[PATCH v2] bpf: Fix bpf_get_smp_processor_id() on !CONFIG_SMP

[Andrea Righi]

On x86-64 calling bpf_get_smp_processor_id() in a kernel with CONFIG_SMP
disabled can trigger the following bug, as pcpu_hot is unavailable:

 [    8.471774] BUG: unable to handle page fault for address: 00000000936a290c
 [    8.471849] #PF: supervisor read access in kernel mode
 [    8.471881] #PF: error_code(0x0000) - not-present page

Fix by inlining a return 0 in the !CONFIG_SMP case.

Fixes: 1ae6921009e5 ("bpf: inline bpf_get_smp_processor_id() helper")
Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Cc: Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx>
Signed-off-by: Andrea Righi <arighi@xxxxxxxxxx>
---
 kernel/bpf/verifier.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

ChangeLog v1 -> v2:
 - generate the proper inline code for the !CONFIG_SMP case (this time
   tested with the right .config)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index f7f892a52a37..77f56674aaa9 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -21281,11 +21281,15 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
 			 * changed in some incompatible and hard to support
 			 * way, it's fine to back out this inlining logic
 			 */
+#ifdef CONFIG_SMP
 			insn_buf[0] = BPF_MOV32_IMM(BPF_REG_0, (u32)(unsigned long)&pcpu_hot.cpu_number);
 			insn_buf[1] = BPF_MOV64_PERCPU_REG(BPF_REG_0, BPF_REG_0);
 			insn_buf[2] = BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_0, 0);
 			cnt = 3;
-
+#else
+			insn_buf[0] = BPF_ALU32_REG(BPF_XOR, BPF_REG_0, BPF_REG_0);
+			cnt = 1;
+#endif
 			new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt);
 			if (!new_prog)
 				return -ENOMEM;
-- 
2.47.1