[PATCH bpf-next v2 0/3] bpf: Disallow interpreter fallback for interpreter-unsupported insns

[PATCH bpf-next v2 0/3] bpf: Disallow interpreter fallback for interpreter-unsupported insns

[Leon Hwang]

Sashiko reported two potential issues about interpreter fallback [1]
[2].

After verifying them by patch #7 of v1, I think they are real issues. With
LLM assistance, the interpreter does not support the internal
BPF_PROBE_ATOMIC insn and the gotox insn (used for indirect jumps),
either.

1) the user BPF_ADDR_SPACE_CAST insn
   the interpreter just ignores it.

2) the arena ST/STX/LDX insn
   the interpreter could hit the BUG_ON() in ___bpf_prog_run().

3) the BPF_MOV64_PERCPU_REG insn
   the interpreter could hit page fault, due to loading memory from
   invalid __percpu pointer.

4) the internal BPF_PROBE_ATOMIC insn
   the interpreter could hit the BUG_ON() in ___bpf_prog_run().

5) the gotox insn used for indirect jumps
   the interpreter could hit the BUG_ON() in ___bpf_prog_run(), too.

Reject these insns on interpreter fallback path in
__bpf_prog_select_runtime() by setting 'jit_required = true'.

Link:
[1] https://lore.kernel.org/bpf/20260608151347.2C77D1F00893@smtp.kernel.org/
[2] https://lore.kernel.org/bpf/20260622150759.EC9071F000E9@smtp.kernel.org/

Changes:
v1 -> v2:
* Drop RFC.
* Change target tree to bpf-next to utilize the 'jit_required' bit.
* Set jit_required as true if there's arena map, then all arena-related
  insns will be rejected if JIT is not available.
* Set jit_required as true if there's insn_array map, then the gotox
  insns will be rejected if JIT is not available.
* Drop the issues-proven patch.
* v1: https://lore.kernel.org/bpf/20260626154330.33619-1-leon.hwang@linux.dev/

Leon Hwang (3):
  bpf: Disallow interpreter fallback for arena-related insns
  bpf: Disallow interpreter fallback for gotox insn
  bpf: Disallow interpreter fallback for BPF_ADDR_PERCPU insn

 include/linux/bpf.h   | 4 ++--
 kernel/bpf/fixups.c   | 5 +++++
 kernel/bpf/verifier.c | 2 ++
 3 files changed, 9 insertions(+), 2 deletions(-)

--
2.55.0

[PATCH bpf-next v2 1/3] bpf: Disallow interpreter fallback for arena-related insns

[Leon Hwang]

Since the interpreter does not support the arena-related insns,
interpreter fallback should not be allowed for these insns in
core.c::__bpf_prog_select_runtime().

Currently, when the interpreter executes the arena ST/LDX/STX insns,
it would hit the BUG_ON() in ___bpf_prog_run() at run time.

[    2.579196] BPF interpreter: unknown opcode a2 (imm: 0x0)
[    2.579998] ------------[ cut here ]------------
[    2.580652] kernel BUG at kernel/bpf/core.c:2349!
[    2.581314] Oops: invalid opcode: 0000 [#1] SMP PTI

Set jit_required as true when arena map is used in the prog to disallow
interpreter fallback for arena-related insns.

Fixes: 6082b6c328b5 ("bpf: Recognize addr_space_cast instruction in the verifier.")
Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
---
 kernel/bpf/verifier.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index de816063ae63..3849d3d00e15 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -17873,6 +17873,7 @@ static int check_map_prog_compatibility(struct bpf_verifier_env *env,
 			return -EOPNOTSUPP;
 		}
 		env->prog->aux->arena = (void *)map;
+		env->prog->jit_required = true;
 		if (!bpf_arena_get_user_vm_start(env->prog->aux->arena)) {
 			verbose(env, "arena's user address must be set via map_extra or mmap()\n");
 			return -EINVAL;
-- 
2.55.0

[PATCH bpf-next v2 2/3] bpf: Disallow interpreter fallback for gotox insn

[Leon Hwang]

The interpreter does not recognize the BPF_JMP|BPF_JA|BPF_X insn, which
is used for insn_array map. Thereafter, it would hit the BUG_ON() in
___bpf_prog_run() at run time.

[    2.563726] BPF interpreter: unknown opcode 0d (imm: 0x0)
[    2.564557] ------------[ cut here ]------------
[    2.565206] kernel BUG at kernel/bpf/core.c:2349!
[    2.565882] Oops: invalid opcode: 0000 [#1] SMP PTI

Set jit_required as true when insn_array map is used in the prog in
order to disallow interpreter fallback for gotox insn in
core.c::__bpf_prog_select_runtime().

Fixes: 493d9e0d6083 ("bpf, x86: add support for indirect jumps")
Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
---
 kernel/bpf/verifier.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 3849d3d00e15..e2ef7836b7b5 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -17927,6 +17927,7 @@ static int __add_used_map(struct bpf_verifier_env *env, struct bpf_map *map)
 			return err;
 		}
 		env->insn_array_maps[env->insn_array_map_cnt++] = map;
+		env->prog->jit_required = true;
 	}
 
 	return env->used_map_cnt - 1;
-- 
2.55.0

[PATCH bpf-next v2 3/3] bpf: Disallow interpreter fallback for BPF_ADDR_PERCPU insn

[Leon Hwang]

The BPF_MOV64_PERCPU_REG insn requires JIT to emit native code to for
'dst_reg = src_reg + <percpu_base_off>'.

However, the interpreter ignores the 'off' at its ALU64_MOV_X label.
The 'off' indicates the insn is BPF_MOV64_PERCPU_REG insn. Then, when
the interpreter loads memory from the register, it will hit a page
fault.

[    2.545572] BUG: unable to handle page fault for address: ffffffffacaaf034
[    2.546485] #PF: supervisor read access in kernel mode
[    2.547167] #PF: error_code(0x0000) - not-present page
[    2.547850] PGD 134e63067 P4D 134e63067 PUD 134e64063 PMD 10021c063 PTE 800ffffeca550062
[    2.548912] Oops: Oops: 0000 [#1] SMP PTI

Set jit_required as true in order to disallow interpreter fallback in
core.c::__bpf_prog_select_runtime(), if any BPF_ADDR_PERCPU insn is
patched to the prog.

BTW, rename the helper bpf_map_supports_cpu_flags() to
bpf_map_is_percpu_map().

Fixes: 7bdbf7446305 ("bpf: add special internal-only MOV instruction to resolve per-CPU addrs")
Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
---
 include/linux/bpf.h | 4 ++--
 kernel/bpf/fixups.c | 5 +++++
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 31181e0c2b80..d9542127dfdf 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -4163,7 +4163,7 @@ bpf_prog_update_insn_ptrs(struct bpf_prog *prog, u32 *offsets, void *image)
 }
 #endif
 
-static inline bool bpf_map_supports_cpu_flags(enum bpf_map_type map_type)
+static inline bool bpf_map_is_percpu_map(enum bpf_map_type map_type)
 {
 	switch (map_type) {
 	case BPF_MAP_TYPE_PERCPU_ARRAY:
@@ -4190,7 +4190,7 @@ static inline int bpf_map_check_op_flags(struct bpf_map *map, u64 flags, u64 all
 		return -EINVAL;
 
 	if (flags & (BPF_F_CPU | BPF_F_ALL_CPUS)) {
-		if (!bpf_map_supports_cpu_flags(map->map_type))
+		if (!bpf_map_is_percpu_map(map->map_type))
 			return -EINVAL;
 		if ((flags & BPF_F_CPU) && (flags & BPF_F_ALL_CPUS))
 			return -EINVAL;
diff --git a/kernel/bpf/fixups.c b/kernel/bpf/fixups.c
index d3be972714b2..a0bddada7964 100644
--- a/kernel/bpf/fixups.c
+++ b/kernel/bpf/fixups.c
@@ -2008,6 +2008,9 @@ int bpf_do_misc_fixups(struct bpf_verifier_env *env)
 					return -EFAULT;
 				}
 
+				if (bpf_map_is_percpu_map(map_ptr->map_type))
+					prog->jit_required = true;
+
 				new_prog = bpf_patch_insn_data(env, i + delta,
 							       insn_buf, cnt);
 				if (!new_prog)
@@ -2112,6 +2115,7 @@ int bpf_do_misc_fixups(struct bpf_verifier_env *env)
 			 * way, it's fine to back out this inlining logic
 			 */
 #ifdef CONFIG_SMP
+			prog->jit_required = true;
 			insn_buf[0] = BPF_MOV64_IMM(BPF_REG_0, (u32)(unsigned long)&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);
@@ -2133,6 +2137,7 @@ int bpf_do_misc_fixups(struct bpf_verifier_env *env)
 		/* Implement bpf_get_current_task() and bpf_get_current_task_btf() inline. */
 		if ((insn->imm == BPF_FUNC_get_current_task || insn->imm == BPF_FUNC_get_current_task_btf) &&
 		    bpf_verifier_inlines_helper_call(env, insn->imm)) {
+			prog->jit_required = true;
 			insn_buf[0] = BPF_MOV64_IMM(BPF_REG_0, (u32)(unsigned long)&current_task);
 			insn_buf[1] = BPF_MOV64_PERCPU_REG(BPF_REG_0, BPF_REG_0);
 			insn_buf[2] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_0, 0);
-- 
2.55.0