[PATCH] x86/static_call: Fix __static_call_fixup()

[Peter Zijlstra <peterz@infradead.org>]

Christian reported spurious module crashes after some of Song's module
memory layout patches.

Turns out that if the very last instruction on the very last page of the
module is a 'JMP __x86_return_thunk' then __static_call_fixup() will
trip a fault and dies.

And while the module rework made this slightly more likely to happen,
it's always been possible.

Fixes: ee88d363d156 ("x86,static_call: Use alternative RET encoding")
Reported-by: Christian Bricart <christian@bricart.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 arch/x86/kernel/static_call.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/arch/x86/kernel/static_call.c b/arch/x86/kernel/static_call.c
index b70670a98597..2e67512d7104 100644
--- a/arch/x86/kernel/static_call.c
+++ b/arch/x86/kernel/static_call.c
@@ -186,6 +186,16 @@ EXPORT_SYMBOL_GPL(arch_static_call_transform);
  */
 bool __static_call_fixup(void *tramp, u8 op, void *dest)
 {
+	/*
+	 * Not all .return_sites are a static_call trampoline (most are not).
+	 * Check if the next 3 bytes are still kernel text, if not, then this
+	 * definitely is not a trampoline and we need not worry further.
+	 *
+	 * This avoids the memcmp() below tripping over pagefaults etc..
+	 */
+	if (!kernel_text_address(tramp+7))
+		return false;
+
 	if (memcmp(tramp+5, tramp_ud, 3)) {
 		/* Not a trampoline site, not our problem. */
 		return false;