uprobes/x86: Harden uretprobe syscall trampoline check

Jann reported a possible issue when trampoline_check_ip returns
address near the bottom of the address space that is allowed to
call into the syscall if uretprobes are not set up:

   https://lore.kernel.org/bpf/202502081235.5A6F352985@keescook/T/#m9d416df341b8fbc11737dacbcd29f0054413cbbf

Though the mmap minimum address restrictions will typically prevent
creating mappings there, let's make sure uretprobe syscall checks
for that.

Fixes: ff474a7 ("uprobe: Add uretprobe syscall to speed up return probe")
Reported-by: Jann Horn <[email protected]>
Signed-off-by: Jiri Olsa <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Reviewed-by: Oleg Nesterov <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Acked-by: Andrii Nakryiko <[email protected]>
Acked-by: Masami Hiramatsu (Google) <[email protected]>
Acked-by: Alexei Starovoitov <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]

Author: olsajiri

arch/x86/kernel/uprobes.c

@@ -357,19 +357,23 @@ void *arch_uprobe_trampoline(unsigned long *psize)
 	return &insn;
 }
 
-static unsigned long trampoline_check_ip(void)
+static unsigned long trampoline_check_ip(unsigned long tramp)
 {
-	unsigned long tramp = uprobe_get_trampoline_vaddr();
-
 	return tramp + (uretprobe_syscall_check - uretprobe_trampoline_entry);
 }
 
 SYSCALL_DEFINE0(uretprobe)
 {
 	struct pt_regs *regs = task_pt_regs(current);
-	unsigned long err, ip, sp, r11_cx_ax[3];
+	unsigned long err, ip, sp, r11_cx_ax[3], tramp;
+
+	/* If there's no trampoline, we are called from wrong place. */
+	tramp = uprobe_get_trampoline_vaddr();
+	if (unlikely(tramp == UPROBE_NO_TRAMPOLINE_VADDR))
+		goto sigill;
 
-	if (regs->ip != trampoline_check_ip())
+	/* Make sure the ip matches the only allowed sys_uretprobe caller. */
+	if (unlikely(regs->ip != trampoline_check_ip(tramp)))
 		goto sigill;
 
 	err = copy_from_user(r11_cx_ax, (void __user *)regs->sp, sizeof(r11_cx_ax));

include/linux/uprobes.h

@@ -39,6 +39,8 @@ struct page;
 
 #define MAX_URETPROBE_DEPTH		64
 
+#define UPROBE_NO_TRAMPOLINE_VADDR	(~0UL)
+
 struct uprobe_consumer {
 	/*
 	 * handler() can return UPROBE_HANDLER_REMOVE to signal the need to

kernel/events/uprobes.c

@@ -2169,8 +2169,8 @@ void uprobe_copy_process(struct task_struct *t, unsigned long flags)
  */
 unsigned long uprobe_get_trampoline_vaddr(void)
 {
+	unsigned long trampoline_vaddr = UPROBE_NO_TRAMPOLINE_VADDR;
 	struct xol_area *area;
-	unsigned long trampoline_vaddr = -1;
 
 	/* Pairs with xol_add_vma() smp_store_release() */
 	area = READ_ONCE(current->mm->uprobes_state.xol_area); /* ^^^ */