[PATCH bpf-next v2 1/6] bpf: Reject load-acquire from pointers requiring fault protection

[Daniel Borkmann <daniel@iogearbox.net>]

A BPF_LOAD_ACQ is not rewritten to a BPF_PROBE_MEM load by the verifier,
unlike a regular BPF_LDX, so the JIT emits a plain load with no exception
table entry and a fault panics the kernel instead of being handled.

Reject the source pointer types that a BPF_LDX would have had that fault
protection applied to, i.e. the ones bpf_convert_ctx_accesses() turns
into BPF_PROBE_MEM: a bare PTR_TO_BTF_ID, PTR_TO_BTF_ID | PTR_UNTRUSTED,
PTR_TO_BTF_ID | MEM_ALLOC | PTR_UNTRUSTED and PTR_TO_MEM | MEM_RDONLY |
PTR_UNTRUSTED.

This is reachable e.g. by loading ->mm out of a trusted task_struct
yields an untrusted pointer to mm_struct, and it is NULL for a kernel
thread:

  [...]
  SEC("tp_btf/sched_switch")
  int BPF_PROG(demo, bool preempt, struct task_struct *prev,
               struct task_struct *next)
  {
      struct mm_struct *mm = next->mm;  /* untrusted */

      out_ldx = (__u64)mm->pgd;         /* BPF_LDX      */
      out_acq = load_acquire(&mm->pgd); /* BPF_LOAD_ACQ */
      return 0;
  }
  [...]

Both dereference the same pointer, but only the BPF_LDX is protected
(x86-64 JIT, jump targets shown prog-relative):

  [...]
  ; out_ldx = (__u64)mm->pgd;
  17:   movq    $-10485760, %r10
  1e:   movq    %rsi, %r11
  21:   addq    $184, %r11
  28:   subq    %r10, %r11
  2b:   movabsq $140737498841088, %r10
  35:   cmpq    %r10, %r11
  38:   ja      0x3e                 <-- kernel addr?
  3a:   xorl    %edi, %edi           <-- no: dst = 0, skip the load
  3c:   jmp     0x45
  3e:   movq    184(%rsi), %rdi      <-- yes: load + extable entry
  [...]
  ; load_acquire(&mm->pgd)
  53:	movq    %rsi, %rdi
  56:	movq    184(%rdi), %rax       <-- no check, no extable entry
  [...]

Note that BPF_PROBE_MEM is not visible in a bpftool xlated dump, as
bpf_insn_prepare_dump() rewrites it back to BPF_MEM.

A PTR_TRUSTED pointer is deliberately not on the list. Such a load is
not converted either, but it does not need to be, since the pointer is
guaranteed live, so load-acquire from it stays allowed.

The check is gated on BPF_LOAD_ACQ so that atomic RMW and store-release
error messages are unchanged; writes (RMW / store-release) to such
pointers are already rejected elsewhere, so only load-acquire needs this.

Fixes: 880442305a39 ("bpf: Introduce load-acquire and store-release instructions")
Reported-by: STAR Labs SG <info@starlabs.sg>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
---
 kernel/bpf/verifier.c | 27 ++++++++++++++++++++++++++-
 1 file changed, 26 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index e6233c0081d1..648c5784178e 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -4923,6 +4923,30 @@ static bool is_arena_reg(struct bpf_verifier_env *env, int regno)
 	return reg->type == PTR_TO_ARENA;
 }
 
+static bool is_load_acq_unsafe(struct bpf_verifier_env *env, int regno,
+			       struct bpf_insn *insn)
+{
+	const struct bpf_reg_state *reg = reg_state(env, regno);
+
+	/*
+	 * A BPF_LOAD_ACQ is not rewritten to a BPF_PROBE_MEM load by the
+	 * verifier, unlike a regular BPF_LDX. The JIT would emit a plain load
+	 * with no exception table entry, so a fault (e.g. NULL deref) crashes
+	 * the kernel instead of being handled.
+	 *
+	 * Reject the source pointer types that a BPF_LDX would have had that
+	 * fault protection applied to, i.e. the ones bpf_convert_ctx_accesses()
+	 * turns into BPF_PROBE_MEM: a bare PTR_TO_BTF_ID and any PTR_UNTRUSTED
+	 * pointer (untrusted btf ids, untrusted MEM_ALLOC, rdonly untrusted
+	 * memory). A PTR_TRUSTED pointer is not among them, is not converted,
+	 * and stays allowed. Same for the other flagged PTR_TO_BTF_ID variants
+	 * (MEM_ALLOC, MEM_RCU, ...), hence the exact match on the base type.
+	 */
+	return insn->imm == BPF_LOAD_ACQ &&
+	       (reg->type == PTR_TO_BTF_ID ||
+		(type_flag(reg->type) & PTR_UNTRUSTED));
+}
+
 /* Return false if @regno contains a pointer whose type isn't supported for
  * atomic instruction @insn.
  */
@@ -4939,7 +4963,8 @@ static bool atomic_ptr_type_ok(struct bpf_verifier_env *env, int regno,
 		return false;
 	if (is_arena_reg(env, regno))
 		return bpf_jit_supports_insn(insn, true);
-
+	if (is_load_acq_unsafe(env, regno, insn))
+		return false;
 	return true;
 }
 
-- 
2.43.0