[PATCH v5 2/2] riscv: patch: skip fixmap mapping when kernel text is already writable

[Xiaofeng Yuan]

patch_map() always creates a temporary writable mapping via fixmap for
kernel text addresses, even when CONFIG_STRICT_KERNEL_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_KERNEL_RWX
is not enabled, since the text pages are already writable in that case.
The module text path is already gated on CONFIG_STRICT_MODULE_RWX and
is kept unchanged.

Reported-by: Klara Modin <klara at kasm.eu>
Closes: https://lore.kernel.org/all/ant_8TaBbov_GS4i@soda.int.kasm.eu/
Reported-by: Lad Prabhakar <prabhakar.csengg at gmail.com>
Closes: https://lore.kernel.org/all/CA+V-a8tQK8rih9SGGTyqrEBGpNkx4H0eX2YccCRrgkVAPr+EBg@mail.gmail.com/
---
v5: fix the fixmap gating for kernel text. The v4 early return on
    CONFIG_STRICT_MODULE_RWX wrongly skipped the fixmap for kernel text
    too, which panics on configs with CONFIG_MODULES=n where
    CONFIG_STRICT_MODULE_RWX is unavailable but CONFIG_STRICT_KERNEL_RWX
    is still enabled and kernel text is read-only.
v3: early return when !CONFIG_STRICT_MODULE_RWX (per Nam Cao's suggestion)
v2: add commit description

Signed-off-by: Xiaofeng Yuan <xiaofengmian at 163.com>
---
 arch/riscv/kernel/patch.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/riscv/kernel/patch.c b/arch/riscv/kernel/patch.c
index 16b243376f..2239c28981 100644
--- a/arch/riscv/kernel/patch.c
+++ b/arch/riscv/kernel/patch.c
@@ -45,6 +45,8 @@ static __always_inline void *patch_map(void *addr, const unsigned int fixmap)
 	phys_addr_t phys;
 
 	if (core_kernel_text(uintaddr) || is_kernel_exittext(uintaddr)) {
+		if (!IS_ENABLED(CONFIG_STRICT_KERNEL_RWX))
+			return addr;
 		phys = __pa_symbol(addr);
 	} else if (IS_ENABLED(CONFIG_STRICT_MODULE_RWX)) {
 		struct page *page = vmalloc_to_page(addr);
-- 
2.43.0