[PATCH bpf-next 1/2] bpf: Drop pkt_end markers on arithmetic to prevent is_pkt_ptr_branch_taken

[Daniel Borkmann <daniel@iogearbox.net>]

When a pkt pointer acquires AT_PKT_END or BEYOND_PKT_END range from
a comparison, and then, known-constant arithmetic is performed,
adjust_ptr_min_max_vals() copies the stale range via dst_reg->raw =
ptr_reg->raw without clearing the negative reg->range sentinel values.

This lets is_pkt_ptr_branch_taken() choose one branch direction and
skip going through the other. Fix this by clearing negative pkt range
values (that is, AT_PKT_END and BEYOND_PKT_END) after arithmetic on
pkt pointers. This ensures is_pkt_ptr_branch_taken() returns unknown
and both branches are properly verified.

Fixes: 6d94e741a8ff ("bpf: Support for pointers beyond pkt_end.")
Reported-by: STAR Labs SG <info@starlabs.sg>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
---
 kernel/bpf/verifier.c | 30 ++++++++++++++++++++++--------
 1 file changed, 22 insertions(+), 8 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 1227b168bb07..9c1135d373e2 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -15448,10 +15448,17 @@ static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env,
 		}
 		dst_reg->var_off = tnum_add(ptr_reg->var_off, off_reg->var_off);
 		dst_reg->raw = ptr_reg->raw;
-		if (!known && reg_is_pkt_pointer(ptr_reg)) {
-			dst_reg->id = ++env->id_gen;
-			/* something was added to pkt_ptr, set range to zero */
-			memset(&dst_reg->raw, 0, sizeof(dst_reg->raw));
+		if (reg_is_pkt_pointer(ptr_reg)) {
+			if (!known)
+				dst_reg->id = ++env->id_gen;
+			/*
+			 * Clear range for unknown addends since we can't know
+			 * where the pkt pointer ended up. Also clear AT_PKT_END /
+			 * BEYOND_PKT_END from prior comparison as any pointer
+			 * arithmetic invalidates them.
+			 */
+			if (!known || dst_reg->range < 0)
+				memset(&dst_reg->raw, 0, sizeof(dst_reg->raw));
 		}
 		break;
 	case BPF_SUB:
@@ -15490,10 +15497,17 @@ static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env,
 		}
 		dst_reg->var_off = tnum_sub(ptr_reg->var_off, off_reg->var_off);
 		dst_reg->raw = ptr_reg->raw;
-		if (!known && reg_is_pkt_pointer(ptr_reg)) {
-			dst_reg->id = ++env->id_gen;
-			/* something was added to pkt_ptr, set range to zero */
-			if (smin_val < 0)
+		if (reg_is_pkt_pointer(ptr_reg)) {
+			if (!known)
+				dst_reg->id = ++env->id_gen;
+			/*
+			 * Clear range if the subtrahend may be negative since
+			 * pkt pointer could move past its bounds. A positive
+			 * subtrahend moves it backwards keeping positive range
+			 * intact. Also clear AT_PKT_END / BEYOND_PKT_END from
+			 * prior comparison as arithmetic invalidates them.
+			 */
+			if ((!known && smin_val < 0) || dst_reg->range < 0)
 				memset(&dst_reg->raw, 0, sizeof(dst_reg->raw));
 		}
 		break;
-- 
2.43.0