bpf: Require CAP_PERFMON for kfuncs reading memory

Mark fault-safe probe reading kfuncs as KF_PERFMON, similarly as we do for
the old-style BPF helper equivalents. bpf_rdonly_cast() is included in this
list as well as it returns PTR_TO_MEM | MEM_RDONLY | PTR_UNTRUSTED for an
unchecked object and is using fault-safe BPF_PROBE_MEM.

Note that only the void form of bpf_rdonly_cast() produced a register that
was readable without CAP_PERFMON. For a struct type id the kfunc returns
PTR_TO_BTF_ID | PTR_UNTRUSTED, whose dereference has always been gated in
check_ptr_to_btf_access(). The flag is not conditional on the type id, so
for the latter it only moves the rejection from the dereference to the call
itself, which is the better place to report it anyway.

The bpf_stream_vprintk() and bpf_stream_print_stack() kfuncs are marked
as well. The former ends up in the same bpf_bprintf_prepare() as the
bpf_snprintf() helper, where %pks, %pus and %pI4 read through a program-
supplied address and %pB resolves one into a symbol. The latter walks the
stack and prints each instruction pointer via %pS.

Lastly, bpf_get_kmem_cache() takes a raw scalar address that the verifier
does not constrain and dereferences the page/slab metadata for it. Field
reads on the returned pointer are still blocked (PTR_TO_BTF_ID | PTR_UNTRUSTED
-> check_ptr_to_btf_access() results in -EPERM without the CAP), but the
NULL/non-NULL result is observable.

Reported-by: STAR Labs SG <info@starlabs.sg>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/r/20260910213510.49358-2-daniel@iogearbox.net
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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