Merge branch 'bpf-verifier-fix-ptr_to_flow_keys-constant-offset-oob'

Nuoqi Gui says:

====================
bpf, verifier: fix PTR_TO_FLOW_KEYS constant-offset OOB

A constant offset added to a PTR_TO_FLOW_KEYS register lands in
reg->var_off, but check_flow_keys_access() bounds-checks only insn->off
and never folds reg->var_off.value.  A BPF_PROG_TYPE_FLOW_DISSECTOR
program can therefore do "flow_keys += 0x1000; *(flow_keys + 0)" and have
it accepted, then read/write kernel stack past struct bpf_flow_keys at
runtime.  Patch 1 folds reg->var_off.value into the offset (and rejects
non-constant offsets), mirroring check_ctx_access(); patch 2 adds verifier
selftests.

This is a regression introduced in the 7.1 development cycle by commit
022ac0750883 ("bpf: use reg->var_off instead of reg->off for pointers"),
which moved the constant offset from reg->off (folded generically before
022ac0750883) into reg->var_off without updating the flow_keys path.  No
released kernel is affected: v7.0.x rejects the program above, and the bug
reproduces only on v7.1-rc1..rc5, so no stable backport is needed.

It was first reported privately to security@kernel.org; per their guidance
it is handled in the open as a normal regression fix.  Found by manual
verifier audit and confirmed dynamically in a disposable QEMU/KVM guest:
the load above is accepted, a runtime read leaked a kernel-stack pointer
0x1000 past bpf_flow_keys, and a runtime write of a marker faulted the
guest in net_rx_action.

An alternative -- forbidding pointer arithmetic on PTR_TO_FLOW_KEYS
outright by dropping "if (known) break;" in adjust_ptr_min_max_vals() --
was rejected because v7.0.x accepted (and correctly bounds-checked)
constant arithmetic on the keys pointer; restoring the fold preserves that
behaviour while closing the divergence.

Signed-off-by: Nuoqi Gui <gnq25@mails.tsinghua.edu.cn>
---
v2 -> v3:
 - Pass existing reg/argno context into check_flow_keys_access(), avoiding
   a stale regno reference in check_mem_access().
 - Add a variable-offset selftest using bpf_get_prandom_u32().

v1 -> v2:
 - Target bpf-next instead of bpf (per reviewer feedback).
 - Base-commit updated to bpf-next/master.

v2: https://lore.kernel.org/bpf/20260604180730.2518088-1-gnq25@mails.tsinghua.edu.cn/
v1: https://lore.kernel.org/bpf/20260604150755.2487555-1-gnq25@mails.tsinghua.edu.cn/
====================

Link: https://patch.msgid.link/20260606-c3-01-v3-v3-0-97c51f592f15@mails.tsinghua.edu.cn
Signed-off-by: Alexei Starovoitov <ast@kernel.org>