Re: [PATCH bpf] bpf: invalidate unused part of bpf_prog_pack

[Peter Zijlstra <peterz@infradead.org>]

> > On Apr 21, 2022, at 3:30 PM, Linus Torvalds <torvalds@linux-foundation.org> wrote:

> > I actually think bpf_arch_text_copy() is another horribly badly done thing.
> > 
> > It seems only implemented on x86 (I'm not sure how anything else is
> > supposed to work, I didn't go look), and there it is horribly badly
> > done, using __text_poke() that does all these magical things just to
> > make it atomic wrt concurrent code execution.
> > 
> > None of which is *AT*ALL* relevant for this case, since concurrent
> > code execution simply isn't a thing (and if it were, you would already
> > have lost).
> > 
> > And if that wasn't pointless enough, it does all that magic "map the
> > page writably at a different virtual address using poking_addr in
> > poking_mm" and a different address space entirely.
> > 
> > All of that is required for REAL KERNEL CODE.
> > 
> > But the thing is, for bpf_prog_pack, all of that is just completely
> > pointless and stupid complexity.

I think the point is that this hole will likely share a page with active
code, and as such there should not be a writable mapping mapping to it,
necessitating the whole __text_poke() mess.

That said; it does seem somewhat silly have a whole page worth of int3
around just for this.

Perhaps we can do something like the completely untested below?

---
 arch/x86/kernel/alternative.c | 48 +++++++++++++++++++++++++++++++++++++------
 1 file changed, 42 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index d374cb3cf024..60afa9105307 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -994,7 +994,20 @@ 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 +1072,7 @@ 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((void *)poking_addr + offset_in_page(addr), src, len);
 	kasan_enable_current();
 
 	/*
@@ -1091,7 +1104,8 @@ static void *__text_poke(void *addr, const void *opcode, size_t len)
 	 * 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)
+		BUG_ON(memcmp(addr, src, len));
 
 	local_irq_restore(flags);
 	pte_unmap_unlock(ptep, ptl);
@@ -1118,7 +1132,7 @@ 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 +1151,7 @@ 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 +1181,29 @@ 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;
+}
+
+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);