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Subject[PATCH] x86/static_call: Fix __static_call_fixup()

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;
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