[PATCH bpf-next v2 04/13] bpf: Track R2 of register-pair returns in precision backtracking

[Yonghong Song <yonghong.song@linux.dev>]

A function returning a value larger than 8 bytes (a struct/union, or an
__int128) uses R2 as a second return register alongside R0. Precision
backtracking treats only R0 as a return register at a call/return boundary,
so once the verifier starts modeling R2 that way, marking the second half
of such a return precise would trip the "unexpected regs" checks in
backtrack_insn() and reject a valid program with -EFAULT. Handle it here,
ahead of the patch that introduces the modeling.

Marking the upper half precise, for example by branching on it after a
call to a static subprogram, walks backtracking into the callee and
reaches its BPF_EXIT with R2 still set in the mask. R2 is part of
BPF_REGMASK_ARGS, so this hits "backtracking exit unexpected regs".
Returning the pair from a global subprogram or from a kfunc instead hits
the equivalent check at the call site.

Handle R2 like R0 in the three boundaries where a call defines the return
registers:

 - static subprog exit (BPF_EXIT): when the callee returns a pair, R2 is a
   return register rather than a clobbered argument, so its precision has to
   cross the frame boundary just like R0's: clear it from the caller's mask
   before the R1-R5 check, then set it again in the callee's mask after
   bt_subprog_enter().

   The clear has to be conditional, which is why the subprogram containing
   the exit insn is looked up and queried. For a callee that does not return
   a pair, check_func_call() has already invalidated the caller's R1-R5 and
   prepare_func_exit() copies back only R0, so nothing after the call can
   depend on R2 and backtracking should never still be asking for it here.
   Clearing it unconditionally would turn that into a silent no-op instead
   of reporting it through the existing "backtracking exit unexpected regs"
   check.
 - global subprog call: a global subprog returning >8 bytes also sets R2;
   clear it before the args check.
 - kfunc call (BPF_CALL): a kfunc returning >8 bytes (model ret_size > 8)
   also sets R2; clear it like R0.

All three are gated on R2 actually being in the mask, so the extra BTF and
kfunc descriptor lookups stay off the common backtracking path.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
---
 include/linux/bpf_verifier.h |  2 ++
 kernel/bpf/backtrack.c       | 59 ++++++++++++++++++++++++++++--------
 kernel/bpf/verifier.c        | 13 ++++++++
 3 files changed, 62 insertions(+), 12 deletions(-)

diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 18a6ecff39c5..adb3f3019a98 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -1442,6 +1442,8 @@ int bpf_jmp_offset(struct bpf_insn *insn);
 struct bpf_iarray *bpf_insn_successors(struct bpf_verifier_env *env, u32 idx);
 void bpf_fmt_stack_mask(char *buf, ssize_t buf_sz, u64 stack_mask);
 bool bpf_subprog_is_global(const struct bpf_verifier_env *env, int subprog);
+int bpf_get_kfunc_ret_size(const struct bpf_prog *prog, u32 func_id,
+			   u16 btf_fd_idx, u8 *ret_size);
 
 int bpf_find_subprog(struct bpf_verifier_env *env, int off);
 bool bpf_is_throw_kfunc(struct bpf_insn *insn);
diff --git a/kernel/bpf/backtrack.c b/kernel/bpf/backtrack.c
index 2f473ad4fd7c..498b15082801 100644
--- a/kernel/bpf/backtrack.c
+++ b/kernel/bpf/backtrack.c
@@ -424,6 +424,14 @@ static int backtrack_insn(struct bpf_verifier_env *env, int idx, int subseq_idx,
 				 */
 				verifier_bug_if(idx + 1 != subseq_idx, env,
 						"extra insn from subprog");
+				/* a global subprog returning more than 8 bytes
+				 * sets R2 as well. R2 is part of the args mask
+				 * checked just below, so it has to be cleared
+				 * here rather than next to R0.
+				 */
+				if (bt_is_reg_set(bt, BPF_REG_2) &&
+				    bpf_ret_reg_pair(env, subprog))
+					bt_clear_reg(bt, BPF_REG_2);
 				/* r1-r5 are invalidated after subprog call,
 				 * so for global func call it shouldn't be set
 				 * anymore
@@ -507,6 +515,17 @@ static int backtrack_insn(struct bpf_verifier_env *env, int idx, int subseq_idx,
 				return -ENOTSUPP;
 			/* regular helper call sets R0 */
 			bt_clear_reg(bt, BPF_REG_0);
+			/* a kfunc returning more than 8 bytes also sets R2 */
+			if (insn->src_reg == BPF_PSEUDO_KFUNC_CALL &&
+			    bt_is_reg_set(bt, BPF_REG_2)) {
+				u8 ret_size;
+
+				if (bpf_get_kfunc_ret_size(env->prog, insn->imm, insn->off,
+							   &ret_size))
+					return -ENOTSUPP;
+				if (ret_size > 8)
+					bt_clear_reg(bt, BPF_REG_2);
+			}
 			if (bt_reg_mask(bt) & BPF_REGMASK_ARGS) {
 				/* if backtracking was looking for registers R1-R5
 				 * they should have been found already.
@@ -521,7 +540,29 @@ static int backtrack_insn(struct bpf_verifier_env *env, int idx, int subseq_idx,
 					return -EFAULT;
 			}
 		} else if (opcode == BPF_EXIT) {
-			bool r0_precise;
+			bool from_subprog_call, r0_precise, r2_precise = false;
+
+			/* BPF_EXIT in subprog or callback always returns
+			 * right after the call instruction, so by checking
+			 * whether the instruction at subseq_idx-1 is subprog
+			 * call or not we can distinguish actual exit from
+			 * *subprog* from exit from *callback*. In the former
+			 * case, we need to propagate the precision of the
+			 * return registers, if necessary. In the latter we
+			 * never do that.
+			 */
+			from_subprog_call = subseq_idx - 1 >= 0 &&
+					    bpf_pseudo_call(&env->prog->insnsi[subseq_idx - 1]);
+			if (from_subprog_call && bt_is_reg_set(bt, BPF_REG_2)) {
+				struct bpf_subprog_info *callee;
+
+				/* 'idx' is the exit insn, so it is in the callee */
+				callee = bpf_find_containing_subprog(env, idx);
+				if (verifier_bug_if(!callee, env,
+						    "no subprog contains exit insn %d", idx))
+					return -EFAULT;
+				r2_precise = bpf_ret_reg_pair(env, callee - env->subprog_info);
+			}
 
 			/* Backtracking to a nested function call, 'idx' is a part of
 			 * the inner frame 'subseq_idx' is a part of the outer frame.
@@ -534,23 +575,15 @@ static int backtrack_insn(struct bpf_verifier_env *env, int idx, int subseq_idx,
 			if (subseq_idx >= 0 && bpf_calls_callback(env, subseq_idx))
 				for (i = BPF_REG_1; i <= BPF_REG_5; i++)
 					bt_clear_reg(bt, i);
+			if (r2_precise)
+				bt_clear_reg(bt, BPF_REG_2);
 			if (bt_reg_mask(bt) & BPF_REGMASK_ARGS) {
 				verifier_bug(env, "backtracking exit unexpected regs %x",
 					     bt_reg_mask(bt));
 				return -EFAULT;
 			}
 
-			/* BPF_EXIT in subprog or callback always returns
-			 * right after the call instruction, so by checking
-			 * whether the instruction at subseq_idx-1 is subprog
-			 * call or not we can distinguish actual exit from
-			 * *subprog* from exit from *callback*. In the former
-			 * case, we need to propagate r0 precision, if
-			 * necessary. In the former we never do that.
-			 */
-			r0_precise = subseq_idx - 1 >= 0 &&
-				     bpf_pseudo_call(&env->prog->insnsi[subseq_idx - 1]) &&
-				     bt_is_reg_set(bt, BPF_REG_0);
+			r0_precise = from_subprog_call && bt_is_reg_set(bt, BPF_REG_0);
 
 			bt_clear_reg(bt, BPF_REG_0);
 			if (bt_subprog_enter(bt))
@@ -558,6 +591,8 @@ static int backtrack_insn(struct bpf_verifier_env *env, int idx, int subseq_idx,
 
 			if (r0_precise)
 				bt_set_reg(bt, BPF_REG_0);
+			if (r2_precise)
+				bt_set_reg(bt, BPF_REG_2);
 			/* r6-r9 and stack slots will stay set in caller frame
 			 * bitmasks until we return back from callee(s)
 			 */
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 4010575d6715..282aee7fc44c 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2504,6 +2504,19 @@ int bpf_get_kfunc_addr(const struct bpf_prog *prog, u32 func_id,
 	return 0;
 }
 
+int bpf_get_kfunc_ret_size(const struct bpf_prog *prog, u32 func_id,
+			   u16 btf_fd_idx, u8 *ret_size)
+{
+	const struct bpf_kfunc_desc *desc;
+
+	desc = find_kfunc_desc(prog, func_id, btf_fd_idx);
+	if (!desc)
+		return -EFAULT;
+
+	*ret_size = desc->func_model.ret_size;
+	return 0;
+}
+
 #define BPF_FD_SLOT_BTF	1UL
 
 static void fd_slot_set_map(struct bpf_fd_array *slot, struct bpf_map *map)
-- 
2.53.0-Meta