bpf, arm64: Fix fp initialization for exception boundary

In the ARM64 BPF JIT when prog->aux->exception_boundary is set for a BPF
program, find_used_callee_regs() is not called because for a program
acting as exception boundary, all callee saved registers are saved.
find_used_callee_regs() sets `ctx->fp_used = true;` when it sees FP
being used in any of the instructions.

For programs acting as exception boundary, ctx->fp_used remains false
even if frame pointer is used by the program and therefore, FP is not
set-up for such programs in the prologue. This can cause the kernel to
crash due to a pagefault.

Fix it by setting ctx->fp_used = true for exception boundary programs as
fp is always saved in such programs.

Fixes: 5d4fa9e ("bpf, arm64: Avoid blindly saving/restoring all callee-saved registers")
Signed-off-by: Puranjay Mohan <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Acked-by: Xu Kuohai <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

Author: puranjaymohan

arch/arm64/net/bpf_jit_comp.c

@@ -412,6 +412,7 @@ static void push_callee_regs(struct jit_ctx *ctx)
 		emit(A64_PUSH(A64_R(23), A64_R(24), A64_SP), ctx);
 		emit(A64_PUSH(A64_R(25), A64_R(26), A64_SP), ctx);
 		emit(A64_PUSH(A64_R(27), A64_R(28), A64_SP), ctx);
+		ctx->fp_used = true;
 	} else {
 		find_used_callee_regs(ctx);
 		for (i = 0; i + 1 < ctx->nr_used_callee_reg; i += 2) {