bpf: fix REG INVARIANTS VIOLATION on speculative pointer arithmetic

An unprivileged socket filter does variable pointer arithmetic on a PTR_TO_MAP_VALUE whose offset collapses to a constant. The Spectre-v1 speculative path used to snapshot the pointer with a const offset and an unbounded r32, which tripped reg_bounds_sanity_check() on the following register move.

Mark the test __success_unpriv (the speculative path only runs unprivileged) and flag it BPF_F_TEST_REG_INVARIANTS so the invariant violation becomes a hard load failure. The unprivileged run fails without the verifier fix and passes with it:

verifier_bounds/spec_ptr_alu_const_offset @unpriv:FAIL # without fix
verifier_bounds/spec_ptr_alu_const_offset @unpriv:OK # with fix

Signed-off-by Jiayuan Chen <[email protected]>

---

Take the following unprivileged program as an example:

	r0 = bpf_map_lookup_elem(...)	/* PTR_TO_MAP_VALUE, offset 0 */
	...
	14: r0 += r1			/* r1 is a bounded scalar */
	15: r9 = r0

Loading it triggers a verifier warning from reg_bounds_sanity_check():

	verifier bug: REG INVARIANTS VIOLATION (alu): const subreg tnum out
	of sync with range bounds r64={.base=0x0, .size=0x0}
	r32={.base=0x0, .size=0xffffffff} var_off=(0x0, 0x0)

What happens:

1. Processing insn 14 (r0 += r1) in adjust_ptr_min_max_vals(), the new offset is computed into dst_reg's var_off and 32/64-bit ranges.

2. Because pointer registers do not track 32-bit subregister bounds, __mark_reg32_unbounded() first sets r32 to the full range; r32 is re-derived from the offset at the end of the function by reg_bounds_sync().

3. On the unprivileged path, sanitize_ptr_alu() is called and, via sanitize_speculative_path() -> push_stack(), snapshots the current register state and schedules the next instruction (insn 15) to be verified directly as a speculative path.

4. That snapshot is taken between step 2 and the final reg_bounds_sync():
at this point dst_reg's var_off still holds the (const) original offset while r32 has just been blanked to the full range, i.e. the two are out of sync. When the speculative path later verifies insn 15 (r9 = r0), the inconsistent state reaches reg_bounds_sanity_check() and trips the warning.

var_off and the 32-bit range must always be consistent. There are two ways to keep the snapshot consistent:

1. sync var_off and r32 before the snapshot so they match, or
2. leave r32 at its original (already consistent) value and blank it only after the snapshot.

The whole point of sanitize_ptr_alu() is to insert a harmless masking sequence that keeps the access in bounds under speculation, so the state it snapshots should faithfully represent that. Take approach 2: move __mark_reg32_unbounded() to after sanitize_ptr_alu(), so the speculative snapshot keeps the pointer's original, consistent r32. The non-speculative path is unchanged: r32 is still blanked before the offset is applied and re-derived by reg_bounds_sync().

Fixes 5f99f312bd3b ("bpf: add register bounds sanity checks and sanitization")

Reported-by Hiker Cl <[email protected]>

Closes https://lore.kernel.org/bpf/CAGM=xGB1fJ9kT8XTitVo74B0WGqgjkoUHdLwzytwV0AyqeVApw@mail.gmail.com/ open here

Signed-off-by Jiayuan Chen <[email protected]>

---

 kernel/bpf/verifier.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

kernel/bpf/verifier.c +0 -3

index d17f14b35b79..d79038a8da10 100644
@@ -14558,9 +14558,6 @@ static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env, struct bpf_insn
 		return -EINVAL;
 	}
 
-	/* pointer types do not carry 32-bit bounds at the moment. */
-	__mark_reg32_unbounded(dst_reg);
-
 	if (sanitize_needed(opcode)) {
 		ret = sanitize_ptr_alu(env, insn, ptr_reg, off_reg, dst_reg,
 				       &info, false);

kernel/bpf/verifier.c +2 -0

+	__mark_reg32_unbounded(dst_reg);
+
 	switch (opcode) {
 	case BPF_ADD:
 		/*

Quoted 50 lines · Jiayuan Chen

Take the following unprivileged program as an example:

	r0 = bpf_map_lookup_elem(...)	/* PTR_TO_MAP_VALUE, offset 0 */
	...
	14: r0 += r1			/* r1 is a bounded scalar */
	15: r9 = r0

Loading it triggers a verifier warning from reg_bounds_sanity_check():

	verifier bug: REG INVARIANTS VIOLATION (alu): const subreg tnum out
	of sync with range bounds r64={.base=0x0, .size=0x0}
	r32={.base=0x0, .size=0xffffffff} var_off=(0x0, 0x0)

What happens:

1. Processing insn 14 (r0 += r1) in adjust_ptr_min_max_vals(), the new offset is computed into dst_reg's var_off and 32/64-bit ranges.

2. Because pointer registers do not track 32-bit subregister bounds, __mark_reg32_unbounded() first sets r32 to the full range; r32 is re-derived from the offset at the end of the function by reg_bounds_sync().

3. On the unprivileged path, sanitize_ptr_alu() is called and, via sanitize_speculative_path() -> push_stack(), snapshots the current register state and schedules the next instruction (insn 15) to be verified directly as a speculative path.

4. That snapshot is taken between step 2 and the final reg_bounds_sync():
at this point dst_reg's var_off still holds the (const) original offset while r32 has just been blanked to the full range, i.e. the two are out of sync. When the speculative path later verifies insn 15 (r9 = r0), the inconsistent state reaches reg_bounds_sanity_check() and trips the warning.

var_off and the 32-bit range must always be consistent. There are two ways to keep the snapshot consistent:

1. sync var_off and r32 before the snapshot so they match, or
2. leave r32 at its original (already consistent) value and blank it only after the snapshot.

The whole point of sanitize_ptr_alu() is to insert a harmless masking sequence that keeps the access in bounds under speculation, so the state it snapshots should faithfully represent that. Take approach 2: move __mark_reg32_unbounded() to after sanitize_ptr_alu(), so the speculative snapshot keeps the pointer's original, consistent r32. The non-speculative path is unchanged: r32 is still blanked before the offset is applied and re-derived by reg_bounds_sync().

Quoted 5 lines

Fixes 5f99f312bd3b ("bpf: add register bounds sanity checks and sanitization")

Reported-by Hiker Cl <[email protected]>

Closes https://lore.kernel.org/bpf/CAGM=xGB1fJ9kT8XTitVo74B0WGqgjkoUHdLwzytwV0AyqeVApw@mail.gmail.com/ open here

Signed-off-by Jiayuan Chen <[email protected]>

---

Acked-by Eduard Zingerman <[email protected]>

Quoted 21 lines

 kernel/bpf/verifier.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

kernel/bpf/verifier.c +0 -3

index d17f14b35b79..d79038a8da10 100644
@@ -14558,9 +14558,6 @@ static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env, struct bpf_insn
 		return -EINVAL;
 	}
 
-	/* pointer types do not carry 32-bit bounds at the moment. */
-	__mark_reg32_unbounded(dst_reg);
-
 	if (sanitize_needed(opcode)) {
 		ret = sanitize_ptr_alu(env, insn, ptr_reg, off_reg, dst_reg,
 				       &info, false);

@@ -14568,6 +14565,14 @@ static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env, struct bpf_insn
 			return sanitize_err(env, insn, ret);
 	}
 

kernel/bpf/verifier.c static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env, struct bpf_insn · line 14573

Quoted 6 lines

kernel/bpf/verifier.c +6 -0

+	/* Pointer types do not carry 32-bit bounds at the moment. Blank r32
+	 * only after sanitize_ptr_alu() may have snapshotted dst_reg into a
+	 * speculative path: otherwise that snapshot freezes a const offset
+	 * with an unbounded r32, which later trips reg_bounds_sanity_check().
+	 * reg_bounds_sync() below re-derives r32 from the updated offset.
+	 */

Nit: comment is a bit too verbose. "... otherwise reg_bounds_sanity_check()
might hit some constraints violations" should have been enough.

Quoted 5 lines

kernel/bpf/verifier.c +2 -0

+	__mark_reg32_unbounded(dst_reg);
+
 	switch (opcode) {
 	case BPF_ADD:
 		/*