[PATCH v4 0/2] riscv: fix kprobes on minimal kernel configs

[PATCH v4 0/2] riscv: fix kprobes on minimal kernel configs

[Xiaofeng Yuan]

When building with an allnoconfig-derived minimal configuration
(without CONFIG_STRICT_MODULE_RWX), kprobes fails because
instruction slot pages from EXECMEM_KPROBES are read-only and
patch_map() bypasses the fixmap writable alias.

These two patches fix the infrastructure.

PATCH 1/2 changelog:
  v2: use CONFIG_STRICT_MODULE_RWX instead of ARCH_HAS_EXECMEM_ROX
  v3: add commit description
  v4: fix indent alignment

PATCH 2/2 changelog:
  v2: add commit description
  v3: early return when !CONFIG_STRICT_MODULE_RWX

Xiaofeng Yuan (2):
  riscv: mm: make EXECMEM_KPROBES writable without
    CONFIG_STRICT_MODULE_RWX
  riscv: patch: skip fixmap mapping when kernel text is already
    writable

 arch/riscv/kernel/patch.c | 4 ++--
 arch/riscv/mm/init.c      | 4 +++-
 2 files changed, 6 insertions(+), 2 deletions(-)

--
2.43.0

[PATCH v4 1/2] riscv: mm: make EXECMEM_KPROBES writable without CONFIG_STRICT_MODULE_RWX

[Xiaofeng Yuan]

When CONFIG_STRICT_MODULE_RWX is not set, execmem cannot create
temporary writable mappings for read-only executable pages. In this
case, the execmem ranges must already have writable permissions.

Currently EXECMEM_KPROBES unconditionally uses PAGE_KERNEL_READ_EXEC,
which causes kprobe instruction slot writes to trigger page faults
on systems where CONFIG_STRICT_MODULE_RWX is not enabled.

Fix this by using PAGE_KERNEL_EXEC when CONFIG_STRICT_MODULE_RWX
is not available.

Signed-off-by: Xiaofeng Yuan <xiaofengmian@163.com>
---
v2: use CONFIG_STRICT_MODULE_RWX instead of CONFIG_ARCH_HAS_EXECMEM_ROX (per Nam Cao's review)
v3: add commit description
v4: fix indent alignment (per Nam Cao's review)
diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
index 5b1b3c88b4..7951d72a12 100644
--- a/arch/riscv/mm/init.c
+++ b/arch/riscv/mm/init.c
@@ -1457,7 +1457,9 @@ struct execmem_info __init *execmem_arch_setup(void)
 			[EXECMEM_KPROBES] = {
 				.start	= VMALLOC_START,
 				.end	= VMALLOC_END,
-				.pgprot	= PAGE_KERNEL_READ_EXEC,
+				.pgprot	= IS_ENABLED(CONFIG_STRICT_MODULE_RWX) ?
+					  PAGE_KERNEL_READ_EXEC :
+					  PAGE_KERNEL_EXEC,
 				.alignment = 1,
 			},
 			[EXECMEM_BPF] = {
-- 
2.43.0

[PATCH v4 2/2] riscv: patch: skip fixmap mapping when kernel text is already writable

[Xiaofeng Yuan]

Currently patch_map() always creates a temporary writable mapping via
fixmap for kernel text addresses, even when CONFIG_STRICT_MODULE_RWX
is disabled and the kernel text is already mapped with _PAGE_WRITE.

This is unnecessary overhead at best, and on minimal configurations
it can cause page faults.

Skip the fixmap path for kernel text when CONFIG_STRICT_MODULE_RWX
is not enabled, since the text pages are already writable in this case.

Signed-off-by: Xiaofeng Yuan <xiaofengmian@163.com>
---
v2: add commit description
v3: early return when !CONFIG_STRICT_MODULE_RWX (per Nam Cao's suggestion)
diff --git a/arch/riscv/kernel/patch.c b/arch/riscv/kernel/patch.c
index 16b243376f..caef41d5ef 100644
--- a/arch/riscv/kernel/patch.c
+++ b/arch/riscv/kernel/patch.c
@@ -44,15 +44,16 @@ static __always_inline void *patch_map(void *addr, const unsigned int fixmap)
 	uintptr_t uintaddr = (uintptr_t) addr;
 	phys_addr_t phys;
 
+	if (!IS_ENABLED(CONFIG_STRICT_MODULE_RWX))
+		return addr;
+
 	if (core_kernel_text(uintaddr) || is_kernel_exittext(uintaddr)) {
 		phys = __pa_symbol(addr);
-	} else if (IS_ENABLED(CONFIG_STRICT_MODULE_RWX)) {
+	} else {
 		struct page *page = vmalloc_to_page(addr);
 
 		BUG_ON(!page);
 		phys = page_to_phys(page) + offset_in_page(addr);
-	} else {
-		return addr;
 	}
 
 	return (void *)set_fixmap_offset(fixmap, phys);
-- 
2.43.0

Re: [PATCH v4 0/2] riscv: fix kprobes on minimal kernel configs

[Nam Cao]

Xiaofeng Yuan <xiaofengmian@163.com> writes:
> When building with an allnoconfig-derived minimal configuration
> (without CONFIG_STRICT_MODULE_RWX), kprobes fails because
> instruction slot pages from EXECMEM_KPROBES are read-only and
> patch_map() bypasses the fixmap writable alias.
>
> These two patches fix the infrastructure.

Reviewed-by: Nam Cao <namcao@linutronix.de>