Commit 9f118095 authored by Daniel Borkmann's avatar Daniel Borkmann Committed by Alexei Starovoitov
Browse files

bpf: Drop pkt_end markers on arithmetic to prevent is_pkt_ptr_branch_taken 



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: 6d94e741 ("bpf: Support for pointers beyond pkt_end.")
Reported-by: default avatarSTAR Labs SG <info@starlabs.sg>
Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/r/20260409155016.536608-1-daniel@iogearbox.net


Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
parent e0fcb42b

kernel/bpf/verifier.c

+22 −8
Original line number Diff line number Diff line
@@ -15448,9 +15448,16 @@ 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)) {

		if (reg_is_pkt_pointer(ptr_reg)) {

			if (!known)

				dst_reg->id = ++env->id_gen;

			/* something was added to pkt_ptr, set range to zero */

			/*

			 * 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;
@@ -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)) {

		if (reg_is_pkt_pointer(ptr_reg)) {

			if (!known)

				dst_reg->id = ++env->id_gen;

			/* something was added to pkt_ptr, set range to zero */

			if (smin_val < 0)

			/*

			 * 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;