riscv, mm: Perform BPF exhandler fixup on page fault

Commit 21855ca ("riscv/mm: Prevent kernel module to access user
memory without uaccess routines") added early exits/deaths for page
faults stemming from accesses to user-space without using proper
uaccess routines (where sstatus.SUM is set).

Unfortunatly, this is too strict for some BPF programs, which relies
on BPF exhandler fixups. These BPF programs loads "BTF pointers". A
BTF pointers could either be a valid kernel pointer or NULL, but not a
userspace address.

Resolve the problem by calling the fixup handler in the early exit
path.

Fixes: 21855ca ("riscv/mm: Prevent kernel module to access user memory without uaccess routines")
Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
Link: https://lore.kernel.org/r/20230214162515.184827-1-bjorn@kernel.org
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Author: bjorn-rivos

arch/riscv/mm/fault.c

@@ -267,10 +267,12 @@ asmlinkage void do_page_fault(struct pt_regs *regs)
 	if (user_mode(regs))
 		flags |= FAULT_FLAG_USER;
 
-	if (!user_mode(regs) && addr < TASK_SIZE &&
-			unlikely(!(regs->status & SR_SUM)))
-		die_kernel_fault("access to user memory without uaccess routines",
-				addr, regs);
+	if (!user_mode(regs) && addr < TASK_SIZE && unlikely(!(regs->status & SR_SUM))) {
+		if (fixup_exception(regs))
+			return;
+
+		die_kernel_fault("access to user memory without uaccess routines", addr, regs);
+	}
 
 	perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, addr);