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bpf: Drop pkt_end markers on arithmetic to prevent is_pkt_ptr_branch_taken

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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: 6d94e741a8 ("bpf: Support for pointers beyond pkt_end.")
Reported-by: STAR Labs SG <info@starlabs.sg>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/r/20260409155016.536608-1-daniel@iogearbox.net
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Daniel Borkmann 2026-04-09 17:50:15 +02:00 committed by Alexei Starovoitov
parent e0fcb42bc6
commit 9f118095dd
1 changed files with 22 additions and 8 deletions
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30
kernel/bpf/verifier.c
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@ -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;