bpf, x64: Fix tailcall infinite loop

[ Upstream commit 2b5dcb31a19a2e0acd869b12c9db9b2d696ef544 ]

From commit ebf7d1f508a73871 ("bpf, x64: rework pro/epilogue and tailcall
handling in JIT"), the tailcall on x64 works better than before.

From commit e411901c0b775a3a ("bpf: allow for tailcalls in BPF subprograms
for x64 JIT"), tailcall is able to run in BPF subprograms on x64.

From commit 5b92a28aae4dd0f8 ("bpf: Support attaching tracing BPF program
to other BPF programs"), BPF program is able to trace other BPF programs.

How about combining them all together?

1. FENTRY/FEXIT on a BPF subprogram.
2. A tailcall runs in the BPF subprogram.
3. The tailcall calls the subprogram's caller.

As a result, a tailcall infinite loop comes up. And the loop would halt
the machine.

As we know, in tail call context, the tail_call_cnt propagates by stack
and rax register between BPF subprograms. So do in trampolines.

Fixes: ebf7d1f508a7 ("bpf, x64: rework pro/epilogue and tailcall handling in JIT")
Fixes: e411901c0b77 ("bpf: allow for tailcalls in BPF subprograms for x64 JIT")
Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Leon Hwang <hffilwlqm@gmail.com>
Link: https://lore.kernel.org/r/20230912150442.2009-3-hffilwlqm@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index a593004..52f36c4 100644 (file)
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -1018,6 +1018,10 @@ static void emit_shiftx(u8 **pprog, u32 dst_reg, u8 src_reg, bool is64, u8 op)
 
 #define INSN_SZ_DIFF (((addrs[i] - addrs[i - 1]) - (prog - temp)))
 
+/* mov rax, qword ptr [rbp - rounded_stack_depth - 8] */
+#define RESTORE_TAIL_CALL_CNT(stack)                           \
+       EMIT3_off32(0x48, 0x8B, 0x85, -round_up(stack, 8) - 8)
+
 static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image, u8 *rw_image,
                  int oldproglen, struct jit_context *ctx, bool jmp_padding)
 {
@@ -1623,9 +1627,7 @@ st:                       if (is_imm8(insn->off))
 
                        func = (u8 *) __bpf_call_base + imm32;
                        if (tail_call_reachable) {
-                               /* mov rax, qword ptr [rbp - rounded_stack_depth - 8] */
-                               EMIT3_off32(0x48, 0x8B, 0x85,
-                                           -round_up(bpf_prog->aux->stack_depth, 8) - 8);
+                               RESTORE_TAIL_CALL_CNT(bpf_prog->aux->stack_depth);
                                if (!imm32)
                                        return -EINVAL;
                                offs = 7 + x86_call_depth_emit_accounting(&prog, func);
@@ -2400,6 +2402,7 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
         *                     [ ...        ]
         *                     [ stack_arg2 ]
         * RBP - arg_stack_off [ stack_arg1 ]
+        * RSP                 [ tail_call_cnt ] BPF_TRAMP_F_TAIL_CALL_CTX
         */
 
        /* room for return value of orig_call or fentry prog */
@@ -2464,6 +2467,8 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
        else
                /* sub rsp, stack_size */
                EMIT4(0x48, 0x83, 0xEC, stack_size);
+       if (flags & BPF_TRAMP_F_TAIL_CALL_CTX)
+               EMIT1(0x50);            /* push rax */
        /* mov QWORD PTR [rbp - rbx_off], rbx */
        emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_6, -rbx_off);
 
@@ -2516,9 +2521,15 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
                restore_regs(m, &prog, regs_off);
                save_args(m, &prog, arg_stack_off, true);
 
+               if (flags & BPF_TRAMP_F_TAIL_CALL_CTX)
+                       /* Before calling the original function, restore the
+                        * tail_call_cnt from stack to rax.
+                        */
+                       RESTORE_TAIL_CALL_CNT(stack_size);
+
                if (flags & BPF_TRAMP_F_ORIG_STACK) {
-                       emit_ldx(&prog, BPF_DW, BPF_REG_0, BPF_REG_FP, 8);
-                       EMIT2(0xff, 0xd0); /* call *rax */
+                       emit_ldx(&prog, BPF_DW, BPF_REG_6, BPF_REG_FP, 8);
+                       EMIT2(0xff, 0xd3); /* call *rbx */
                } else {
                        /* call original function */
                        if (emit_rsb_call(&prog, orig_call, prog)) {
@@ -2569,7 +2580,12 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
                        ret = -EINVAL;
                        goto cleanup;
                }
-       }
+       } else if (flags & BPF_TRAMP_F_TAIL_CALL_CTX)
+               /* Before running the original function, restore the
+                * tail_call_cnt from stack to rax.
+                */
+               RESTORE_TAIL_CALL_CNT(stack_size);
+
        /* restore return value of orig_call or fentry prog back into RAX */
        if (save_ret)
                emit_ldx(&prog, BPF_DW, BPF_REG_0, BPF_REG_FP, -8);

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 49f8b69..7605518 100644 (file)
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1029,6 +1029,11 @@ struct btf_func_model {
  */
 #define BPF_TRAMP_F_SHARE_IPMODIFY     BIT(6)
 
+/* Indicate that current trampoline is in a tail call context. Then, it has to
+ * cache and restore tail_call_cnt to avoid infinite tail call loop.
+ */
+#define BPF_TRAMP_F_TAIL_CALL_CTX      BIT(7)
+
 /* Each call __bpf_prog_enter + call bpf_func + call __bpf_prog_exit is ~50
  * bytes on x86.
  */

diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
index 53ff50c..e97aeda 100644 (file)
--- a/kernel/bpf/trampoline.c
+++ b/kernel/bpf/trampoline.c
@@ -415,8 +415,8 @@ static int bpf_trampoline_update(struct bpf_trampoline *tr, bool lock_direct_mut
                goto out;
        }
 
-       /* clear all bits except SHARE_IPMODIFY */
-       tr->flags &= BPF_TRAMP_F_SHARE_IPMODIFY;
+       /* clear all bits except SHARE_IPMODIFY and TAIL_CALL_CTX */
+       tr->flags &= (BPF_TRAMP_F_SHARE_IPMODIFY | BPF_TRAMP_F_TAIL_CALL_CTX);
 
        if (tlinks[BPF_TRAMP_FEXIT].nr_links ||
            tlinks[BPF_TRAMP_MODIFY_RETURN].nr_links) {

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 873ade1..9f180ae 100644 (file)
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -19641,6 +19641,9 @@ static int check_attach_btf_id(struct bpf_verifier_env *env)
        if (!tr)
                return -ENOMEM;
 
+       if (tgt_prog && tgt_prog->aux->tail_call_reachable)
+               tr->flags = BPF_TRAMP_F_TAIL_CALL_CTX;
+
        prog->aux->dst_trampoline = tr;
        return 0;
 }