Hi Peter,
> On May 15, 2022, at 10:40 PM, Song Liu <song@xxxxxxxxxx> wrote:
>
> Introduce a memset like API for text_poke. This will be used to fill the
> unused RX memory with illegal instructions.
>
> Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Song Liu <song@xxxxxxxxxx>
Could you please share your comments on this?
Thanks,
Song
> ---
> arch/x86/include/asm/text-patching.h | 1 +
> arch/x86/kernel/alternative.c | 70 ++++++++++++++++++++++++----
> 2 files changed, 61 insertions(+), 10 deletions(-)
>
> diff --git a/arch/x86/include/asm/text-patching.h b/arch/x86/include/asm/text-patching.h
> index d20ab0921480..1cc15528ce29 100644
> --- a/arch/x86/include/asm/text-patching.h
> +++ b/arch/x86/include/asm/text-patching.h
> @@ -45,6 +45,7 @@ extern void *text_poke(void *addr, const void *opcode, size_t len);
> extern void text_poke_sync(void);
> extern void *text_poke_kgdb(void *addr, const void *opcode, size_t len);
> extern void *text_poke_copy(void *addr, const void *opcode, size_t len);
> +extern void *text_poke_set(void *addr, int c, size_t len);
> extern int poke_int3_handler(struct pt_regs *regs);
> extern void text_poke_bp(void *addr, const void *opcode, size_t len, const void *emulate);
>
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index d374cb3cf024..732814065389 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -994,7 +994,21 @@ static inline void unuse_temporary_mm(temp_mm_state_t prev_state)
> __ro_after_init struct mm_struct *poking_mm;
> __ro_after_init unsigned long poking_addr;
>
> -static void *__text_poke(void *addr, const void *opcode, size_t len)
> +static void text_poke_memcpy(void *dst, const void *src, size_t len)
> +{
> + memcpy(dst, src, len);
> +}
> +
> +static void text_poke_memset(void *dst, const void *src, size_t len)
> +{
> + int c = *(int *)src;
> +
> + memset(dst, c, len);
> +}
> +
> +typedef void text_poke_f(void *dst, const void *src, size_t len);
> +
> +static void *__text_poke(text_poke_f func, void *addr, const void *src, size_t len)
> {
> bool cross_page_boundary = offset_in_page(addr) + len > PAGE_SIZE;
> struct page *pages[2] = {NULL};
> @@ -1059,7 +1073,7 @@ static void *__text_poke(void *addr, const void *opcode, size_t len)
> prev = use_temporary_mm(poking_mm);
>
> kasan_disable_current();
> - memcpy((u8 *)poking_addr + offset_in_page(addr), opcode, len);
> + func((u8 *)poking_addr + offset_in_page(addr), src, len);
> kasan_enable_current();
>
> /*
> @@ -1087,11 +1101,13 @@ static void *__text_poke(void *addr, const void *opcode, size_t len)
> (cross_page_boundary ? 2 : 1) * PAGE_SIZE,
> PAGE_SHIFT, false);
>
> - /*
> - * If the text does not match what we just wrote then something is
> - * fundamentally screwy; there's nothing we can really do about that.
> - */
> - BUG_ON(memcmp(addr, opcode, len));
> + if (func == text_poke_memcpy) {
> + /*
> + * If the text does not match what we just wrote then something is
> + * fundamentally screwy; there's nothing we can really do about that.
> + */
> + BUG_ON(memcmp(addr, src, len));
> + }
>
> local_irq_restore(flags);
> pte_unmap_unlock(ptep, ptl);
> @@ -1118,7 +1134,7 @@ void *text_poke(void *addr, const void *opcode, size_t len)
> {
> lockdep_assert_held(&text_mutex);
>
> - return __text_poke(addr, opcode, len);
> + return __text_poke(text_poke_memcpy, addr, opcode, len);
> }
>
> /**
> @@ -1137,7 +1153,7 @@ void *text_poke(void *addr, const void *opcode, size_t len)
> */
> void *text_poke_kgdb(void *addr, const void *opcode, size_t len)
> {
> - return __text_poke(addr, opcode, len);
> + return __text_poke(text_poke_memcpy, addr, opcode, len);
> }
>
> /**
> @@ -1167,7 +1183,41 @@ void *text_poke_copy(void *addr, const void *opcode, size_t len)
>
> s = min_t(size_t, PAGE_SIZE * 2 - offset_in_page(ptr), len - patched);
>
> - __text_poke((void *)ptr, opcode + patched, s);
> + __text_poke(text_poke_memcpy, (void *)ptr, opcode + patched, s);
> + patched += s;
> + }
> + mutex_unlock(&text_mutex);
> + return addr;
> +}
> +
> +/**
> + * text_poke_set - memset into (an unused part of) RX memory
> + * @addr: address to modify
> + * @c: the byte to fill the area with
> + * @len: length to copy, could be more than 2x PAGE_SIZE
> + *
> + * Not safe against concurrent execution; useful for JITs to dump
> + * new code blocks into unused regions of RX memory. Can be used in
> + * conjunction with synchronize_rcu_tasks() to wait for existing
> + * execution to quiesce after having made sure no existing functions
> + * pointers are live.
> + */
> +void *text_poke_set(void *addr, int c, size_t len)
> +{
> + unsigned long start = (unsigned long)addr;
> + size_t patched = 0;
> +
> + if (WARN_ON_ONCE(core_kernel_text(start)))
> + return NULL;
> +
> + mutex_lock(&text_mutex);
> + while (patched < len) {
> + unsigned long ptr = start + patched;
> + size_t s;
> +
> + s = min_t(size_t, PAGE_SIZE * 2 - offset_in_page(ptr), len - patched);
> +
> + __text_poke(text_poke_memset, (void *)ptr, (void *)&c, s);
> patched += s;
> }
> mutex_unlock(&text_mutex);
> --
> 2.30.2
>
