch will add kfuncs for storing a task kfunc as a kptr,
and then another patch will add selftests to validate.
Signed-off-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20221120051004.3605026-3-void@manifault.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
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| .. | ||
| libbpf | ||
| bpf_design_QA.rst | ||
| bpf_devel_QA.rst | ||
| bpf_licensing.rst | ||
| bpf_prog_run.rst | ||
| btf.rst | ||
| clang-notes.rst | ||
| classic_vs_extended.rst | ||
| drgn.rst | ||
| faq.rst | ||
| helpers.rst | ||
| index.rst | ||
| instruction-set.rst | ||
| kfuncs.rst | ||
| linux-notes.rst | ||
| llvm_reloc.rst | ||
| map_array.rst | ||
| map_cgroup_storage.rst | ||
| map_cgrp_storage.rst | ||
| map_cpumap.rst | ||
| map_devmap.rst | ||
| map_hash.rst | ||
| map_lpm_trie.rst | ||
| map_of_maps.rst | ||
| map_queue_stack.rst | ||
| maps.rst | ||
| other.rst | ||
| prog_cgroup_sockopt.rst | ||
| prog_cgroup_sysctl.rst | ||
| prog_flow_dissector.rst | ||
| prog_lsm.rst | ||
| prog_sk_lookup.rst | ||
| programs.rst | ||
| redirect.rst | ||
| ringbuf.rst | ||
| s390.rst | ||
| syscall_api.rst | ||
| test_debug.rst | ||
| verifier.rst | ||