[PATCH v4 1/2] bpf: Skip scalar adjustment for BPF_NEG if dst is a pointer

[Brahmajit Das]

In check_alu_op(), the verifier currently calls check_reg_arg() and
adjust_scalar_min_max_vals() unconditionally for BPF_NEG operations.
However, if the destination register holds a pointer, these scalar
adjustments are unnecessary and potentially incorrect.

This patch adds a check to skip the adjustment logic when the destination
register contains a pointer.

Reported-by: syzbot+d36d5ae81e1b0a53ef58@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=d36d5ae81e1b0a53ef58
Fixes: aced132599b3 ("bpf: Add range tracking for BPF_NEG")
Suggested-by: KaFai Wan <kafai.wan@xxxxxxxxx>
Suggested-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
Signed-off-by: Brahmajit Das <listout@xxxxxxxxxxx>
---
Changes v4:
Cleaning up, instead of using __is_pointer_value it's further
simplified by checking if regs[insn->dst_reg].type of SCALAR_VALUE
Link: 

Changes in v3:
using __is_pointer_value to check if register if of pointer type
Link: https://lore.kernel.org/all/20251001095613.267475-1-listout@xxxxxxxxxxx/

Changes in v2: 
Checking if reg->map_ptr is NULL in bpf/log.c but with cleaner approach
(wrong approach)
Link: https://lore.kernel.org/all/20250923174738.1713751-1-listout@xxxxxxxxxxx/

Changes in v1: 
Checking if reg->map_ptr is NULL in bpf/log.c (wrong approach)
Link: https://lore.kernel.org/all/20250923164144.1573636-1-listout@xxxxxxxxxxx/
---
 kernel/bpf/verifier.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index e892df386eed..f3d8ba142faa 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -15645,7 +15645,8 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
 		}
 
 		/* check dest operand */
-		if (opcode == BPF_NEG) {
+		if (opcode == BPF_NEG &&
+		    regs[insn->dst_reg].type == SCALAR_VALUE) {
 			err = check_reg_arg(env, insn->dst_reg, DST_OP_NO_MARK);
 			err = err ?: adjust_scalar_min_max_vals(env, insn,
 							 &regs[insn->dst_reg],
-- 
2.51.0