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

Mark Rutland
2024-04-03T15:02:04Z

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.

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));
}

static void __kprobes patch_unmap(int fixmap)
--
2.30.2