Re: [PATCH] riscv: patch: fix handling of bpf-jit execmem addresses

[Paul Walmsley <pjw@kernel.org>]

On Mon, 14 Sep 2026, Wei-Jie Hung wrote:

> Commit 8718e5a3090b ("riscv: patch: skip fixmap mapping when kernel text
> is already writable") gated the core_kernel_text() branch of patch_map()
> on CONFIG_STRICT_KERNEL_RWX, and explicitly left the vmalloc branch
> unchanged. That branch has a separate problem.
> 
> That branch only creates a temporary writable fixmap alias when
> CONFIG_STRICT_MODULE_RWX is enabled, and otherwise assumes the target is
> directly writable and returns the address unchanged. That assumption no
> longer holds: bpf_prog_pack_alloc calls set_memory_rox() on every pack
> unconditionally in alloc_new_pack(), independently of any
> CONFIG_STRICT_*_RWX option, so BPF text in the vmalloc area is read-only
> regardless of what CONFIG_STRICT_MODULE_RWX says.
> 
> With CONFIG_STRICT_MODULE_RWX=n, patch_map() returns the read-only
> address directly, the subsequent copy_to_kernel_nofault() takes a
> page fault and returns -EFAULT. bpf_arch_text_copy() turns that into
> -EINVAL, which propagates through bpf_jit_binary_pack_finalize() to the
> WARN_ON() in bpf_int_jit_compile() and leaves the program un-JITed.
> 
> Note that BPF cannot be fixed the way kprobes was in
> commit bdc46e507b59 ("riscv: mm: make EXECMEM_KPROBES writable without
> CONFIG_STRICT_MODULE_RWX"). EXECMEM_BPF is already PAGE_KERNEL, i.e.
> writable at allocation time; the read-only mapping is established
> afterwards by generic code in alloc_new_pack(), which arch code cannot
> influence. The only place this can be handled is patch_map().
> 
> This is the same problem that was fixed on arm64 by commit b1480ed230ac
> ("arm64: patching: fix handling of execmem addresses"), and the fix is
> the same: CONFIG_EXECMEM is what actually tracks whether the vmalloc
> area can hold executable memory that needs a temporary alias to be
> written. CONFIG_BPF_JIT, CONFIG_KPROBES and CONFIG_MODULES all select
> it.
> 
> Note that this is not limited to CONFIG_MODULES=n. Unlike arm64, riscv
> selects CONFIG_ARCH_OPTIONAL_KERNEL_RWX, so CONFIG_STRICT_MODULE_RWX can
> be disabled with CONFIG_MODULES=y as well, and the bug is reachable in
> that configuration too.
> 
> The !CONFIG_MMU build is unaffected: it has its own __patch_insn_set()
> and __patch_insn_write() implementations and never calls patch_map().

Thanks, queued for v7.3-rc.


- Paul