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

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

[Daniel Borkmann]

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

[PATCH bpf-next 2/2] selftests/bpf: Add test for stale pkt range after scalar arithmetic

[Daniel Borkmann]

Extend the verifier_direct_packet_access BPF selftests to exercise the
verifier code paths which ensure that the pkt range is cleared after
add/sub alu with a known scalar. The tests reject the invalid access.

  # LDLIBS=-static PKG_CONFIG='pkg-config --static' ./vmtest.sh -- ./test_progs -t verifier_direct
  [...]
  #592/35  verifier_direct_packet_access/direct packet access: pkt_range cleared after sub with known scalar:OK
  #592/36  verifier_direct_packet_access/direct packet access: pkt_range cleared after add with known scalar:OK
  #592/37  verifier_direct_packet_access/direct packet access: test3:OK
  #592/38  verifier_direct_packet_access/direct packet access: test3 @unpriv:OK
  #592/39  verifier_direct_packet_access/direct packet access: test34 (non-linear, cgroup_skb/ingress, too short eth):OK
  #592/40  verifier_direct_packet_access/direct packet access: test35 (non-linear, cgroup_skb/ingress, too short 1):OK
  #592/41  verifier_direct_packet_access/direct packet access: test36 (non-linear, cgroup_skb/ingress, long enough):OK
  #592     verifier_direct_packet_access:OK
  [...]
  Summary: 2/47 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
---
 .../bpf/progs/verifier_direct_packet_access.c | 61 +++++++++++++++++++
 1 file changed, 61 insertions(+)

diff --git a/tools/testing/selftests/bpf/progs/verifier_direct_packet_access.c b/tools/testing/selftests/bpf/progs/verifier_direct_packet_access.c
index 4ee3b7a708f7..915a9707298b 100644
--- a/tools/testing/selftests/bpf/progs/verifier_direct_packet_access.c
+++ b/tools/testing/selftests/bpf/progs/verifier_direct_packet_access.c
@@ -859,4 +859,65 @@ l0_%=:	r0 = 1;					\
 	: __clobber_all);
 }
 
+SEC("tc")
+__description("direct packet access: pkt_range cleared after sub with known scalar")
+__failure __msg("invalid access to packet")
+__naked void pkt_range_clear_after_sub(void)
+{
+	asm volatile ("					\
+	r9 = *(u32*)(r1 + %[__sk_buff_data]);		\
+	r8 = *(u32*)(r1 + %[__sk_buff_data_end]);	\
+	r9 += 256;					\
+	if r9 >= r8 goto l0_%=;				\
+	r0 = 0;						\
+	exit;						\
+l0_%=:	/* r9 has AT_PKT_END (pkt + 256 >= pkt_end) */	\
+	r9 -= 256;					\
+	/*						\
+	 * AT_PKT_END must not survive the arithmetic.	\
+	 * is_pkt_ptr_branch_taken must validate both	\
+	 * branches when visiting the next condition.	\
+	 */						\
+	if r9 < r8 goto l1_%=;				\
+	r0 = 0;						\
+	exit;						\
+l1_%=:	r0 = *(u8*)(r9 + 0);				\
+	r0 = 0;						\
+	exit;						\
+"	:
+	: __imm_const(__sk_buff_data, offsetof(struct __sk_buff, data)),
+	  __imm_const(__sk_buff_data_end, offsetof(struct __sk_buff, data_end))
+	: __clobber_all);
+}
+
+SEC("tc")
+__description("direct packet access: pkt_range cleared after add with known scalar")
+__failure __msg("invalid access to packet")
+__naked void pkt_range_clear_after_add(void)
+{
+	asm volatile ("					\
+	r9 = *(u32*)(r1 + %[__sk_buff_data]);		\
+	r8 = *(u32*)(r1 + %[__sk_buff_data_end]);	\
+	r9 += 256;					\
+	if r9 >= r8 goto l0_%=;				\
+	r0 = 0;						\
+	exit;						\
+l0_%=:	/* r9 has AT_PKT_END (pkt + 256 >= pkt_end) */	\
+	r9 += -256;					\
+	/*						\
+	 * Same as sub, but goes through BPF_ADD path.	\
+	 * AT_PKT_END must not survive the arithmetic.	\
+	 */						\
+	if r9 < r8 goto l1_%=;				\
+	r0 = 0;						\
+	exit;						\
+l1_%=:	r0 = *(u8*)(r9 + 0);				\
+	r0 = 0;						\
+	exit;						\
+"	:
+	: __imm_const(__sk_buff_data, offsetof(struct __sk_buff, data)),
+	  __imm_const(__sk_buff_data_end, offsetof(struct __sk_buff, data_end))
+	: __clobber_all);
+}
+
 char _license[] SEC("license") = "GPL";
-- 
2.43.0

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

[patchwork-bot+netdevbpf]

Hello:

This series was applied to bpf/bpf-next.git (master)
by Alexei Starovoitov <ast@kernel.org>:

On Thu,  9 Apr 2026 17:50:15 +0200 you wrote:
> 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.
> 
> [...]

Here is the summary with links:
  - [bpf-next,1/2] bpf: Drop pkt_end markers on arithmetic to prevent is_pkt_ptr_branch_taken
    https://git.kernel.org/bpf/bpf-next/c/9f118095dd34
  - [bpf-next,2/2] selftests/bpf: Add test for stale pkt range after scalar arithmetic
    https://git.kernel.org/bpf/bpf-next/c/8697bdd67be8

You are awesome, thank you!
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