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

[Pu Lehui <pulehui@huawei.com>]

On 2026/9/14 19:56, 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().
> 
> Fixes: 2c9e5d4a0082 ("bpf: remove CONFIG_BPF_JIT dependency on CONFIG_MODULES of")
> Signed-off-by: Wei-Jie Hung <imbigking12@gmail.com>
> ---
> Based on riscv/fixes (b94cec5761d2).
> 
> Reproduced on qemu-system-riscv64 -M virt with CONFIG_BPF_JIT=y,
> CONFIG_BPF_JIT_ALWAYS_ON=y and CONFIG_STRICT_MODULE_RWX=n.
> ptp_classifier_init() builds a cBPF filter from sock_init(),
> so the failure happens during boot without any userspace involved:
> 
>    WARNING: arch/riscv/net/bpf_jit_core.c:156 at bpf_int_jit_compile+0x3dc/0x43e
>    Call Trace:
>     bpf_int_jit_compile+0x3dc/0x43e
>     __bpf_prog_select_runtime+0xec/0x186
>     bpf_prog_select_runtime+0x12/0x1a
>     bpf_prepare_filter+0x368/0x46a
>     bpf_prog_create+0x66/0x90
>     ptp_classifier_init+0x3e/0x60
>     sock_init+0xc4/0xe6
>     do_one_initcall+0x78/0x14a
> 
>    kernel BUG at net/core/ptp_classifier.c:227!
>    Kernel panic - not syncing: Fatal exception in interrupt
> 
> The BUG_ON() is reached because CONFIG_BPF_JIT_ALWAYS_ON=y turns the
> silent interpreter fallback into -ENOTSUPP. With
> CONFIG_BPF_JIT_ALWAYS_ON=n the failure is silent: writing 1 to
> /proc/sys/net/core/bpf_jit_enable and loading any program reproduces the
> same warning, but the program simply falls back to the interpreter and
> the kernel keeps running.
> 
> Tested with CONFIG_BPF_JIT=y:
>    - CONFIG_MODULES=n, CONFIG_BPF_JIT_ALWAYS_ON=y:
>      panics before this patch, boots after
>    - CONFIG_MODULES=y, CONFIG_STRICT_MODULE_RWX=n,
>      CONFIG_BPF_JIT_ALWAYS_ON=y: same panic before this patch, boots after
>    - CONFIG_BPF_JIT_ALWAYS_ON=n: the warning above is emitted when a
>      program is loaded from userspace before this patch, and gone after
>    - riscv defconfig (CONFIG_STRICT_MODULE_RWX=y): unaffected, boots
>      before and after
>    - kprobes (kretprobe on __riscv_sys_openat via kprobe_events):
>      works before and after this patch
> 
>   arch/riscv/kernel/patch.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/riscv/kernel/patch.c b/arch/riscv/kernel/patch.c
> index 2239c28981bc..2b5adf01c550 100644
> --- a/arch/riscv/kernel/patch.c
> +++ b/arch/riscv/kernel/patch.c
> @@ -48,7 +48,7 @@ static __always_inline void *patch_map(void *addr, const unsigned int fixmap)
>   		if (!IS_ENABLED(CONFIG_STRICT_KERNEL_RWX))
>   			return addr;
>   		phys = __pa_symbol(addr);
> -	} else if (IS_ENABLED(CONFIG_STRICT_MODULE_RWX)) {
> +	} else if (IS_ENABLED(CONFIG_EXECMEM)) {
>   		struct page *page = vmalloc_to_page(addr);
>   
>   		BUG_ON(!page);

Reviewed-by: Pu Lehui <pulehui@huawei.com>
Tested-by: Pu Lehui <pulehui@huawei.com>