Re: [PATCH v2 1/4] arm64: patching: always use fixmap

Jarkko Sakkinen
2024-04-03T16:20:35Z

On Wed Apr 3, 2024 at 6:01 PM EEST, Mark Rutland wrote:
> For historical reasons, patch_map() won't bother to fixmap non-image
> addresses when CONFIG_STRICT_MODULE_RWX=n, matching the behaviour prior
> to the introduction of CONFIG_STRICT_MODULE_RWX. However, as arm64
> doesn't select CONFIG_ARCH_OPTIONAL_KERNEL_RWX, CONFIG_MODULES implies
> CONFIG_STRICT_MODULE_RWX, so any kernel built with module support will
> use the fixmap for any non-image address.

Not familiar with the config flag but I'd guess it is essentially
w^x enforcement right for the sections?

> Historically we only used patch_map() for the kernel image and modules,
> but these days its also used by BPF and KPROBES to write to read-only
> pages of executable text. Currently these both depend on CONFIG_MODULES,
> but we'd like to change that in subsequent patches, which will require
> using the fixmap regardless of CONFIG_STRICT_MODULE_RWX.
>
> This patch changes patch_map() to always use the fixmap, and simplifies
> the logic:
>
> * Use is_image_text() directly in the if-else, rather than using a
> temporary boolean variable.
>
> * Use offset_in_page() to get the offset within the mapping.
>
> * Remove uintaddr and cast the address directly when using
> is_image_text().
>
> For kernels built with CONFIG_MODULES=y, there should be no functional
> change as a result of this patch.
>
> For kernels built with CONFIG_MODULES=n, patch_map() will use the fixmap
> for non-image addresses, but there are no extant users with non-image
> addresses when CONFIG_MODULES=n, and hence there should be no functional
> change as a result of this patch alone.
>
> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> ---
> arch/arm64/kernel/patching.c | 10 +++-------
> 1 file changed, 3 insertions(+), 7 deletions(-)
>
> Catalin, Will, this is a prerequisite for the final two patches in the
> series. Are you happy for this go via the tracing tree?
>
> Mark.
>
> diff --git a/arch/arm64/kernel/patching.c b/arch/arm64/kernel/patching.c
> index 2555349303684..f0f3a2a82ca5a 100644
> --- a/arch/arm64/kernel/patching.c
> +++ b/arch/arm64/kernel/patching.c
> @@ -30,20 +30,16 @@ static bool is_image_text(unsigned long addr)
>
> static void __kprobes *patch_map(void *addr, int fixmap)
> {
> - unsigned long uintaddr = (uintptr_t) addr;
> - bool image = is_image_text(uintaddr);
> struct page *page;
>
> - if (image)
> + if (is_image_text((unsigned long)addr))
> page = phys_to_page(__pa_symbol(addr));
> - else if (IS_ENABLED(CONFIG_STRICT_MODULE_RWX))
> - page = vmalloc_to_page(addr);
> else
> - return addr;
> + page = vmalloc_to_page(addr);
>
> BUG_ON(!page);
> return (void *)set_fixmap_offset(fixmap, page_to_phys(page) +
> - (uintaddr & ~PAGE_MASK));
> + offset_in_page(addr));

nit: could be a single line but i guess it is up to the taste (and
subsystem maintainer). I.e. checkpatch will allow it at least.

I don't mind it too much just mentioning for completeness.

> }
>
> static void __kprobes patch_unmap(int fixmap)

If my assumption about the config flag holds this makes sense:

Reviewed-by: Jarkko Sakkinen <jarkko@kernel.rg>

BR, Jarkko